Recent progress and perspective on lithium metal anode protection

Author’s Accepted Manuscript Recent Progress and Perspective on Lithium Metal Anode Protection Huijun Yang, Cheng Guo, Ahmad Naveed, Jingyu Lei, Jun Yang, Yanna Nuli, Jiulin Wang www.elsevier.com/locate/ensm

PII: DOI: Reference:

S2405-8297(18)30029-1 https://doi.org/10.1016/j.ensm.2018.03.001 ENSM328

To appear in: Energy Storage Materials Received date: 10 January 2018 Revised date: 1 March 2018 Accepted date: 2 March 2018 Cite this article as: Huijun Yang, Cheng Guo, Ahmad Naveed, Jingyu Lei, Jun Yang, Yanna Nuli and Jiulin Wang, Recent Progress and Perspective on Lithium Metal Anode Protection, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2018.03.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Recent Progress and Perspective on Lithium Metal Anode Protection Huijun Yang, Cheng Guo, Ahmad Naveed, Jingyu Lei, Jun Yang, Yanna Nuli, Jiulin Wang* Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Corresponding author: Prof. Jiulin Wang, [email protected] Key words: Li metal anode, Structure design, Interface regulation, Liquid electrolytes, Solid electrolyte

Abstract Lithium (Li) metal is treated as ultimate anode for the most promising next-generation high energy density of Lithium-metal batteries (LMBs). However, uncontrolled Li dendrite growth and low Coulombic efficiency during the Li plating/stripping process in the batteries have severely hindered its practical application. In rechargeable batteries, Li dendrite growth not only leads to rapid capacity decay and short lifetime, but also results in severe safety concerns. Although this phenomenon is known for more than 40 years, none of the reported strategies are able to fully solve the problem till now. This review updates the reports and classifies the Li anode protection strategies into four parts: structure design, interface regulation, liquid and solid electrolytes tailoring. To get visual proof of dendrite growth in cells, new technique are utilized to characterize the Li deposition morphology has also been reviewed.

Key words: Li metal anode, Structure design, Interface regulation, Liquid electrolytes, Solid electrolyte

Abstract Lithium (Li) metal is treated as ultimate anode for the most promising next-generation high energy density of Lithium-metal batteries (LMBs). However, uncontrolled Li dendrite growth and low Coulombic efficiency during the Li plating/stripping process in the batteries have severely hindered its practical application. In rechargeable batteries, Li dendrite growth not only leads to rapid capacity decay and short lifetime, but also results in severe safety concerns. Although this phenomenon is known for more than 40 years, none of the reported strategies are able to fully solve the problem till now. This review updates the reports and classifies the Li anode protection strategies into four parts: structure design, interface regulation, liquid and solid electrolytes tailoring. To get visual proof of dendrite growth in cells, new technique are utilized to characterize the Li deposition morphology has also been reviewed.

1. Introduction Lithium (Li) metal is regarded as the ultimate anode for energy-storage systems for its extremely high theoretical specific capacity (3860 mAh g-1), the lowest redox potential (-3.040 V vs the standard hydrogen electrode) and a low gravimetric density (0.534 g cm-3).[1-3] Rechargeable lithium metal batteries (LMBs) have been extensively studied in the past 40 years and recently

attracted worldwide attention because of the extensive demands for high-energy LMBs.[4] It is greatly considered to explore high-energy density systems like Li-S batteries and Li-O2 batteries for next-generation energy storage systems.[5] The state-of-the-art Li-ion batteries (LIBs) can achieve a specific energy of ~250 Wh kg-1 by using graphite anode and lithium transitional-metal oxide (LMO). Replacing the graphite with Li, the Li-LMO batteries can reach a specific energy of ~400 Wh kg-1. By solving the key issue of Li metal, sulfur and oxygen-based cathodes, the Li-S and Li-O2 systems their practical application would get accelerated with specific energy higher than 500 Wh kg-1.[6, 7] Unfortunately, rechargeable LMBs based Li metal anode suffer from low Coulombic efficiency (CE) which leads to short cycle lifetime and the potential safety problems e.g; short-circuiting and subsequent thermal runaway.[8] The main problems impeding its successful development are the unpredictable shape of Li dendrites growth during repeated plating/stripping processes and low Coulombic efficiency. During the charge/discharge process, Li couldn`t form a homogeneous and stable layer on the lithium surface rather resulting in various ramified dendrites, usually one-dimensional needle-like or three-dimensional branched-like.[3] The continuous growth of Li dendrites could pierce the separator and leads to short circuits. Li being thermodynamically unstable in all liquid electrolytes immediately forms a solid-electrode interphase (SEI) when immersed into electrolyte. An ideal SEI would inhibit the further reaction between Li and electrolytes and suppress the Li dendrite formation. Nonetheless, the vulnerable SEI could not shield the attack of electrolyte, resulting into ceaseless generation of SEI and electrolyte drying-up. Consequently, the breaking parts of Li (dead Li) would lose the battery capacity, resulting into low Coulombic efficiency. More seriously, the shape change derived from the infinite volume change of Li metal anode would increase its porosity, generally changing from compact structure to porous morphology. The loose architecture of Li metal anode lacks of elastic connection after high-areal capacity and long-term cycling.

+

Cathode

Anode

-

LTMO Graphite LFP O2 S

Li metal

……

Dendrite

Fig. 1. Schematic illustration of Li ion battery and Li metal batteries; the failure mechanism of LMBs and the practical strategies to suppress dendrites. Where LNMO represents lithium transition metal oxide; LFP represents LiFePO4.

Fig. 1 shows the schematic illustration of LMBs using lithium transition-metal oxide, sulfur or oxygen as cathode. The difference between LIBs and LMBs lies in the fact that Li ions can be intercalated into the layered structure of graphite and Li dendrite growth can be prevented. When the non pre-lithiated cathodes like sulfur or oxygen will be used in LMBs, Li dendrite would form and grow during the charging process (Li deposition on the Li foil).[9] Due to the intrinsic inhomogeneous nature of Li metal and the uncontrolled deposition behavior, Li ion preferred to

deposit on the hot spots because of fast reaction kinetics and shorter diffusion length.[10] In Fig. 1, the problems related to Li metal anode can be concluded into dendritic Li formation, unstable interface and changing shape of Li metal anode. In this work, we have briefly reviewed the models of Li dendrite growth mechanism, and discussed various solutions presented in recent years for Li metal anode protection. We have divided these strategies into four parts, along with brief theoretical support. For compact Li foil, structure design via forming Li-alloy or embedding into a matrix would render uniform deposition. Interface regulation consists of ex-situ Li foil treatment, separator modification and copper coating. The coating layers on Li foil or separator work as artificial SEI. The artificial layer induced by in-situ or ex-situ method plays a crucial role to lessen side-reactions. Liquid electrolytes would be competitive choice for future battery electrolyte via reasonable electrolyte tailoring. Alternatively, polymer layer or solid-state electrolyte is coated on Li foil to block dendrite penetration because of their high shear modulus. Solid electrolytes are preferred over liquid electrolytes due to their advantage of suppressing dendrite formation. Finally, we updated the new techniques to characterize Li deposition morphology and provided a summary for future development.

2. Theory of dendrite growth The Li plating/stripping process in LMBs is totally different from the intercalation/de intercalation reaction mechanism in LIBs. When graphite carbon serves as anode, Li will intercalate into the suitable layer structure, hence limiting the capacity.[11] Many reviews have been reported for the theory summarization of dendrite growth and solutions for suppressing Li dendrites.[4, 10, 12] Dendrite formation during electro-deposition of Cu, Zn, Ag seems to have 3D tree-like, bush-like, moss-like and even needle-like growth.[13] Mechanisms have been conducted and verified based on aqueous electrolytes with no obvious SEI formation and it seems to be not applicable for Li dendrite formation in organic systems. In addition, Li metal in a rechargeable Li metal battery needs to be repeatedly plated on or stripped from substrates during charge/discharge processes unlike one-time-only process occurring in other metal electrodepositions. Li dendrites will accumulate on the anode and finally lead to many serious problems, thus hinder the practical applications of rechargeable Li metal batteries. Therefore, a good understanding of the mechanism of Li dendrite formation and growth is required to mitigate or eliminate Li dendrites. In order to reveal the behavior of Li deposition and dissolution, several models have been proposed to strengthen the understanding about Li dendrite from thermodynamic [14], electrochemistry[15] and kinetic point of view [16]. Firstly, the thermodynamic mode is actually the deposition/stripping model which illustrates that the Li-ions in the electrolyte travels to the Li-metal electrode surface through the SEI film and is plated underneath the SEI. Due to the higher Li+ conductivity of deposition sites on the SEI, the crystal defects and grain boundaries in the SEI induce the continual deposition of Li-metal. An asymmetric Li-deposition is inducted by the mechanical stress within the Li-anode that results in dendrite-formation. The charge induced growth model suggests that Li-deposition takes place in combination with surface charge; the inhomogeneous distribution of charge on the Li-metal surface initiates the Li-dendrite growth. Based on this model, another metal cation (as electrolyte additive) with an effective reduction potential lower than Li+ can provide a self-healing electrostatic shield for dendrite-free deposition. This model does not provide information about the origin of dendrite-growth but provides a self-shielding method for the prevention of dendritic propagation. Thirdly, the kinetic model

proposes that the presence of Li-ions in the vicinity of Li-metal anode is due to the charge at high current density and drain of Li-salt anions in Sand’s time.[17] The ionic concentration approaches zero from positive electrode to negative at a time called “Sand`s time”. Consequently, the lacking of Li+ layer paired with local space charge layer is considered the main reason for dendritic Li. Chazalviel proposed a widely accepted diffusion model for kinetic mode that empirically relates the Sand’s time with the transfer nature of Li-ions.[17]

𝐽∗ = 2𝑒𝐶𝑜 𝐷/𝑡𝑎 𝐿 𝜏 = 𝜋𝐷

𝑒𝐶𝑜 2𝐽𝑡𝑎

1

2

𝑡𝑎 ≈ 1 − 𝑡𝐿𝑖 + =

2 𝑈𝑎 𝑈𝑎 + 𝑈𝐿𝑖 +

3

Where “D” is the ambipolar diffusion coefficient, “e” is the electronic charge, “Ua”and “ULi+” are the anionic and Li+ mobility, respectively. To simplify the simulation process, most models were conducted on a binary electrolyte with Li salt and polymer electrolyte. During the process of deposition, the concentration of cations will drop at the negative electrode and rise at the positive electrode. The limiting current density J* means the largest current density to sustain theoretically for unlimited time, as shown in equation (1). Brissot and Chazalviel et al.[18, 19] described the equation (2) from the symmetrical Li|PEO|Li cell. In the equation (2) and (3), ta and tLi+ represent the anionic and Li+ transference number, respectively. This model will be inapplicable under low current density conditions, no Sand`s behavior is expected due to the unapparent ionic concentration gradient. However, Rosso et al. observed dendrites formations without Sand behavior.[20] The author attributed this result to the existence of surface local heterogeneities on the Li metal. Ionic concentration gradient is an important factor in the electrolyte. A moving, dynamic electrolyte would lessen the concentration gradient and then be able to suppress the dendrite growth. Yang et al. investigated the behavior of Li deposition in the moving electrolyte, 1 M LiPF6 in carbonate solvents by continuous stirring.[21] The dendritic Li was effectively suppressed at the current density as high as 2.0 mA cm-2 compared to the electro-deposition at 0.5 mA cm-2 under static conditions. In addition, the charging style was also suggested to significantly influence the Li deposition. Matthew et al.[22]reported that time-dependent pulsed charging can effectively suppressed Li dendrite formation by as much 96%. They introduced a coarse-grained simulation model to confirm that dendrite formation increases with applied electrode overpotential. According to the model, the pause between two pulses can successfully relax the cation diffusion, thus reducing the applied overpotential in favor of lower dendrite formation propensity.

3. Structure design For compact Li foil, structure design via Li-alloy or confining into matrix would be effective to guide uniform Li deposition. It is well-known that the effective current density on Li anode during the discharge/charge process has a significant influence on the Li dendrite formation and growth. In general, low current density leads to relatively uniform deposition and stable cycling

performance and contrarily the deposition become more dendritic at high current density. By applying the idea, various 3D current collectors with higher specific surface area has been developed to decrease the effective current density and offer more sites for Li nucleation. 3.1. Lithium alloy Metallic Li is an attractive and ultimate anode for LMBs, especially for Li-free cathodes with high energy density. However, a sole strategy/solution cannot overcome these serious concerns related to fundamental nature of Li. In consideration of the huge mechanical strain and volume changes, Li-alloy composite anodes have been introduced as an alternative. Moreover, the safety concerns would be reduced by employing Li-alloy anode due to less dendrite formation and lower reactivity as compared to Li foil. Up to now, various structure designs by forming Li-alloy has been investigated in order to retard the unordered deposition of Li. Prior to 2014, simple ball milling or co-heating methods to form Li-B alloy showed certain advantage over pure Li.[23-25] In consideration of the effectiveness of matrix, Cheng et al. reported fibrous Li7B6 framework to suppress the dendrite growth and it offers better transfer properties (Fig. 2a).[25] Different from two plateaus in Li-B alloy, Li-C alloy seems better choices without voltage differences. Huang et al. introduced graphite coating on Li anode and made the composite anode short-circuit when immersed into electrolyte (Fig. 2b).[26] Using this concept, lithiated graphite functioned as physical barrier transport Li+. During discharge process, Li+ would firstly extract from the graphite and bulk Li behind serves as reservoir to automatically refill the graphite. This novel anode structure design can also be utilized for other energy storage systems. While due to the benign conductivity of graphite, excessive Li would prefer to deposit on the surface and thus leading to uneven deposition. Other appropriate coating could be explored in the future having good Li+ transport property and modest electronic conductivity. Recently, Zhao et al. reported a facile and effective method for large-scale Li-alloy (including Si, Sn, Al) as alternative anodes (Fig. 2c).[27] Due to the densely packed structure by few layers of graphene, LixSi alloy foil can withstand the air and deliver ultra-high volumetric capacity (near to theoretical value of pure Li). The uniqueness of this work lies in the compact structure via poly(styrene-butadiene-styrene) (SBS) binder and graphene. The LixSi/graphene freestanding foil has been successfully coupled with sulfur or V2O5 cathode, showing stable cycling ability.

(a)

(b) Metal anode

Li Dendrite

Dendrite

Hybrid anode

S cathode

-

+

Li7B6 framework

Nano anode

Dendrite-free

Li Deposit Graphite

Li

(c) Graphene

LixM cluster (M=Si, Sn, Al)

M nanoparticles (M=Si, Sn, Al)

Molten Li

Freestanding LixM/grapheme foil

PET film

Fig. 2 Schematic illustration. (a) Comparison of Li plating/stripping model of pure Li anode and with Li7B6 framework to suppress the dendrite growth from Ref.[25]; (b) Graphite coating on Li anode render self-short-circuit composite anode, coupled with sulfur cathode from Ref. [26]; (c) Illustration of LixM alloy/graphene composite anode (M=Si, Sn, Al) and its preparation procedure from Ref.[27].

3.2. Three dimensional current collectors It is generally considered that smaller true current density would be beneficial to uniform Li deposition. To decrease the effective current density on the anode, Rahul Mukherjee et al. reported the defect-induced graphene network to entrap Li deposition (Fig. 3a).[28] The author demonstrated that graphene defects served as seed to initiate Li deposition within the interior of graphene structure, offering efficient electron transfer. Similarly, Liu et al. reported a vertical nanoscale channels coated on Li surface to confine Li deposition (Fig. 3b).[29] Comparing with the common two-dimensional electrode, homogeneous Li+ ion flux distribution and enlarged confinement for Li growth could be achieved by polyimide-modified layer. In consideration of large surface area of graphene, Cheng et al. proposed a graphene-based anode to render dendrite-inhibition even after 70 hours of lithiaton (Fig. 3c).[30] The high-speed ion pathways and electronic conductivity ensure stable and efficient Li deposition even after 2000 cycles (200 hours

at 10 mA cm-2). Later, the same group reported an unstacked graphene framework coupling with LiTFSI-LiFSI dual salts to improve the Coulombic efficiency of Li deposition (Fig. 3d).[31] They attributed the enhanced performance to large surface area (1666 m2 g-1), large pore volume (1.63 cm3 g-1), high electrical conductivity (435 S cm-1) and excellent SEI forming ability of dual-salts electrolyte. Different from simple carbon-based conductive matrix, 3D graphitized carbon utilizing the large surface area and pore structure would largely reduce local current density. Moreover, in combination with Li interaction and electrochemical deposition can enhance its Li storage capacity. Recently, Zuo et al. have illustrated that graphitized carbon fibers (GCF) serves as a 3D current collector to store a huge amount of Li via intercalation and electrodeposition (Fig. 3e).[32] Benefited from enlarged surface area and porous framework, the Li composite anode delivered an areal capacity of 8 mAh cm-2 by lowing the effective current density and accommodating the volume expansion. The same group then reported another 3D graphitized carbon using similar mechanism. Ye et al. reported an onion-like, graphitized spherical C granule with a 3D conducting skeleton to enhance the negativity of surface charge of the C host (Fig. 3f).[33] Graphitic carbon materials have been selected as the anode for the state-art-of the Li ion batteries due to their distinctive layer structure to ensure Li+ (de)intercalation. At the same time, this reaction mechanism also limited the capacity from the root. By introducing spherical C granules on a 3D conducting carbon layers, Li+ are preferably intercalated into the graphitic layers to form Li/C compound and then plated in the nano-gaps during the Li deposition process. The electron deviation from Li to C further enhances the negativity of the surface charge of C spheres, which distribute Li+ flux and contribute a uniform Li plating. The Li stored in the Li-C compound will offset the irreversible Li loss during each cycle and the resultant Li anode maintains a high Coulombic efficiency (>95%), stable plating/stripping process for 500 cycles (500 hours at 1.0 mA cm-2) and ultra-long lifespan of 1000 cycles against LiFePO4 cathode. Recently, a light-weight, flexible and free-standing hollow carbon fiber was reported to suppress Li dendrite formation and enhances Coulombic efficiency.[34] The excellent electrochemical performance was attributed to high electroactive surface area and low effective current density. Beside this, Li can be confined to the inner space of hollow carbon and interspace of fiber. Apart from carbon-based 3D current collectors, porous metal current collector also proved to be efficient to accommodate Li deposition. Yang et al. prepared a 3D current collector with a submicron skeleton and high electro-active surface area to improve the Li deposition behavior (Fig. 3g).[35] By etching the Cu foil to obtain higher pore volume and higher surface area, it improved the amount of Li deposition and decreased the current density. Due to the sufficient volume to accommodate Li deposition and low effective current density, the Li dendrites were effectively suppressed. Li anode in the 3D copper skeleton can cycle over 600 h without short circuit and maintain a good plating/stripping efficiency of ~98.5%. Yun et al. developed a simple chemical de-alloying process to produce a 3D copper current collector for Li accommodation.[36] Unlike conventional 2D planar current collector, the voids derived from zinc dissolution offer a large surface area and volume to accommodate Li deposition. The bicontinuous microstructure largely decreases the effective current density and results into a uniform lithium deposition with high Coulombic efficiency of 97%. Moreover, when assembled into full cell with LiFePO4 cathode, the 3D current collector maintains high capacity retention of 89.7% after 300 cycles, better than the 58.2% of 2D planar current collector at the same test condition. By applying the same idea of lowering the effective current density, Lu et al. reported a rational design of

freestanding Cu nanowire (CuNW) network to suppress the Li dendrite growth via accommodating Li deposition into 3D nanostructures (Fig. 3h).[37] A high capacity of 7.5 mAh cm-2 of Li deposition without dendrite growth and average Coulombic efficiency of 98.6% during 200 cycles (1 mA cm-2/2 mAh cm-2) was achieved due to the suppression of Li dendrite growth and high conductivity of CuNW network. Recently, Wang et al. systematically analyzed the morphology evolution of Li deposits on porous Cu current collectors.[38] Due to the tip effect, Li would preferentially deposit on the microchannel walls to restrain Li dendrite growth. A stable cycling performance and high average Coulombic efficiency of 98.5% within 200 cycles (1 mA cm-2/3 mAh cm-2) could be achieved by fully utilizing the lightening rod theory of classical electromagnetism. Inspired by molten Li infusion into rGO host[39] and polyimide (PI) matrix[40], prestoring Li by thermal infusion method shows more advantages over simple 3D current collectors to avoid employing of sacrificing cells. Recently, Chi et al. demonstrated a Li composite anode by infusing molten Li into nickel foam host (Fig. 3i).[41] The author attributed the stable deposition within Ni foam host to the improved confinement and lower surface energy during electrochemical cycling.

(a)

(b) Li foil

Nucleation

Depostion

Cycling

(c)

PGN Nucleation

(d)

Depostion

Cycling

(e) Lithium

Lithium

Depositing

Stripping

(f)

(g)

(h)

(i) Ni foam

CuNWs Membrane

Li-Ni composite

Li-CuNWs composite

Fig. 3. Schematic illustration of different structures for Li deposits. (a) Defect-induced graphene network from Ref.[28]; (b) Vertical nanoscale channels seducing Li deposition comparing with planar Li foil deposition from Ref.[29]; (c) Graphene on copper foil for Li deposition from Ref.[30]; (d) Unstacked graphene framework comparing with planar structure for Li deposition from Ref.[31]; (e) Graphitized carbon fibers for both Li insertion and deposition from Ref.[32]; (f) Onion-like graphitized

spherical carbon for both Li insertion and deposition from Ref.[33] (g) 3D copper current collector with electro-active surface for Li deposition from Ref.[35]; (h) Copper nanowire render dendrite-free deposition comparing with planar copper foil deposition from Ref.[37]; (i) Infusing molten Li into nickel foam host for Li-Ni composite anode from Ref.[41].

3.3. Lithiophilic matrix Due to inhomogeneous Li ion flux distribution and tip effect of Li deposition, dendritic Li would grow even under lowered effective current density by 3D conductive matrix. Liang et al. demonstrated a novel electrode design by introducing a three-dimensional oxidized polyacylonitrile nanofiber network interlayer on the top of current collector (Fig. 4a).[42] The polymer fiber with large surface area and functional groups could guide the uniform Li metal deposits. Inspired by the idea of lithiophilic coating, Cheng et al. reported a 3D glass fiber (GF) cloth with functional groups (Si-O, O-H and O-B) to guide the Li deposition (Fig. 4b).[43] Different from common copper current collectors with lots of protuberances, GF cloth can avoid the Li ion accumulating around the protuberances and then evenly distribute the Li ion flux. After this, various similar fibers like poly(acrylonitrile) (PAN) submicron fiber array[44], fibrous metal felt (FMF)[45] proved to be effective to guide Li deposits. Recently, lithiophilic sites served as nucleation on graphene for uniform Li anode deposition has been reported.[46] N-containing functional groups, like pyridinic and pyrrolic nitrogen in graphene structure provides active sites for Li nucleation and deposition. In 2016, Lin et al. introduced layered reduced graphene oxide (rGO) as lithiophilic nanoscale gaps to host Li (Fig. 4c).[39] The novel anode composed of 7wt% rGO, contributed near 3390 mAh g-1 of capacity and low overpotential (80 mV at 3 mAh cm-2) in carbonate electrolyte. The layered Li-rGO can not only maintain stable voltage profiles with cycles, but also reduced overpotential changes both at charging and discharge states. Generally, voltage plateau would increase in final discharge state due to the fresh Li reacting with carbonate electrolyte. While in the case using Li-rGO anode, the flat profile indicates no obvious side-reaction between Li with carbonate solvents, greatly enhanced stability of Li anode from the root. The rGO may play a role of host material, which not only provides confinement but also strongly stick to Li anode surface to shied carbonate solvents. At the same time, this research group reported another zinc oxide (ZnO)-contained lithiophilic matrix (Fig. 4d).[40] Molten Li would firstly react with ZnO on the surface of polyimide matrix and then infuse into the matrix, rather than common physical adsorption. Flat profile at initial discharge state proves less side-reactions occurred on the Li anode surface. The author attributed the difference between bare electrode with Li-coated PI matrix to the lower effective current density and higher kinetics. Similar to lithiophilic property of rGO, uniformly dispersed zinc originated from ZnO reduction serves as seeds to guide Li deposition. Similarly, Zhang et al. infuse molten Li into a highly porous conductive carbonized wood with surface ZnO treatment (Fig. 4e).[47] Due to the dual-function of high surface area and even Li deposition, lower overpotential (90 mV at 3 mA cm-2) and better performance (150 hours at 3 mAh cm-2) have been achieved. More importantly, it reappear the flat profile at the initial discharge state, which means less side-reaction and perfect Li deposition. It is different from molten Li infusing into ZnO-modified matrix, Jin et al. demonstrate a porous carbon with ZnO quantum dots to render uniform Li deposition.[48] The lithiophilic agent of ZnO actually takes effect due to the Li-Zn alloy formation to decrease the energy barrier during the heat-treatment. In 2006, Kan et al. systematically analyzed

the difference of Li deposition with various substrate (Fig. 4f).[49] Substrates like Au, Ag, Zn, Mg, Al and Pt show no overpotential of nucleation due to Li-alloy formation. Metal solubility into Li would reduce its deposition barrier to render selective deposition. Then the author designed carbon-coated gold hollow structure to guide selective Li deposition. Similarly, Yang et al. investigated nano-sized silver on 3D carbon to induce the uniform deposition by using the same mechanism (Fig. 4g).[50] While this solution is still defective due to the accumulating SEI of carbonate solvent on the matrix. After thick deposition product covering the inducing agent, it would lose original effect. The lithiophilic effects also have application on improving compatibility of solid-state electrolyte. Solid-state electrolyte seems notorious due to poor contact with Li anode. Li-Al[51], Li-Au[52], Li-Ge[53] alloy were designed to improve the poor contact of garnet structure solid-state electrolyte through forming Li-alloy. There are substantial reports on host materials for improving Li anode deposition. While there is no unified evaluation criterion for Li metal anode test. Electrolyte components, current density and areal capacity are highly relevant to electrochemical performance. For example, carbonate solvents are more corrosive than ether solvents. Higher current density and higher areal capacity pose great challenges for future Li metal anode. In consideration of the terrible performance of Li metal anode without any solutions, any host material could improve the electrochemical performance to same extent. Some reported literatures combined the ideas of both three “dimensional matrix” and “lithiophilic matrix”. They synergistically functioned for better Li deposition from different perspectives. To make it more specific, host material of 3D matrix and lithiophilic matrix, including test condition are summarized in Table 1.

(a)

(b)

(c)

GO film

Sparked rGO film

Layered Li-rGO composite film

(d) High heat-resistant non-conducting polymide fibres

(e)

Core-shell polymide-ZnO

C-wood with Low tortuosity

Li-coated polymide composite

Li/C-wood

ZnO coated C-wood

C-wood

ZnO coated C-wood

Li/C-wood

(g)

(f) Hollow carbon shell

Unifirom AgNPs on CNFs with strong binding

Seeded Li nucleation and uniform Li anode

Lithiated seed with dissolving surface

Li deposition

Li

seed seed

Au NP

Carbon

Bare CNF substrate

Non-uniform Li deposition

Fig. 4. Schematic illustration and its performance. (a) 3D oxidized PAN nanofiber network interlayer render uniform Li ion flux comparing with planar Li anode deposition from Ref.[42]; (b) 3D glass fiber (GF) cloth with functional groups render uniform Li ion flux comparing with planar Li anode deposition from Ref.[43]; (c) Layered reduced graphene oxide (rGO) as lithiophilic nanoscale gaps and its symmetric cell performance comparison with common Li anode from Ref.[39]; (d) ZnO-contained lithiophilic matrix and its symmetric cell performance comparison with common Li anode from Ref.[40]; (e) Conductive carbonized wood with surface zinc oxide (ZnO) treatment and its symmetric cell performance comparison with common Li anode from Ref.[47]; (f) Gold@Carbon induced Li deposition from Ref.[49]; (g) Silver@carbon fiber induced Li deposition from Ref.[50]. Table 1 The summarized host material for Li anode deposition and its performance Host material (scale)

Electrolyte(a)

CE(b) or symmetric cell performance

(c)

Features

Ref.

(current/CE/cycle times, current/capacity/lifespan) Column nano-

LiTFSI/DOL-DME+1 wt %

1/97.6%/240;

Uniform Li+

channel

LiNO3

2/92.9%/150;

flux

(diameter: 350

[29]

3/88.6%/140;

nm) Scaffolding

LiTFSI-LiFSI/DOL-DME

graphene

0.5/93%/50; 1/93%/50;

Large specific

[31]

2

area (1666 m g-1)

(lateral size: 2μm and 10 nm gap) Graphitized

LiTFSI/DOL-DME+1 wt %

Carbon (pore

LiNO3

0.5/98%/1500 h;

Both Li

[32]

insertion and

size: 72 μm)

deposition

Onion-like

LiTFSI/DOL-DME+1 wt %

graphitized

LiNO3

1/98.5%/600;

Both Li

[33]

insertion and

carbon

deposition

(diameter: 1-2 μm) Copper

LiTFSI/DOL-DME+1 wt %

nanowires

LiNO3+5 mM Li2S8

0.2/98.5/200;

Dendrite-free

[37]

after high capacity (7.5 mAh cm-2)

3D nickel foam

LiPF6/EC-EMC-DMC

1/3/200;

Prestoring Li

[41]

composite

3/3/67;

into Ni foam by

anode (width:

5/3/40;

heat infusing

1/97.9%/120;

Both lithiophilic

3/97.4%/120;

+

100 μm) Oxidized PAN nanofiber

LiTFSI/DOL-DME+2 wt % LiNO3

interlayer

and uniform Li

[42]

flux

(diameter: 300 nm) 3D glass fiber cloth interlayer

LiTFSI/DOL-DME+2 wt % LiNO3

(10 μm)

0.5/98%/90;

Both lithiophilic

5/93%/50;

and uniform Li

10/91%/40;

flux

1/98%/200;

Lithiophilic

[43]

+

N-doped

LiTFSI/DOL-DME+5 wt %

[46]

graphene

LiNO3

graphene oxide

LiTFSI/DOL-DME+1 wt %

1/1/222;

Lithiophilic

(rGO)

LiNO3

3/1/69;

deposition

LiTFSI/DOL-DME+1 wt %

1/1/194;

Both lithiophilic

inducing [39]

composite anode PI-ZnO composite matrix

LiNO3

3/1/67;

and uniform Li

5/1/39;

flux

+

[40]

(diameter: 400 nm) Carbonized

LiPF6/EC-DEC

wood with

1/1/300;

Both lithiophilic

3/1/133;

+

surface ZnO Au@ hollow

flux LiPF6/EC-DEC

0.5/98%/300;

carbon (1 μm) Ag@ carbon

Lithiophilic

[49]

deposition LiTFSI/DOL-DME

fiber (280 nm) (a)

and uniform Li

[47]

0.5/1/500;

Lithiophilic

[50]

deposition

All electrolyte mole concentration is 1 M and volume ratio is same with each solvent in this table;

(b)

CE means Coulombic efficiency; CE performance is evaluated by current density (mA), CE and

cycle times, one special example is Ref.[32], using plating/stripping working time (1500 h); (c)

Symmetric cell performance is evaluated by current density (mA), capacity during each cycling

(mAh), lifespan (h).

4. Interface regulation The interfacial layer on Li anode surface is crucial for Li deposition regulation. In general, there are mainly two solutions: firstly, creating an artificial SEI on Li anode surface via in-situ or ex-situ method to strengthen the surface and prevent side-reaction; secondly, developing interfacial layer on separators or current collectors to lessen dendritic Li formation. The artificial SEI layer should be chemically and electrochemically compatible in common electrolytes, robust enough to suppress Li dendrites, and highly ionically conductive as well. 4.1. Artificial SEI film The SEI film forms rapidly at the anode/electrolyte interphase as long as the Li metal is soaked in a non-aqueous electrolyte.[54] However, the in-situ formed SEI layer is usually unstable so as to break during the cycling process, which results in the low Coulombic efficiency and Li dendrite penetration. Therefore, forming a robust and stable SEI is a straightforward method to solve these problems. Ideally, the SEI should possess high ionic conductivity, relatively small thickness, high Young’s modulus and a homogeneous surface morphology for deposited lithium. However, the in-situ formed SEI can hardly meet all of the requirements, which need the design of artificial SEI film. One adopted strategy is to construct an ex-situ formed interlayer before cell assembly, which are also called “Artificial SEI film”. In the early stages, tetraethoxysilane (TEOS) were utilized to react directly with free hydroxyls on the surface of Li metal, forming a mesoporous SiO2 film.[55] As a result, the impedance of the surface does not have an obvious change for over 100 cycles of stripping and plating. However, the TEOS coating is porous and lacks of relatively low elastic strength. To avoid these disadvantages, an artificial Li3PO4 SEI layer was fabricated via an in situ treatment of polyphosphoric acid with Li metal and its pristine film (Fig. 5a).[56] The Li3PO4 SEI layer is stable and uniform due to the control of reaction time. Simultaneously, the Li3PO4 SEI layer possesses a high ionic conductivity and high Young’s modulus (~10-11GPa) which is sufficient to restrain the Li dendrite growth after 200 cycles in a Li|LiFePO4 battery. While the sole inorganic layer could not shield the electrolyte attack after coating crack. Then Cui’s group designed and prepared an artificial SEI film, which is formed via the Cu3N nanoparticles with

styrene butadiene rubber (SBR) directly contact with metallic Li (Fig. 5d).[57] This uniform layer with a thickness of ca. 400 nm remarkably eliminates the uneven lithium-ion flux due to the high mechanical strength, good flexibility, and high Li-ion conductivity. Similarly, Li polyacrylic acid (LiPAA) was introduced as flexible SEI to accommodate the Li deposition with self-adapting interface capacity (Fig. 5c).[58] This artificial SEI is different from common SEI induced by liquid electrolytes, it owns higher binding ability and better stability due to the elastic LiPAA. Different from the feature of LiPAA, high-polarity β-phase PVDF binder was introduced as artificial layer on both Li anode and Cu foil to achieve dendrite-free Li plating/stripping behavior (Fig. 5b).[59] The fluorine (F) provides preferential diffusion paths via in-situ formed highly diffusive LiF on the Li anode surface. Recently, in-situ formed Li-alloy attracted great consideration due to its superiority over the other ex-situ treatment. Linda F. Nazar et al. reported a simple and effective approach to form Li-M (M=In, Zn, Bi, As) alloy to prevent Li dendrite formation (Fig. 5e).[60] The uniqueness of the composite SEI layer lies on that the conductive alloy can largely decrease the overpotential and electronically insulating LiCl uniformly formed on the surface directly prevent the Li dendrite growth. Different from excessive deposition on the surface of conductive matrix like carbon, the appropriate conductivity of layer can ensure the Li+ deposit underlying it. The alloy-protected Li anode maintains normal profile after 1400 hours at 2 mA cm-2. The core-idea of this series paper is the conductivity of artificial layer.[61, 62] Although its effectiveness in guiding Li growth underneath protective layer, it could not bear the long-term and high current in corrosive carbonate solvents. The artificial layer would lose its effectiveness after long-term accumulating SEI on the anode surface, excepting the less reactive of electrolytes. In the future, artificial composite layer with self-healing function induced by in-situ method would be explored to safer utilize Li anode. While for most of the research concentrated on ex-situ treatment ignore the treated solvents, owing to its natural reactivity towards Li anode. Dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), tetrahydrofuran (THF) would react with Li when initially immersed into the solvents, which are not just agent to dissolve active materials. In future, the effect of solvents should be the highlight. Recently, an ultra-stable implantable SEI on Li metal anode was deliberately designed by electrochemical construction of LiNO3, LiTFSI and Li2S5 (Fig. 5f).[63] Dense and flat SEI layer on Li metal anode can be applied on LMBs with different cathodes (such as sulfur and LiNi0.5Co0.2Mn0.3O2 cathode). Dense inorganic species such as N-rich and S-rich components possess higher Li-ion diffusion and better electron transfer.

(b)

(a)

(c)

(d)

Smart SEI

(e)

Stress increasing

Self-adapting interface regulation

Alloy / LiCl -protected Li

(f)

Fig. 5. Schematic illustration of its mechanism. (a) Artificial Li3PO4 SEI layer render uniform Li deposition comparing with dendritic Li deposition on common Li metal surface from Ref.[56]; (b) β-phase PVDF composed SEI layer from Ref.[59]; (c) Li polyacrylic acid (LiPAA) as flexible SEI layer from Ref. [58]; (d) Li-ion conductive SEI layer via Cu3N coating from Ref.[57] ; (e) Li-alloy for Li dendrite prevention and its cross-section SEM of Li before and after stripping from Ref.[60]; (f) implantable SEI designed by electrochemical construction of LiNO3, LiTFSI and Li2S5 from Ref.[63].

Apart from the reaction between Li metal and chosen chemicals, the surface coating can also be constructed via direct coating. Unlike lithiophilic coating, highly carbonated material without functional groups could serves as interlayer to increase the Young`s modulus without reducing its conductivity.[64] However, due to benign conductivity of carbon-based interlayer, Li would preferentially deposited on the surface, not underneath the artificial SEI layer. Unlike conductive coating, Noked’s group reported an 14 nm thick Al2O3 layer by application of atomic layer deposition(ALD) coatings directly on Li metal.[65] This stable Al2O3 layer can protect the Li surface from the corrosion upon air, sulfur, and organic solvent exposure resulting from the high mechanical strength of Al2O3 layer and the formation of ionically conductive LixAl2O3 alloy.

However, it is hard to guarantee that an SEI film is homogeneous in all aspects (properties, compositions, etc.), which may lead to the non-uniform lithium-ion flux and crack upon prolonged battery cycling. Li3N was found effective to suppress Li dendrite by its high surface Li+ diffusive property. N2 has been utilized to form Li3N protection layer at ambient temperature.[66-68] Ma et al. applied the Li3N to suppress the shuttle effect based on anode protection in Li-S batteries without the essential additive of LiNO3.[68] Park et al. considered that the deposition of Li on the bed of Li3N particles would promote the dendrite growth into 3D Li network.[69] The Li3N surface modification has induced interconnected Li network, rather than the typical needle-like Li dendrite. Moreover, the Li3N-modified Li anode showed stable Li plating/stripping cycle behavior at 1.0 mA cm-2. 4.2. Separator modification In the tightly assembled cells, the separator surface modification can also provide similar function on the Li anode surface. Commercial separators can only play the basic role of physical barrier with nano-sized or even bigger pores. Most of separator modifications have been reported on the polysulfides barrier in Li-S batteries.[70-72] Few studies were concentrated on the separator improvement on dendrite suppressing. In 2012, Myung-Hyun Ryou et al. reported polydomamine-coating separator for dendrite suppression via functional group bonding with Li+ ion.[73] Catecholic adhesion offer stronger interaction between Li anode and separators, largely relieving the surface tension. Based on the adhesive feature, hybrid polydopamine/octaamonium (PDA/POSS) coating was introduced to improve surface characteristics of separators without damaging its microporous structure.[74] Improvements on ionic conductivity (from 0.36 to 0.45 ms cm-1) and Li+ ion transference number (from 0.37 to 0.47) contributed to better Li deposition. However, this ex-situ treatment could not change its natural porous feature. Different kinds of separator have different failure mechanism, especially coupling with different electrolytes.[75] Sometimes, serious Li corrosion would happen before the dendrite penetration into the separator. Comparing with Li anode surface treatment, separator modifications are much easier and more feasible. For example, simple carbon coating not only cover most of pores, which could lead into dendrite penetration, but also enhance its conductivity and reduce impedance. Recently, Liu et al. put forward a functionalized nano carbon (FNC) interlayer on the separator (near the Li anode side) to guide the Li deposition and control dendrite growth (Fig. 6a).[76] The FNC cannot inhibit the formation of Li dendrite; however, change the direction of dendrite growth. The controlled growth reduces the possibility of short circuit and improves the cycle life of Li-metal cell. The concept is different from simple carbon coating due to the immobilized Li+ severing as seed for guiding Li deposition. Moreover, excessive Li+ in the carbon coating would enhance its Li+ transfer property. This simple and effective treatment proved universal in different electrolytes. 4.3. Current collectors with modified surface covering Compared to the SEI coated on planar lithium foil, covering the planar copper foil with a functional layer is another adopted strategy to suppress the Li dendrite. Ideally, in addition to chemical inertness, the covering layer should exhibit high Young's modulus to suppress lithium dendrite, or good flexibility to accommodate the volumetric expansion for Li deposition. Based on this concept, hexagonal boron nitride (h-BN) was adopted to directly coat on Cu metal current collector by the method of chemical vapor deposition (CVD) (Fig. 6b).[77] The two-dimensional

(2D) atomic crystal layers have very strong mechanical strength (Young’s modulus approaching 1.0 TPa) and highly flexible due to their ultrathin thickness (from sub-1 nm to sub-10 nm). This specially designed structure could effectively suppress the reactions between Li metal anode and the liquid electrolyte, and thereby improve the cycling efficiency. Moreover, h-BN film is very effective as an electronic and ionic insulator, thus Li ion cannot penetrate through it when stripping easily. Cui’s group prepared a monolayer of interconnected amorphous hollow carbon nanospheres coated on the copper foil (Fig. 6c).[78] The Li ions pass through the carbon nano spheres and then deposit on the copper foil, largely lessen side-reaction between Li metal anode and electrolyte. The uniform hollow carbon nanospheres with a high Young’s modulus of ∼200 GPa restrain the Li dendrite growth. In addition, the top surface of carbon layer is insulating due to the tetrahedral bondings. This strategy reduces the direct Li deposition on the carbon surface, prone to pass through the nanospheres and then deposit on the loose gap between copper foil and carbon layer. Besides, the carbon nanosphere coating is weakly tied to the copper foil, and thereby buffers the volume changes well. The coating structure exhibits high Coulombic efficiency at ∼99% for more than 150 cycles at 0.25 mA cm−2. LiF has been proved to be a vital component to form a rigid and compact SEI layer.[79] Thus, LiF has widely been utilized to protect lithium anode and remarkable improvements have been achieved. Zhang et al. prepared a LiF-rich protective layer based on pyrolyzed poly(acrylonitrile) (PAN) binder on the copper foam (Fig. 6d).[80] The high Young’s moduli provided by the PAN binder network and LiF backbone exhibits stable operation of lithium plating/stripping even after 350 cycles. Similarly, Zhang et al. reported a LiF-rich Cu by in situ hydrolysis of lithium hexafluorophosphate (LiPF6) (Fig. 6e).[81] Benefited from the effect of LiF in the nucleation/growth of lithium and very high surface diffusivity for Li ions, an ordered columnar Li metal film is obtained. Such structure show outstanding performance in stable cycling with high CE and long cycles in both carbonate and ether electrolytes. Apart from the LiF, the PAN submicron fiber array were also demonstrated to be very effective in guiding Li ion deposited uniformly.[44] An average CE of 97.4% for 250 cycles is achieved due to the homogeneous deposition of Li metal induced by PAN fibers with polar groups. Furthermore, other modified strategies have also been adopted to address the lithium dendrites, including preparation of poly(dimethylsiloxane) thin film as a stable interfacial Layer [82]and building a flexible and electrochemically stable coating of (SiO2@PMMA) core−shell nanospheres[83]. However, anodes with protective layers on the current collector can only be paired with cathodes with preloaded Li ions; otherwise, there is a need for the pre-deposition process, which is not viable for practical applications. Similarly, in-situ formed LiF via electrochemical reduction of nickel fluoride (NiF2) coated on copper were utilized to improve the Coulombic efficiency (Fig. 6f).[84] Transplantable LiF-rich layer could block the side-reaction of electrolyte via forming nanoscale LiF domains.

(a)

Li deposition

Li-dendrite formation

Li dendrite growth with cycling

Li dendrite penetrate through separator

Micro-shorting and dead Li formation

Li deposition

Li-dendrite formation

Li dendrite growth with cycling

Li dendrite growth in plane

Li dendrite growth to a layer

(b)

Cathode Separator Anode

Li Deposition

Cathode Separator Anode

Separator FNC layer

(c)

Li depostion

Li dissolution

H-BN on Cu

Cycling

(d)

SEI layer

Deposition on LiF/PAN 120 Carbonspheres thin film

Deposition on LiF/PAN 400

(e)

(f)

Fig. 6. Schematic illustration of its mechanism. (a) Comparison between common separator and FNC-coated separator in suppressing Li dendrite from Ref.[76]; (b) Hexagonal boron nitride (h-BN) coated on copper by CVD from Ref.[77]; (c) Hollow carbon nano spheres coated on the copper foil render uniform Li deposition from Ref.[78]; (d) Comparison between common copper and LiF-rich protective layer based on pyrolyzed PAN binder on the copper foam from Ref.[80]; (e) LiF-rich Cu by ex-situ hydrolysis of lithium hexafluorophosphate (LiPF6) and its column Li deposition morphology from Ref.[81]; (f) In-situ formed LiF via electrochemical reduction of nickel fluoride (NiF2) coated on copper from Ref.[84].

5. Liquid electrolyte As Li metal can react with almost all organic solvents, a surface film is formed during the initial charge/discharge. Peled made first realization of ionically conductive and electrically insulating interface in 1979 and named it solid electrolyte interface (SEI).[85] Generally, the thickness of SEI is approximately 20 nm and is composed of organic and inorganic components. SEI layer makes Li dynamically more stable in various organic solvents resulting in the improvement of cycle life performance. Since the SEI layer on the graphitic anode can be highly stable over several thousands of cycles, thus the LIBs using graphite anode have been successfully commercialized.[11] Conversely, the SEI layer on Li metal is usually unstable due to hostless nature of Li metal, heterogeneous Li-surface with numerous defects where Li deposition/stripping takes place preferentially leading to uneven deposits. The non-uniform Li-depositions leads to dendritic growth, which is the fundamental challenge in Lithium metal batteries (LMBs). Ideally, a SEI layer should have high Li ion conductivity, appropriate thickness with compact structure and high elastic strength to suppress the Li dendrites from puncturing the separator hence avoiding

internal short circuits.[86] 5.1. Solvents and lithium salts The presently employed electrolytes in LMBs mostly contain solvents and Li salts. The commonly used solvents are ethers: 1,2-dimethoxy ethane (DME), polyethylene glycol dimethyl ethers (PEGDME), tetraethylene glycol dimethyl ether (TEGDME), 1,3-dioxalane (DOL), tetrahydrofuran (THF); esters: ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and sulfones (dimethylsulfoxide and trimethylene sulfite). As the reduction potential of most of organic solvents is below 1.0 V vs. (Li+/Li). Therefore, immediate reactions between Li and electrolyte takes place in a time constant of milliseconds when bare Li is exposed to an electrolytic solution[87]. Although there are several mechanisms of SEI formation present in the literature, yet there are many controversies. The high stability and uniformity of SEI plays significant role in long cycle life, high Coulombic efficiency and safety of LMBs [88]. It has been long understood that solvent selection is one of the most critical factors affecting the stability of LMBs. In the class of liquid electrolytes, organic carbonate-based electrolytes are the electrolytes of choice in Li-ion battery technology with acceptable battery stability. Whereas, the performance of Li-metal in carbonate solvents e.g. propylene carbonate (PC) and (EC) is poor typically resulting in dendritic morphology and low Coulombic efficiency (<80%).[89, 90] Alkyl carbonates (ROCOOLi) are the main components of initial SEI composition because of one-electron reduction of alkyl carbonates, which in the presence of trace water amounts are further decomposed to Li2CO3. The inner layer of SEI is dominated by more stable components like Li2O, Li2CO3 and Li-halides while metastable ROCOOLi composes the outer layer. Along with carbonate solvents, researchers have screened many other liquid electrolyte systems and suggested that ether-based electrolytes show much improved performance due to the formation of oligomers in SEI layers on Li surface. The most commonly employed ether solvents used for Li anode are DOL, DME, THF and their combinations. Ethers show relatively better compatibility with Li electrode than carbonates, particularly cyclic ether solvents, 1,3-dioxolane (DOL) decompose on Li anode and generate polymer passivated films of high elasticity for effective inhibition of Li dendritic growth. The uniformity of the SEI can be highly enhanced by the addition of EC/PC to the DOL electrolytes hence obtaining homogeneous Li plating/stripping.[91] Miao et al. introduced 1,4-dioxane as co-solvent (DX) into LiFSI/DME electrolyte system to exhibit stable Li cycling with dendrite Li deposition even at relatively high current rate and high efficiency of ca. 98% at 0.25 mA cm-2.[92] As compared about surface morphology of Li deposition, sharp-end needle-like dendrites in 1 M LiFSI/PC, flat and pancake-like structure in 1 M LiFSI/DX and dendrite-free and stable in 1 M LiFSI/DX-DME. The remarkable performance can explained by the low reduction potential of DX (-1.95 V vs. Li+/Li) to make the solvent less reactive. Meanwhile, the LiF-rich SEI originated from FSI- anion reduction could mitigate the formation of Li dendrite. However, ethers are regarded unfit for most of the commercial batteries primarily due to their flammability and low anodic decomposition voltage (< 4 V vs. Li+/Li). Markevich et al. reported a highly stable cycling of Li|Li cells at a high current density (2.0 mA cm-2) and high areal capacity (3.3 mAh cm-2) by using FEC as co-solvent (Fig. 7a).[93] Full cell paired with LiNi0.6Co0.2Mn0.2O2 (NMC) cathode with high areal loading, it demonstrates stable cycling with the same capacity during 90 cycles. The reason for this high performance is the formation of a stable and efficient solid electrolyte interphase (SEI)

on the surface of the Li metal electrodes cycled in the FEC-based solution. The characteristics a Li-salt should possess is high chemical and electrochemical stability, high dissociation ability in specific solvent to ensure high ionic conductivity and adequate solubility. In all attempts to tailor the electrolyte chemistries in order to stabilize Li-metal surface, the solvent and Li-salt decompose to form SEI layer. The inorganic components of the SEI are formed by the anionic decomposition of Li-salts and are widely discussed by the researchers. [94] Thinner unstable SEI layers on Li-anode are witnessed employing LiBF4, resulting in low charge transfer resistance facilitating battery operations at low temperature and high rates. [95] In effort of making suitable SEI on Li-metal to prevent continues consumption of electrolyte there are numerous studies available in the literature. Firstly, for the passivation of Li-anode there are very few solvent/salt systems known which result in the formation of stable SEI for long cycling.[96] Unlike commercial LIBs, the electrolyte systems for LMBs are not purely based on carbonate/LiPF6. A stable electrolyte having LiClO4 or LiAsF6 in DOL provides high passivation with excellent cycling efficiencies.[97] Dendritic suppression and smooth Li-deposits is attributed to the polymerization of DOL forming breathable SEI [98], yet the system suffers during high rate applications. In another report EC/DME mixture with LiN(SO2CF2CF3)2 (LiBETI) salt can cycle Li with about 97% efficiency.[99] The above-mentioned studies revealed that cyclic oxygen containing solvents with fluorinated Li-salts form LiF that is essential for stable and dense SEI. The correlation of Li-passivation with chemical reactivity of electrolytic components was investigated by Odziemkowski and Irish. [100, 101] The reactivity of a number of solvents (THF, 2-MeTHF and PC) containing Li-salts LiAsF6, LiClO4, LiBF4, LiPF6 and LiN(CF3SO2)2 with bare Li was studied. LiAsF6 was found to be the most rapidly SEI forming salt due to faster reaction with Li while the other salts were reacting slower hence taking longer time in making passivating film. The results illustrated that the anion reduction of the salt is the most important factor in passivating Li. Lithium bis(fluorosulfonyl)imide (LiFSI) is another attractive salt as FSI- anion termed as ‘magic anion’ can form robust SEI protecting layer on Li surface showing promise in commercial electrolytes with comparatively lower viscosity and higher chemical stability. [102] As a result, LiFSI in DOL/DME electrolyte forms stable SEI and dendrite free morphology on Li-metal. [103-105] LiFSI-DOL electrolyte resulted in robust, thin and homogenous inner SEI and oligomeric elastic outer SEI [106] along with higher Coulombic efficiency (≈99%) as compared to LiTFSI-based single salt and mixed electrolyte systems. Miao et al. reported one dual-salts system coupling with DOL to achieve dendrite-free Li deposition with high Coulombic efficiency (Fig. 7b).[107] Considering all the reports on LMBs, most commonly employed salt is LiPF6 despite it lacking the ability to passivate Li effectively. In consequence, there is an ever-growing demand of new Li-salts being able to form stable SEI through flexible decomposition products at Li-surface. An interesting new class of Li-salts are based on borate structures. The simplest of borate salts is LiBOB that forms effective SEI layer. The BOB anion polymerizes into an extended network facilitating a more flexible SEI layer but unfortunately, the SEI layer formed is resistive making the system unsuitable for long term cycling at high rates [95]. The other drawback associated with LiBOB is the lower solubility in carbonate solvents[95]. The fluorinated borate salts LiBFMB [108, 109] and LiDOFB [110-112] have resulted in higher solubility in carbonate solvents than LiBOB, higher transference numbers (tLi+=0.24 LiPF6; 0.33 LiDFOB; 0.47 LiBFMB) [109] and Coulombic efficiency as compared to LiPF6.[111]

5.2. Additives Apart from the limited function of solvent and Li salts, some effective electrolyte additive has been explored and applied for Li protection. Some additives like VC, FEC has been used to stabilize SEI film on silicon (Si) anode.[113] Reviving the Li metal anode, various electrolyte additives have yet to emerge to improve the Coulombic efficiency and protect the Li anode. Recently, Zheng et al. illustrated the optimal amount (0.05 M) of LiPF6 as an additive into LiTFSI–LiBOB dual-salt/carbonate-solvent based electrolytes significantly enhances the charging capability and cycling stability of LMBs (Fig. 7c).[114] A spoonful LiPF6 additive could not only stabilize Al foil due to the corrosion feature of LiTFSI in high voltage, but also alter the nature of SEI layer by catalytic polymerization of carbonate solvent. Nevertheless, the average Coulombic efficiency of Li anode (~90.6%) is still not satisfactory for practical applications. Lu yingying et al. reported halogenated salt blends to exhibit stable long-term cycling at room temperature, no signs of deposition instabilities were observed.[16] The electrochemical impedance spectra (EIS) analysis indicated that the lower interfacial impedance is a result of the LiF-based SEI, which is consistent with the theoretical predictions based on joint density functional theoretical (JDFT) calculations. Later, this group found 0.5wt% LiF into conventional carbonate electrolyte can remarkably suppress the dendrite growth and improve the lifetime of battery.[115] All the results reveal that the LiF-dominated SEI layer has higher surface diffusion of Li ions over other SEI components. While low solubility of LiF in electrolyte solvent and the thickness of LiF-based SEI seems difficult to control by directly using LiF or HF make it unpractical.[116] The common carbonate solvent FEC is a better choice. Zhang et al. used 10% FEC as additive into common carbonate-based electrolyte to form a LiF-rich SEI.[79] This FEC-induced SEI layer is compact and stable, and thus beneficial to achieve high Coulombic efficiency of 98% and uniform and dendrite-free Li deposition. This FEC-protected Li metal anode matches a high-loading NMC cathode with high initial capacity and stable performance. Liu et al. utilized pre-treated Li in FEC-based solvent to obtain an artificial protective film and then realized superior cycle stability in Li-O2 batteries.[117] FEC is an effective compound, which itself does not contain vinyl group, but can lose a HF molecule to form a VC molecule. VC has been confirmed as a polymerizable additive and HF can effectively improve the cycle life of Li metal.[116] Similar study was further explored by the synergistic effects of LiAsF6 and cyclic carbonate additives (FEC or VC).[118] The LixAs alloy and LiF acted as seeds to guide the uniform Li deposition, cyclic carbonate additives would improve Coulombic efficiency and the flexibility of SEI layer. Lithium nitrate (LiNO3) is a widely used electrolyte additive especially for Li-S batteries to suppress shuttle effect. Zhang et al. systematically studied the effect of LiNO3 on the Li anode and sulfur cathode, respectively.[119] LiNO3 formed a stable passivation film with the continuous consumption. On the cathode side, LiNO3 undergoes a large and irreversible reduction starting at 1.6 V (vs Li+/Li) only at the first discharge. It indicates the operational voltage of Li-S batteries should be higher than 1.6 V. Xiong et al. revealed the synergistic effect of LiNO3 and polysulfide to form an effective SEI film (Fig. 7d).[120] The author assumed SEI film has two sub layers, the external layer composed of oxidized product from polysulfide and the internal layer composed of the reduced products of polysulfide and LiNO3. Similarly, Li et al. proved the synergetic effect of both Lithium polysulfide and LiNO3 as additive to form a stable and uniform SEI.[88] Then various electrolyte systems based on the mechanism has been applied on protection of Li anode by

forming complex organic and inorganic hybrid components.[121-123] The sacrificing agent of LiNO3 may prove effective in coin-cell type under careful using, while lacking the practical applicability due to its potential safety and gas emission.[124, 125] It is important to note that the sulfur-rich SEI layer owns higher Li+ ionic conductivity, but lacks elasticity due to small inorganic Li2S/Li2S2 molecular species. Recently, a self-formed flexible organosulfides-rich interphase poses as “plasticizer” to enable dendrite-free Li deposition.[126] The author compared small molecules like dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS) are incapable to form dense and stable SEI to suppress the electrolyte consumption. The unique and robust SEI layer induced by large sulfur-rich compound would accommodate the volume change during the repeated Li plating/stripping process, showing superiority over inorganic sulfur-rich layer and LiNO3-induced protective layer. In addition, some additives do not have preferential reduction ability to form stable SEI, however, it can also stabilize the Li metal by creating a uniform layer and decrease the effective current density. Ding et al. firstly put forward self-healing electrostatic shield mechanism to render an even Li deposition by utilizing 0.05 M CsPF6 additive into 1 M LiPF6/PC (Fig. 7e).[15] During Li deposition, the Cs+ cations form a positively charged electrostatic shield around the initial growth tip of the protuberances to force further deposition of Li to adjacent regions. This strategy solved the uneven deposition from the root, unlike changing the SEI component by preferential reduction on Li anode. While Cs+ could not enhance the Coulombic efficiency of Li deposition in PC-based electrolyte (~76.5%), which even obtains high Coulombic efficiency (99.86%) in full cell with excess amount Li metal anode. However, this system has two main problems: firstly, being only effective at low current density (0.2 mA cm-2), Li+ and Cs+ would co-deposit at high current density; secondly, the low solubility of CsPF6 in common solvents severely hinder its further development. Later, this group found trace-water in LiPF6-containing electrolyte would lead to dendrite-free deposition.[127] The author attributed it to dense LiF-rich SEI derived from reaction of LiPF6 with trace amount of H2O. Chen et al. reported nano-diamonds as an electrolyte additive to co-deposit with Li ions and produce dendrite-free Li deposits (Fig. 7f).[128] The uniform Li deposition morphology renders enhanced electrochemical cycling performance. Li prefers to adsorb onto nano-diamond surfaces with a low diffusion energy barrier by first-principles calculations. The co-deposition can alter the Li plating behavior, offering a promising and effective route to suppress Li dendrite growth in LMBs. Similar idea is metal-compound addition to form in-situ Li-alloy. Lynden A. Archer et al. reported a simple electroless ion-exchange solution to form indium (In) coating on Li surface (Fig. 7g).[129] Indium coating owns fast surface diffusion of Li ions by joint density functional theory. This Li-In alloy anode preserves compact and uniform deposition confirmed by direct visualization. Then the same group reported another precursor (SiCl4) to form hybrid SEI composed of Si-interlinked elastic component and LiCl inorganic component.[130] It is similar to the ex-situ formed Li-M alloy to achieve both elastic and robust artificial layer.[60] Due to the low solubility of SiCl4, the choice for electrolyte solvent is relatively narrow.

(a)

(b)

(c) Li Metal Competitive Reaction E-control

Dual-salt

Dual-salt+additive

Li Metal

LiF layer Li Metal

(d)

(e)

(f)

Fig. 7. (a) Long-term cycling performance of Li|Li cell using FEC-based electrolyte from Ref.[93]; (b) Schematic illustration of interfacial modification via competitive reaction of FSI anion and DOL from Ref.[107]; (c) SEM of Li metal cycling in different electrolyte (function of LiPF 6 additive) from Ref.[114]; (d) Synergic effect of LiNO3 and polysulfides on Li metal anode protection from

Ref.[120]; (e) Illustration of Li deposition process based on the SHES mechanism from Ref.[15]; (f) Nanodiamonds suppress the growth of lithium dendrite from Ref.[131]; (g) SEM of Li anode surface with In(TFSI)3 electrolyte additive render smooth Li deposition from Ref.[129]. 5.3. High concentration of lithium salts Dendrite formation time is proportional to the concentration of Li salts, which means the dendrite formation can be delayed if the lithium salt concentration is increased. Fleury et al.[132] and Chazalviel et al.[17] established mathematical growth considering a binary electrolyte and found once the ionic concentration has dropped to zero, the ramified structures are expected to form. Yang et al.[21]investigated the behavior of Li deposition in the moving electrolyte 1 M LiPF6 in EC/DMC by using a magnetic stirrer. The dendritic Li was effectively suppressed even at the current density of 2.0 mA cm-2 due to the moving and dynamic electrolyte. The commonly used diluted electrolyte is within 1~2 M based on the prior optimization experience about conductivity, viscosity and cost, while it seems incapable to provide efficient protection of Li anode. Various novel electrolyte systems with high ionic concentration has been proposed to suppress the Li dendrite and enhance the performance of LMBs. Jeong et.al examined that the SEI products were thinner in higher concentration lithium salt electrolyte (3.27 M LiBETI/PC), comparing to the same dilute electrolyte (1.28 M LiBETI/PC) without the interface-forming solvents or additives.[133] The growth of Li dendrites was suppressed in the highly concentrated solutions to some extent. This result indicates a new direction for Li dendrites formation. Qian et al. reported 4 M LiFSI in DME enables the high-rate cycling of Li metal anode with Coulombic efficiency (99.1%) without dendrite growth (Fig. 8a).[134] In the highly-concentrated electrolyte system, Li|Li cell last more than 6000 cycles (600 hours) at 10 mA cm-2 and Li|Cu cell achieved an average Coulombic efficiency of 98.4% at 4

mA cm-2 for over 1000 cycles (～500 hours). By contrast, to the extensive dendritic Li metal deposition in LiPF6/PC electrolyte, the nodule-like structure with smooth edges would be less prone to penetrate separator and decrease the side-reactions with Li due to its smaller surface area. According to the molecular dynamic (MD) simulations, the larger fraction of DME solvent are uncoordinated for 4 M LiFSI/DME electrolyte, which leads to improved electrolyte reductive stability. The key factor of high stability of the electrolyte system is the solvent and Li salts. DME has the lowest reduction potential of 1.68 V (vs Li+/Li) among various linear ethers, indicating the lesser reactivity with Li metal. LiFSI is more soluble than other common Li salts, implying the high ionic conductivity and high transference number. Moreover, increased Li ion concentration also enables high-rate Li plating/stripping at high current density. Recently, the same group exploited the same electrolyte system (4 M LiFSI/DME) to further confirm the advantages of fast discharge and demonstrated the efficient operation of anode-free Li rechargeable (Cu||LiFePO4) with high Coulombic efficiency and enhanced cycling performance comparing to conventional carbonate-based electrolyte.[135] Originated from the idea of high concentration Li salts in DME solvent, Ma et al. reported a concentrated electrolyte system with 2 M LiFSI+2 M LiTFSI in DME can achieve good Coulombic efficiency of Li deposit.[136] Later, the same group reported a similar result that the dual-salts concentrated electrolyte (1 M LiFSI+2 M LiTFSI and 2 M LiFSI+1 M LiTFSI) exhibited better electrochemical performance than the single-salt electrolyte (3 M LiTFSI).[137] The authors attributed this to more compact and thinner SEI layer due to the uniform inorganic layer of LiF decomposed from LiFSI. Li-S batteries and Li-O2 have attracted intensive attention due to their high theoretical energy density and environmental friendliness. Sulfur(S) and oxygen (O2) are practical and promising cathode materials for next-generation high-energy rechargeable lithium batteries. However, for Li-S batteries, low conductivity of pure sulfur and the dissolution of intermediate discharge product like Li2Sn (2
In the Li-O2 battery electrolyte system, the common carbonate solvent would be attacked by the superoxide radical anions (O2.-). Then the choice of solvent has to turn to the ethers or sulfones. Norihiro et al. reported a novel electrolyte design for enhancing the cycling performance of Li anode with high concentration of LiNO3 in DMSO solvent.[142] Li anode would be rapidly exhausted due to the side reaction in DMSO solvent. By comparing different Li salts (LiClO4, LiTFSI, LiFSI) and increasing Li salt concentration, 4 M LiNO3/DMSO electrolyte systems display the most enhanced and stable cycling performance. The improvement can be attributed to the inorganic SEI layer on Li anode and free DMSO solvent in the electrolyte due to the unique structure of the electrolyte. Li et al. revealed that the cycling performance of Li-O2 batteries have a strong dependence on the concentration of LiTFSI in triglyme (G3) and tetraglyme (G4).[143] The molar ratio (LiTFSI:Gx) of 1:5 ensures the high stability over 20 discharge/charge cycles. The author attributed the superior cycling performance to the stronger competitive accessibility of O2-, between Li+ ions and triglyme molecules. Later, Liu et al. further discussed that the possible growth mechanism of discharge products.[144] At low Li+ ion concentration, Li2O2 crystals grown as a thin film covered on the surface of electrode, hindering the charge transfer reaction and leading to low capacity. In the high concentration (4-5 M), oxygen passages were easily blocked by discharge and resulting into low volume utilization. Only at middle Li+ ions concentrations (2-3 M), the Li2O2 crystals grown three dimensionally, which ensure high utilization of electrode volume and accommodate more discharge products, thus exhibiting highest discharge capacity. Liu et al. similarly studied the influence of different LiTFSI concentrations in DME solvent.[145] In the high concentrated electrolyte systems, these cells exhibit enhanced cycling stability, which is consistent to less reaction residue on the air-electrode surface and less Li corrosion. By density functional theory calculations, DME molecules are highly coordinated with salt cations and the C-H bond scission of DME becomes more difficult, which suppressed the decomposition. The common solvents used in Li-S batteries and Li-O2 batteries are ethers. The reason is that the intermediate discharge product of polysulfide and O2- would react with conventional carbonate solvents. For high-energy systems of LMBs, the protection of Li anode would be meaningful for their practical applications. The highly concentrated electrolyte systems may provide a route to solve the Li dendrites. However, the high Li+ concentration though excessive Li salts would severely hinder its practical application due to its significantly decreased ionic conductivity, increased viscosity, ultra-high cost and salting-out after cycling. Zheng et al. put forward one simple solution via high discharge rate and slow charge rate to realize transient high concentration electrolyte layer on Li anode.[9] It proved effective to enhance cycling performance of LMBs while seems meaningless due to the relatively common scenario that slow discharge rate and high charge rate using behaviors happened in practical applications. Most importantly, this solution did not distinguish the influence of thermodynamics or dynamics. If it was originated from concentration polarization, then it would be useless when using in portable devices. When it comes to scale-up application in pouch Li-S cells, Li metal anode corrosion, not the polysulfides shuttle is more responsible for cell failure.[146] The dead Li formation and consequently increased resistance pose greater challenges in pouch cells comparing with coin cells. In the future, more economic solutions toward building dendrite-free Li metal anode would be very meaningful to render high energy density LMBs with long-term lifespan.

(a)

(b)

MD simulation 1M LiFSI-DME and 4M LiFSI-DME

Fig. 8. Performance of high concentration Li salts systems. (a) 4 M LiFSI/DEM enables the high-rate cycling of Li metal anode with Coulombic efficiency, Li deposition morphology and molecular simulation from Ref.[134]; (b) 7 M LiTFSI in DOL-DME solvent to inhibit polysulfide dissolution from Ref.[138]. Table 2 Summary of electrolyte component optimization and cell performance electrolyte component(a)

Current (mA)/

Current (mA)/ -2

capacity(mAh.cm )/lifespan(h) 1M LiPF6/EC-DEC+5% FEC

/

(b)

Ref.

CE/cycle number 0.1/98%/100;

[79]

0.5/92%/100; 1 M LiPF6/FEC-DMC (1:4)

0.5/3.3/2000;

0.5/98.5%/40;

[93]

2/3.3/4000; 0.5 M LiTFSI+0.5 M LiFSI/DOL:DME(2:1)

10/15/180;

0.25/0.625/99%;

[107]

1 M LiODFB/EC-DMC-FEC (4.5:4.5:1)

0.28/0.42/2000;

0.25/98%/150;

[112]

4 M LiFSI/DME

10/0.5/600;

0.2/99.2%/500;

[134]

4/98.4%/1000; 10/97%/500; (a)

(b)

Solvent ratio was calculated by volume; Symmetrical cell performance was evaluated by current, capacity each cycle and the whole lifespan; (c)CE was evaluated by current, CE number and the whole cycle life.

6. Solid electrolytes Young's modulus, known as a measurement of the rigidity of the elastic solid materials, is

employed as a parameter to describe the stability of the lithium/electrolyte interface. On the basis of Monroe’s model[147], Li dendrite can be suppressed effectively when the interface Young's modulus is about two times higher than that of the Li anode. This conclusion is directly the theoretical basis for the design of modified lithium anode and electrolyte. The polymer layer coating on Li foil or separator can serve as artificial SEI with higher shear modulus compared to the nature of SEI induced by liquid electrolyte. It can alleviate and guide the dendrite formation and improve the Coulombic efficiency, however, it can`t change the features of dendrite growth. 6.1. Polymer-based electrolytes Compared to the liquid electrolytes, solid electrolytes possess numerous advantages such as high Young's modulus, broad electrochemical stability window and good adhesion with lithium anode. Young's modulus, known as a measurement of the rigidity of the elastic solid materials, is employed for a parameter to describe the stability of the lithium/electrolyte interface. Li dendrite can be suppressed effectively when the interface Young's modulus is about two times higher than that of the Li anode.[147] This conclusion directly provides the theoretical basis for the design of modified lithium anode and electrolyte. The polymer layer coating on Li foil or separator can serve as artificial SEI with higher shear modulus compared to the nature of SEI induced by liquid electrolyte. It can alleviate and guide the dendrite formation and improve the Coulombic efficiency, however, it can’t change the features of dendrite growth.[148] Thus, non-liquid electrolytes, including inorganic solid electrolytes and polymer solid/gel electrolytes, have potential applications in LMBs. Higher heat tolerance, increased mechanical strength and wide electrochemical window are the properties that make solid electrolytes attractive replacements of the liquid electrolytes. Solid electrolytes possess very high Young's modulus that meet the Young's modulus threshold level in C. Monroe’s model. [147] Moreover, it is a comparatively an easier method to suppress dendrite growth. Generally, Polymer-based electrolytes can be classified into solid polymer electrolytes (SPEs), gel polymer electrolytes (GPEs) and composite electrolytes in accordance with the composition and physical state. In 1981, Armand et al. predicted that linear polyethers like PEO can be used for energy storage battery electrolytes.[149] Since then, there have been continual research efforts toward new PEO-based solid electrolytes due to the ability to coordinate with lithium ion. In general, PEO offers some interesting features, for instance, good stability with fresh Li, high capacity in salt complexion and relatively low cost. However, PEO-based solid electrolytes exhibits comparatively low Young's modulus which is of the order of 107 Pa at 30℃. [150] Their ionic conductivity is also lower than that of liquids. Under this condition, lithium dendrites growth becomes difficult to be suppressed effectively. One effective strategy is to add inorganic fillers, for instance, SiO2 [151], Al2O3 [152], ZrO2 [153], into SPEs. In the early research by Zhou et al based on low-molecular weight PEO-based solid electrolytes, addition of fumed silica significantly enhanced the mechanical strength due to the solid-like structure formed by surface functionalized fumed silica. Consequently, the Young's modulus increased by more than two orders of magnitude and the ionic conductivity reached 10-3 S cm-1. Ahmad et al reported the addition of cellulose could greatly improve the mechanical strength of the polymer electrolytes (10.9 MPa for tensile strength and 995 MPa for Young’s modulus).[154] Using crosslinked electrolytes is another feasible strategy to mitigate or even suppress

lithium dendrite penetration. Li and co-workers[155] reported a cross-linked material of polyhedral oligomeric silsesquioxane (POSS) and poly(ethylene glycol) by a facile one-pot reaction, which possess high storage modulus and good ionic conductivity, showing superior resistance to lithium dendrite growth even under high current densities of 0.5 mA cm-2 and 1.0 mA cm-2. Given that Li dendrites are micron sized, the uniform cross-linked structures with nanoscale allow the transport of Li ion, but inhibit Li dendrites growth. However, agglomeration of ceramic fillers and weak polymer-ceramic interaction limit the further improvement of electrolytes.[156] Cui’s group reported a new method to prepare ceramic-polymer electrolyte constructed a cross-linked network via in situ synthesis ceramic particles inside polymer electrolyte. As a result, no internal short circuit caused by Li dendrite formation was observed after 80 cycles, indicating the good mechanical properties that can effectively suppress lithium dendrite penetration. Recently, Zhou et al. reported a polymer/ceramic/polymer sandwich electrolyte (PCPSE), which integrates the benefits from the inorganic and polymer electrolytes (Fig. 9a).[157] In this special structure, owing to the blocked salt anion transfer, the double-layer electric field between the lithium and electrolyte is alleviated. In addition, the polymer layer guides the Li ions deposited more uniformly due to the good adhesion to the lithium anode. As a result, this all-solid-state Li/LiFePO4 cells deliver stable cycle performance over 640 cycles with a high Coulombic efficiency of 99.8−100%. Choudhury et al. designed a crosslinked polymer electrolyte, which combines the advantages of silica nanocomposites and polymer electrolytes (Fig. 9d).[158] The hairy nanoparticles were used for polymer crosslinking, which can selectively pass Li ions and effectively inhibit lithium dendrite growth. Solid electrolytes containing block copolymers, especially PEO and polystyrene (PS), has also been studied for years. Block copolymers consist of a soft block and a hard block, which provide the lithium ion transport channel and inhibit the dendrite growth respectively. Recently, Balsara et al. combined the inorganic fillers with block copolymers.[159] They added surface-modified TiO2 nanoparticles to a mixture of PEO-block-PS with LiTFSI as lithium salts. The results showed that electrolytes with 24 wt% nanoparticles exhibited good interfacial stability and the amount of charge passed before dendrite formation observed was a factor of 4.7 larger than that of the neat block copolymer electrolyte. Another impressive work in which PEO acts as a) block and poly(styrene trifluoromethane-sulphonylimide of lithium) P(STFSILi) acts as the b) block were reported by Bouchet et al.[160] This design overcomes numerous limitations and possess many advantages include high Li+ transference number, excellent ionic conductivity and good mechanical strength. Compared with solid polymer electrolytes, gel polymer electrolytes possess higher ionic conductivity and stronger adhesion lithium surfaces. In general, GPE is composed of liquid electrolytes and a polymer matrix, which includes poly(ethylene oxide) (PEO), poly(ethylene glycol) (PEG), poly(propylene oxide) (PPO), poly(methyl methacrylate) (PMMA), PAN, and poly(vinylidene fluoride hexafluoro propylene) (PVdF-HFP)[161, 162]. Recently, Yang et al. reported a novel cross-linked 3D network gel polymer electrolyte [163]. They used diglycidyl ether of bisphenol-A (DEBA) as the supporting framework, poly(ethylene glycol) diglycidyl ether (PEGDE) and diamino-poly(propylene oxide) (DPPO) crosslinking agent, PVDF-HFP as the polymer host. DEBA was able to improve the mechanical strength of the polymer network, together with the dense cross-linked structure, forming a highly uniform SEI layer and effectively prevents Li dendrite growth.

Due to the good compatibility with the lithium anode and high ionic conductivity, polymerized ionic liquid (PILs) have been demonstrated to be attractive materials for polymer-based electrolytes. Zhang et al. synthesized an interesting class of PILs with polymerized ionic networks (PINs) which possess a high charge density.[164] Such electrolytes with PINs as the polymer hosts exhibit good ionic conductivities, wide electrochemical windows and good compatibility with both electrode. A PIL-based gel polymer for lithium metal batteries at low– medium temperatures was reported by Zhang et al. [165] Using poly(N,N,N-trimethyl-N-(1-vinlyimidazolium-3-ethyl)-ammonium bis(trifluoromethanesulfonyl) imide) as a polymer host, the GPE showed good ion conductivity, high electrochemical stability and good interfacial stability with lithium metal. However, it is notable that GPE is not entirely thermodynamically stable with Li anode due to the component of liquid electrolytes exist. Thus, there is still a long way for GPE to realize the commercial application. 6.2. Inorganic electrolytes High elastic modulus, strong impenetrable barrier of inorganic electrolytes prevent dendrite penetration[166] along with providing increased battery safety.[167-169] Inorganic solid electrolytes include sulfides, oxides and oxynitride type materials.[170-172] Sulfide and oxide solid electrolytes enjoys higher ionic conductivity (10-2-10-3 S cm-1) but are electrochemically unstable towards Li metal.[173, 174] On the other hand, the oxynitrides are electrochemically stable against Li metal but lower ionic conductivity limits their applicability to only thin-film batteries.[175] Respective advantages and disadvantages of inorganic solid-state oxide, sulfide and oxynitride electrolytes pave way to search for the materials with a balance of ionic conductivity and stability towards Li-metal. Garnet Li7La3Zr2O12 (LLZO) emerges as an attractive inorganic solid-state electrolyte for Li-metal batteries as it fits well on both grounds with admirable ionic conductivity (10-3-10-4 S cm-1)[176] as well as best electrochemical stability with Li-metal.[177, 178] However, the serious challenge for garnet electrolyte is high interfacial impedance[179, 180] towards both electrodes (positive and negative) mainly governed by the poor contact (wetting) with the electrodes.[181] The contact issues are easier to overcome in case of liquid electrolytes by suitable engineering of the interface[182] but to do the same with solid-state electrolytes is challenging. Although there are many ways suggested in the literature[183, 184] for minimizing the contact problem but here, we will discuss the most recent strategies developed to decrease the interfacial resistance of LLZO electrolyte. Recently, Luo et al. have improved the Li/LLZO interface by depositing an ultrathin layer of amorphous Si through PECVD (Fig. 9b).[185] The wettability of LLZO have transformed amazingly from superlithiophobic to superlithiophilic because of the reaction between Li and Si. The modified Si-coated LLZO symmetric cells hence exhibited a 7-fold decrease in the interfacial resistance as compared to bare LLZO. In another attempt, conformal surface modification layer of ZnO significantly increased the wettability of garnet electrolyte for molten Li.[186] The continuous and tight contact between Li and garnet have decreased the interfacial resistance to as low as 20 Ω cm2 much lower than untreated sample (2000 Ω cm2). Ultrathin Al2O3 deposition by atomic layer deposition decreased the interfacial impedance between Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZN) garnet electrolyte and metallic Li-anode significantly from 1710 Ω cm2 to 1 Ω cm2 (Fig. 9c). [187] Experimental and computational findings suggests that the oxide coating enables wetting of Li-metal in contact with garnet electrolyte. The Li-alloy lithiophilic layer had been fully

exploited to improve the contact between Li anode and inorganic garnet electrolyte. Xu et al. have introduced Li3PO4 in the garnet type electrolyte Li6.5Zr1.5Ta0.5O12 as additive thereby improving the interfacial compatibility along with suppression of Li-dendrites.[188] Li3PO4 added symmetric Li/garent/Li cells have successfully cycled for 60 h at a current density of 0.1 mA cm-2 almost twice longer than Li3PO4 free garnet electrolyte that short-circuited completely after 33 h. Li et al. proposed that the introduction of LiF to Li6.5La3Zr0.5O12 (LLZT) garnet electrolyte increases the stability of the electrolyte towards moist atmosphere.[189] The increased stability of garnet electrolyte means less Li2CO3 on the surface that is responsible for interfacial resistance. Li/polymer/LLZT-2LiF/LiFePO4 cell shows high Coulombic efficiency and long cycle life. In Li-S cell, LLZT-2LiF electrolyte as a separator blocks the polysulfide transport towards Li-metal with high Coulombic efficiency and 93% capacity retention after 100 cycles. Combining the advantages of both polymer and solid state electrolytes, Fu et al. introduced LLZO into PEO-based composite (Fig. 9e).[190] This flexible composite provides enhanced mechanical strength and decreased interfacial resistance. Symmetric cells using this composite electrolyte extends lifespan to 500 h at 0.2 mA cm-2 and 300 h at 0.5 mA cm-2, effectively exhibiting dendrites suppressing. This research group reported another hybrid electrolyte consists of solid-state electrolyte and liquid electrolyte for Li-S batteries.[191] 3D bilayer solid-state electrolyte framework can address the issue of both chemical and physical short circuits of Li dendrite after long-term cycling. Additionally, the garnet membrane can host electrode material and liquid electrolyte for continuous Li ion conductive pathways. Recently, similar ceramic-polymer composite electrolyte was researched to build up safer LMBs.[192] Al-doped LLZTO was used as ceramic filler into LiTFSI/PEO polymer to synthesis solid electrolyte membrane. The fillers is essential to provide barrier to short circuit and extend the electrochemical windows to 5.5 V. In the future, more composite electrolyte systems would be explored to combine the advantages for Li anode protection. The surface chemistry impact on the interfacial resistance of LLZO and Li-metal has been recently investigated.[193] Mechanistic insights explored by XPS, first principle calculations, Li contact angle measurements and impedance studies revealed that the presence of surface contaminations on LLZO surface like Li2CO3 and LiOH are mainly responsible for poor wettability. Sharafi et al. hence proposed a simple procedure for removing these contaminated layers and nearly eliminated all of the interfacial resistance with dramatically high Li-wetting.[193] For inorganic solid-state electrolytes, the interface still needs further studies; however, the above-mentioned recent findings to minimize the interfacial resistance coupled with their higher ionic conductivity can be the way forward for developing scalable Li-metal based energy storage devices. Recently, neutron depth profiling (NDP) was introduced into garnet electrolyte to reveal the interfacial behavior.[194] Common characterizations seem incapable to determine the short circuit condition of working cells with garnet electrolyte. However, the NDP measurement can further demonstrates possible capabilities for diagnosing short circuit through in-situ monitoring of Li plating/stripping processes.

(a)

(b)

(c) Voids

ALD interlayer

(d)

(e)

Fig. 9. Illustration of solid electrolyte. (a) all-solid-state battery design with the PCPSE electrolyte and electric potential profile across the sandwich electrolyte from Ref.[157] ; (b) Lithiophobic layer render low interfacial resistance from Ref.[185]; (c) Ultrathin Al2O3 deposition by ALD decreased the interfacial impedance and its SEM images from Ref. [187]; (d) Silica/PEO composite electrolyte and its TEM from Ref. [158]; (e) Schematic of hybrid solid-state composite electrolyte, conducting polymer served as matrix for ceramic garnet nanofibers from Ref.[190].

7. Characterization techniques of Li deposition Dendrite growth being the core issue for Li metal battery safety demands extensive and detailed characterization to explore the mechanisms of dendrite growth. Variety of characterization techniques have been developed to investigate the fundamentals of dendrite growth i.e. nucleation, formation and growth. The techniques used to characterize Li dendrites can be classified into two categories: 1) morphological characterizations; 2) chemical composition characterizations. The former including scanning electron microscopy (SEM), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), atomic force microscopy (AFM) while the later comprises of Fourier transform infrared spectroscopy (FTIR), X-Ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) which are discussed in detail in the previous reviews.[195, 196] Most of the characterizations performed in the earlier years focused on in-situ or ex-situ in static conditions. More valuable information on the dynamics of Li-dendrite growth can be extracted by employing in operando characterizations. However, the higher reactivity and low

atomic number of metallic Lithium (weak electron and X-Ray scatterer) renders the in operando diagnostics relatively immature. In the earlier days, open-cell in-situ TEM [197-199] and in-situ SEM [200] techniques were used to examine the Li dendrite formation and non-uniform Li depositions respectively. The problem associated with these open-cell characterizations is that they employed solid electrolytes or ionic liquids as liquid electrolytes would be more volatile hence making the situation far away from practical ones. As an alternate optical, microscopy provides an in-situ method to observe Li dendrite growth at the expense of compromising the resolution (≈ 200 nm).[88, 201, 202] Yet the resolution is enough to detect the surface changes and dendrite growth at the same time. In the in-situ examination, parallel direction of Li-surface is mostly analyzed to investigate the Li-dendrite growth.[203, 204] In a recent report, liquid cell was used to test in-situ electrochemical TEM, uncovering the mechanisms of electrolyte decomposition and characterizing the dynamics of SEI formation and growth.[203] In a similar kind of liquid cell with commercial carbonate electrolyte, structural and chemical changes of the SEI and Li depositions were analyzed by coupling the electrochemical (quantitative) measurements with STEM imaging (quantitative).[205] Combined effects of EC-STEM tracked Li nucleation, mechanisms of SEI formation and Li growth making EC-STEM a powerful and informative technique. Following investigations focused on the dependence of Li deposition morphology on the electron beam effects and current density, observation of the inhomogeneous nature of the SEI prior to dendrite formation. Recently, Zhang et al. developed an in situ electrochemical SEM (EC-SEM) method to comprehensively observe and study Li plating/stripping behaviors (Fig. 10a).[206] The author revealed the synergistic effect of LiNO3 and polysulfides via visual explanation. Along with morphological investigations, X-ray techniques e.g. X-ray diffraction and X-ray absorption spectroscopy [207], X-ray microscopy [208] are regarded as powerful characterization techniques to analyze chemical compositions of surface films. Unfortunately, fewer literature reports particularly focused on Li metal. These studies comprises on the examination of changes in composition of anode at different cycling stages using spatially and temporally resolved synchrotron X-ray diffraction.[209] Harry et al.[210] observed the nucleation of subsurface dendritic structures inside the Li electrode as well as early stages of dendritic formation in symmetric Li/polymer/Li cells using X-ray microtomography. Additionally among the in operando techniques, 7Li-NMR spectroscopy, 7Li-magnetic resonance imaging (MRI) and in operando electron paramagnetic resonance spectroscopy have emerged as new analytical techniques for quantitative analysis of Li nucleation and growth[211-213]. Recently, cryo–electron microscopy avoiding air-contact has been utilized to characterize the high-resolution Li dendrite (Fig. 10b).[214] It is meaningful to reveal the different SEI forming mechanism in different electrolytes, offering hints to explore suitable electrolytes. Finally, we think that in future effective combinations of in operando morphological and surface characterization techniques can provide inclusive knowledge about the dynamics of Li anode.

(a)

(b)

Liquid N2

Cu TEM grid

Shutter control Cryo TEM holder

Li depostion

Li dendrite Plunge freeze under Ar Electron beam

Sample in liquid N2

Load sample Liquid N2

Close shutter

Transfer at -170 oC

Fig. 10. Schemes of advanced technique to characterize Li deposition. (a) In situ electrochemical SEM (EC-SEM) from Ref.[206]; (b) Preserving and stabilizing Li metal by cryo-EM from Ref.[214].

8. Summary and perspectives As we discussed above, Li metal still considered an ideal anode for rechargeable batteries, including Li-S, Li-O2, and LMBs with lithium transitional-metal oxide cathode. However, uncontrolled Li dendrite growth and low Coulombic efficiency during the Li plating/stripping process are the major obstacles denting the progress of LMBs towards widespread commercialization. During long-time charge/discharge process, it seems difficult for Li to build a homogeneous and stable SEI layer to avoid further side-reactions due to the infinite volume expansion and thermodynamic instability towards the liquid electrolyte. Moreover, the safety problem derived from Li dendrites and short circuit overwhelmingly hindered its development. To now, after near 40 years of continuous research efforts, various solutions have been proposed to improve the Li deposition morphology and enhance its plating/stripping efficiency. Comparing to the safety problem, the relatively low Coulombic efficiency can be softened by providing excessive Li. However, even if 99.9% of Coulombic efficiency has been achieved, threefold excess of Li is still needed to maintain its initial capacity after 1000 cycles from the perspective of theoretical calculation. However, possible exception would be that the 0.1% Li lose during cycling could be recycling in the next process if there is nearly no side reaction between Li with electrolytes. That means the interface stability matters more than high Coulombic efficiency. In this review, we have summarized the strategies into four parts: structure design, interface

regulation, liquid electrolytes and solid electrolytes. Some reported literature may have combined two or above solutions to improve the electrochemical performances. In future, more efforts are needed to address the origin problems from the Li dendrite growth mechanism. The root causes of Li dendrite growth are the Li ion concentration gradient and non-uniformity of deposition. The low Coulombic efficiency is mainly due to thermodynamic instability of fresh Li metal with the electrolyte. It would be promising to increase the Li+ ion transfer in bulk electrolytes to reduce the Li+ concentration gradient. Meanwhile, the SEI layer on the Li anode surface still remains critical due to the side reaction and uneven deposition. Embedding Li into conductive matrix can provide large volume for deposition, while lead into more side-reaction simultaneously. Solutions about creating the artificial SEI layer are effective to a certain extent, still lacking the ability after long-term cycling. In the future, many issues should be considered for Li metal anode protection: 1. Interface regulation including in-situ and ex-situ method were mainly concentrated on seeds to guide its Li deposition. In other words, it would lose its effect after the thick SEI coating due to its accumulating side reaction. In the future, how to combine the interface regulation and suppressing side reaction would be more meaningful. 2. Liquid electrolytes are common used in LIBs due to its benign film forming ability for graphite anode, which are different from Li. The fundamental understanding SEI forming mechanism on Li should be deeply understand, including components analysis, in-situ observation and interface regulation. The function of solvents, Li salts and additives should be carefully testified with detailed proofs. In the future, combination of advanced test and traditional electrochemistry would provide more insights and ideas. 3. Solid electrolyte still remains critical, not only for the poor compatibility with Li anode, but also with cathodes. Its low room temperature conductivity and poor contact with cathode sides should be largely improved to compete with liquid electrolyte. 4. The advanced Li anode protection solution should be based on the full-cell application, which means limited Li and electrolyte compatibility. More importantly, the pouch cell performance should be the final trial. 5. Most importantly, safety should be paid at first priority during developing effective strategies to evaluate the electrochemical performances of Li metal anode. Safety concerns about the highly reactive Li, flammable electrolyte and unstable cathode should be carefully considered. Although good electrochemical performance has been achieved by novel design, it should be prohibited in LMBs system due to potential safety problems. In one word, the origin problems of Li have been fully discussed during the past 40 years. Nowadays, more and more creative works are already providing a great opportunity for the revival of Li metal anode, although there is still a long way to go before LMBs’ practical applications.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (U1705255, 21333007, 21373137), and New Century Excellent Talents in University (NCET-13-0371).

References

[1] B. Zhang, M. Metzger, S. Solchenbach, M. Payne, S. Meini, H.A. Gasteiger, A. Garsuch, B.L. Lucht, Role of 1,3-Propane Sultone and Vinylene Carbonate in Solid Electrolyte Interface Formation and Gas Generation, J. Phys. Chem. C 119(21) (2015) 11337-11348. [2] J.B. Goodenough, K.S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135(4) (2013) 1167-76. [3] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.-G. Zhang, Lithium metal anodes for rechargeable batteries, Energy Environ. Sci. 7(2) (2014) 513-537. [4] X.B. Cheng, R. Zhang, C.Z. Zhao, Q. Zhang, Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review, Chem. Rev. 117(15) (2017) 10403-10473. [5] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Li-O2 and Li-S batteries with high energy storage, Nat. Mater. 11(1) (2011) 19-29. [6] D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries, Nat. Nanotechnol. 12(3) (2017) 194-206. [7] X.B. Cheng, J.Q. Huang, Q. Zhang, Review-Li Metal Anode in Working Lithium-Sulfur Batteries, J. Electrochem. Soc. 165(1) (2018) A6058-A6072. [8] M.D. Tikekar, S. Choudhury, Z. Tu, L.A. Archer, Design principles for electrolytes and interfaces for stable lithium-metal batteries, Nat. Energy 1(9) (2016) 16114. [9] J. Zheng, P. Yan, D. Mei, M.H. Engelhard, S.S. Cartmell, B.J. Polzin, C. Wang, J.-G. Zhang, W. Xu, Highly Stable Operation of Lithium Metal Batteries Enabled by the Formation of a Transient High-Concentration Electrolyte Layer, Adv. Energy Mater. 6(8) (2016) 1502151-1502160. [10] K.N. Wood, M. Noked, N.P. Dasgupta, Lithium Metal Anodes: Toward an Improved Understanding of Coupled Morphological, Electrochemical, and Mechanical Behavior, ACS Energy Lett. 2(3) (2017) 664-672. [11] N.A. Kaskhedikar, J. Maier, Lithium Storage in Carbon Nanostructures, Adv. Mater. 21(25-26) (2009) 2664-2680. [12] P. Barai, K. Higa, V. Srinivasan, Effect of Initial State of Lithium on the Propensity for Dendrite Formation: A Theoretical Study, J. Electrochem. Soc. 164(2) (2017) A180-A189. [13] J.N. Chazalviel, Electrochemical aspects of the generation of ramified metallic electrodeposits, Phys. Rev. A 42(12) (1990) 7355-7367. [14] Yamaki, J. I., Tobishima, S. I., Hayashi, K., Saito, K., Nemoto, Y., & Arakawa, M. A consideration of the morphology of electrochemically deposited lithium in an organic electrolyte, J. Power Sources, 74(2) (1998), 219-227. [15] F. Ding, W. Xu, G.L. Graff, J. Zhang, M.L. Sushko, X. Chen, Y. Shao, M.H. Engelhard, Z. Nie, J. Xiao, X. Liu, P.V. Sushko, J. Liu, J.G. Zhang, Dendrite-free lithium deposition via self-healing electrostatic shield mechanism, J. Am. Chem. Soc. 135(11) (2013) 4450-6. [16] Y. Lu, Z. Tu, L.A. Archer, Stable lithium electrodeposition in liquid and nanoporous solid electrolytes, Nat. Mater. 13(10) (2014) 961-9. [17] J.N. Chazalviel, Electrochemical aspects of the generation of ramified metallic electrodeposits, Phys. Rev. A (Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics) 42(12) (1990) 7355-7367. [18] Brissot, C., Rosso, M., Chazalviel, J. N., & Lascaud, S. Dendritic growth mechanisms in lithium/polymer cells, J. Power Sources 81(1999) 925-929. [19] M. Rosso, C. Brissot, A. Teyssot, M. Dollé, L. Sannier, J.-M. Tarascon, R. Bouchet, S. Lascaud, Dendrite short-circuit and fuse effect on Li/polymer/Li cells, Electrochim. Acta 51(25) (2006)

5334-5340. [20] M. Rosso, T. Gobron, C. Brissot, J-N. Chazalviel, S. Lascaud. Onset of dendritic growth in lithium/polymer cells, J. Power Sources 97(2001), 804-806. [21] X. Yang, Z. Wen, X. Zhu, S. Huang, Electrodeposition of lithium film under dynamic conditions and its application in all-solid-state rechargeable lithium battery, Solid State Ionics 176(11-12) (2005) 1051-1055. [22] M.Z. Mayers, J.W. Kaminski, T.F. Miller, Suppression of Dendrite Formation via Pulse Charging in Rechargeable Lithium Metal Batteries, J. Phys. Chem. C 116(50) (2012) 26214-26221. [23] S. Liu, J. Yang, L. Yin, Z. Li, J. Wang, Y. Nuli, Lithium-rich Li2.6BMg0.05 alloy as an alternative anode to metallic lithium for rechargeable lithium batteries, Electrochim. Acta 56(24) (2011) 8900-8905. [24] B. Duan, W. Wang, H. Zhao, A. Wang, M. Wang, K. Yuan, Z. Yu, Y. Yang, Li-B Alloy as Anode Material for Lithium/Sulfur Battery, ECS Electrochem. Lett. 2(6) (2013) A47-A51. [25] X.B. Cheng, H.J. Peng, J.Q. Huang, F. Wei, Q. Zhang, Dendrite-free nanostructured anode: entrapment of lithium in a 3D fibrous matrix for ultra-stable lithium-sulfur batteries, Small 10(21) (2014) 4257-63. [26] C. Huang, J. Xiao, Y. Shao, J. Zheng, W.D. Bennett, D. Lu, L.V. Saraf, M. Engelhard, L. Ji, J. Zhang, X. Li, G.L. Graff, J. Liu, Manipulating surface reactions in lithium-sulphur batteries using hybrid anode structures, Nat. Commun. 5 (2014) 3015. [27] J. Zhao, G. Zhou, K. Yan, J. Xie, Y. Li, L. Liao, Y. Jin, K. Liu, P.C. Hsu, J. Wang, H.M. Cheng, Y. Cui, Air-stable and freestanding lithium alloy/graphene foil as an alternative to lithium metal anodes, Nat. Nanotechnol. 12(10) (2017) 993-999. [28] R. Mukherjee, A.V. Thomas, D. Datta, E. Singh, J. Li, O. Eksik, V.B. Shenoy, N. Koratkar, Defect-induced plating of lithium metal within porous graphene networks, Nat. Commun. 5 (2014) 3710. [29] W. Liu, D. Lin, A. Pei, Y. Cui, Stabilizing Lithium Metal Anodes by Uniform Li-Ion Flux Distribution in Nanochannel Confinement, J. Am. Chem. Soc. 138(47) (2016) 15443-15450. [30] Cheng, X. B., Peng, H. J., Huang, J. Q., Zhang, R., Zhao, C. Z., & Zhang, Q, Dual-phase lithium metal anode containing a polysulfide-induced solid electrolyte interphase and nanostructured graphene framework for lithium–sulfur batteries, ACS Nano 9(6) (2015), 6373-6382. [31] R. Zhang, X.B. Cheng, C.Z. Zhao, H.J. Peng, J.L. Shi, J.Q. Huang, J. Wang, F. Wei, Q. Zhang, Conductive Nanostructured Scaffolds Render Low Local Current Density to Inhibit Lithium Dendrite Growth, Adv. Mater. 28(11) (2016) 2155-2162. [32] T.T. Zuo, X.W. Wu, C.P. Yang, Y.X. Yin, H. Ye, N.W. Li, Y.G. Guo, Graphitized Carbon Fibers as Multifunctional 3D Current Collectors for High Areal Capacity Li Anodes, Adv. Mater. 29(29) (2017) 1700389-1700394. [33] H. Ye, S. Xin, Y.-X. Yin, J.-Y. Li, Y.-G. Guo, L.-J. Wan, Stable Li Plating/Stripping Electrochemistry Realized by a Hybrid Li Reservoir in Spherical Carbon Granules with 3D Conducting Skeletons, J. Am. Chem. Soc. 139(16) (2017) 5916-5922. [34] L. Liu, Y.-X. Yin, J.-Y. Li, N.-W. Li, X.-X. Zeng, H. Ye, Y.-G. Guo, L.-J. Wan, Free-Standing Hollow Carbon Fibers as High-Capacity Containers for Stable Lithium Metal Anodes, Joule 1(3) (2017) 563-575. [35] C.P. Yang, Y.X. Yin, S.F. Zhang, N.W. Li, Y.G. Guo, Accommodating lithium into 3D current collectors with a submicron skeleton towards long-life lithium metal anodes, Nat. Commun. 6 (2015)

8058. [36] Q. Yun, Y.-B. He, W. Lv, Y. Zhao, B. Li, F. Kang, Q.-H. Yang, Chemical Dealloying Derived 3D Porous Current Collector for Li Metal Anodes, Adv. Mater. 28(32) (2016) 6932-6939. [37] L.L. Lu, J. Ge, J.N. Yang, S.M. Chen, H.B. Yao, F. Zhou, S.H. Yu, Free-Standing Copper Nanowire Network Current Collector for Improving Lithium Anode Performance, Nano lett. 16(7) (2016) 4431-4437. [38] S.H. Wang, Y.X. Yin, T.T. Zuo, W. Dong, J.Y. Li, J.L. Shi, C.H. Zhang, N.W. Li, C.J. Li, Y.G. Guo, Stable Li Metal Anodes via Regulating Lithium Plating/Stripping in Vertically Aligned Microchannels, Adv. Mater. 29(40) (2017) 1703729-1703735. [39] D. Lin, Y. Liu, Z. Liang, H.-W. Lee, J. Sun, H. Wang, K. Yan, J. Xie, Y. Cui, Layered reduced graphene oxide with nanoscale interlayer gaps as a stable host for lithium metal anodes, Nat. Nanotechnol. 11(7) (2016) 626-632. [40] Y. Liu, D. Lin, Z. Liang, J. Zhao, K. Yan, Y. Cui, Lithium-coated polymeric matrix as a minimum volume-change and dendrite-free lithium metal anode, Nat. Commun. 7 (2016) 10992. [41] S.-S. Chi, Y. Liu, W.-L. Song, L.-Z. Fan, Q. Zhang, Prestoring Lithium into Stable 3D Nickel Foam Host as Dendrite-Free Lithium Metal Anode, Adv. Funct. Mater. 27(24) (2017) 1700348-1700357. [42] Z. Liang, G. Zheng, C. Liu, N. Liu, W. Li, K. Yan, H. Yao, P.C. Hsu, S. Chu, Y. Cui, Polymer nanofiber-guided uniform lithium deposition for battery electrodes, Nano lett. 15(5) (2015) 2910-2196. [43] X.B. Cheng, T.Z. Hou, R. Zhang, H.J. Peng, C.Z. Zhao, J.Q. Huang, Q. Zhang, Dendrite-Free Lithium Deposition Induced by Uniformly Distributed Lithium Ions for Efficient Lithium Metal Batteries, Adv. Mater. 28(15) (2016) 2888-2895. [44] J. Lang, J. Song, L. Qi, Y. Luo, X. Luo, H. Wu, Uniform Lithium Deposition Induced by Polyacrylonitrile Submicron Fiber Array for Stable Lithium Metal Anode, ACS Appl. Mater. Inter. 9(12) (2017) 10360-10365. [45] H. Lee, J. Song, Y.J. Kim, J.K. Park, H.T. Kim, Structural modulation of lithium metal-electrolyte interface with three-dimensional metallic interlayer for high-performance lithium metal batteries, Sci. Rep. 6 (2016) 30830. [46] R. Zhang, X.R. Chen, X. Chen, X.B. Cheng, X.Q. Zhang, C. Yan, Q. Zhang, Lithiophilic Sites in Doped Graphene Guide Uniform Lithium Nucleation for Dendrite-Free Lithium Metal Anodes, Angew. Chem. 56(27) (2017) 7764-7768. [47] Y. Zhang, W. Luo, C. Wang, Y. Li, C. Chen, J. Song, J. Dai, E.M. Hitz, S. Xu, C. Yang, Y. Wang, L. Hu, High-capacity, low-tortuosity, and channel-guided lithium metal anode, Proc. Natl. Acad. Sci. USA 114(14) (2017) 3584-3589. [48] C. Jin, O. Sheng, J. Luo, H. Yuan, C. Fang, W. Zhang, H. Huang, Y. Gan, Y. Xia, C. Liang, J. Zhang, X. Tao, 3D lithium metal embedded within lithiophilic porous matrix for stable lithium metal batteries, Nano Energy 37 (2017) 177-186. [49] K. Yan, Z. Lu, H.-W. Lee, F. Xiong, P.-C. Hsu, Y. Li, J. Zhao, S. Chu, Y. Cui, Selective deposition and stable encapsulation of lithium through heterogeneous seeded growth, Nat. Energy 1(3) (2016) 16010. [50] C. Yang, Y. Yao, S. He, H. Xie, E. Hitz, L. Hu, Ultrafine Silver Nanoparticles for Seeded Lithium Deposition toward Stable Lithium Metal Anode, Adv. Mater. 29(38) (2017) 1702714-1702721. [51] K. Park, B.-C. Yu, J.-W. Jung, Y. Li, W. Zhou, H. Gao, S. Son, J.B. Goodenough, Electrochemical Nature of the Cathode Interface for a Solid-State Lithium-Ion Battery: Interface between LiCoO2 and

Garnet-Li7La3Zr2O12, Chem. Mater. 28(21) (2016) 8051-8059. [52] C.L. Tsai, V. Roddatis, C.V. Chandran, Q. Ma, S. Uhlenbruck, M. Bram, P. Heitjans, O. Guillon, Li7La3Zr2O12 Interface Modification for Li Dendrite Prevention, ACS Appl. Mater. Inter. 8(16) (2016) 10617-10626. [53] W. Luo, Y. Gong, Y. Zhu, Y. Li, Y. Yao, Y. Zhang, K.K. Fu, G. Pastel, C.F. Lin, Y. Mo, E.D. Wachsman, L. Hu, Reducing Interfacial Resistance between Garnet-Structured Solid-State Electrolyte and Li-Metal Anode by a Germanium Layer, Adv. Mater. 29(22) (2017) 1606042-1606048. [54] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104(10) (2004) 4303-4417. [55] G.A. Umeda, E. Menke, M. Richard, K.L. Stamm, F. Wudl, B. Dunn, Protection of lithium metal surfaces using tetraethoxysilane, J. Mater. Chem. 21(5) (2011) 1593-1599. [56] N.W. Li, Y.X. Yin, C.P. Yang, Y.G. Guo, An Artificial Solid Electrolyte Interphase Layer for Stable Lithium Metal Anodes, Adv. Mater. 28(9) (2016) 1853-1858. [57] Y. Liu, D. Lin, P.Y. Yuen, K. Liu, J. Xie, R.H. Dauskardt, Y. Cui, An Artificial Solid Electrolyte Interphase with High Li-Ion Conductivity, Mechanical Strength, and Flexibility for Stable Lithium Metal Anodes, Adv. Mater. 29(10) (2017) 1605531-1605538. [58] N.-W. Li, Y. Shi, Y.-X. Yin, X.-X. Zeng, J.-Y. Li, C.-J. Li, L.-J. Wan, R. Wen, Y.-G. Guo, A Flexible Solid Electrolyte Interphase Layer for Long‐Life Lithium Metal Anodes, Angew. Chem. 57(6) (2018) 1505-1509. [59] J. Luo, C.-C. Fang, N.-L. Wu, High Polarity Poly(vinylidene difluoride) Thin Coating for Dendrite-Free and High-Performance Lithium Metal Anodes, Adv. Energy Mater. 8(2) (2018) 1701482-1701488. [60] X. Liang, Q. Pang, I.R. Kochetkov, M.S. Sempere, H. Huang, X. Sun, L.F. Nazar, A facile surface chemistry route to a stabilized lithium metal anode, Nat. Energy 2(9) (2017) 17119. [61] J. Zhu, J. Yang, J. Zhou, T. Zhang, L. Li, J. Wang, Y. Nuli, A stable organic–inorganic hybrid layer protected lithium metal anode for long-cycle lithium-oxygen batteries, J. Power Sources 366 (2017) 265-269. [62] D.J. Lee, H. Lee, J. Song, M.-H. Ryou, Y.M. Lee, H.-T. Kim, J.-K. Park, Composite protective layer for Li metal anode in high-performance lithium–oxygen batteries, Electrochem. Commun. 40 (2014) 45-48. [63] X.-B. Cheng, C. Yan, X. Chen, C. Guan, J.-Q. Huang, H.-J. Peng, R. Zhang, S.-T. Yang, Q. Zhang, Implantable Solid Electrolyte Interphase in Lithium-Metal Batteries, Chem 2(2) (2017) 258-270. [64] Y. Zhao, Q. Sun, X. Li, C. Wang, Y. Sun, K.R. Adair, R. Li, X. Sun, Carbon paper interlayers: A universal and effective approach for highly stable Li metal anodes, Nano Energy 43 (2018) 368-375. [65] A.C. Kozen, C.F. Lin, A.J. Pearse, M.A. Schroeder, X. Han, L. Hu, S.B. Lee, G.W. Rubloff, M. Noked, Next-Generation Lithium Metal Anode Engineering via Atomic Layer Deposition, ACS Nano 9(6) (2015) 5884-5892. [66] M. Wu, Z. Wen, Y. Liu, X. Wang, L. Huang, Electrochemical behaviors of a Li3N modified Li metal electrode in secondary lithium batteries, J. Power Sources 196(19) (2011) 8091-8097. [67] Y.J. Zhang, W. Wang, H. Tang, W.Q. Bai, X. Ge, X.L. Wang, C.D. Gu, J.P. Tu, An ex-situ nitridation route to synthesize Li3N-modified Li anodes for lithium secondary batteries, J. Power Sources 277 (2015) 304-311. [68] G. Ma, Z. Wen, M. Wu, C. Shen, Q. Wang, J. Jin, X. Wu, A lithium anode protection guided highly-stable lithium-sulfur battery, Chem. Commun. 50(91) (2014) 14209-14212.

[69] K. Park, J.B. Goodenough, Dendrite-Suppressed Lithium Plating from a Liquid Electrolyte via Wetting of Li3N, Adv. Energy Mater. 7(19) (2017) 1700732-1700738. [70] Z.A. Ghazi, X. He, A.M. Khattak, N.A. Khan, B. Liang, A. Iqbal, J. Wang, H. Sin, L. Li, Z. Tang, MoS2 /Celgard Separator as Efficient Polysulfide Barrier for Long-Life Lithium-Sulfur Batteries, Adv. Mater. 29(21) (2017) 1606817-1606822. [71] J.-Q. Huang, Q. Zhang, H.-J. Peng, X.-Y. Liu, W.-Z. Qian, F. Wei, Ionic shield for polysulfides towards highly-stable lithium–sulfur batteries, Energy Environ. Sci. 7(1) (2014) 347-353. [72] H. Yao, K. Yan, W. Li, G. Zheng, D. Kong, Z.W. Seh, V.K. Narasimhan, Z. Liang, Y. Cui, Improved lithium–sulfur batteries with a conductive coating on the separator to prevent the accumulation of inactive S-related species at the cathode–separator interface, Energy Environ. Sci. 7(10) (2014) 3381-3390. [73] M.-H. Ryou, D.J. Lee, J.-N. Lee, Y.M. Lee, J.-K. Park, J.W. Choi, Excellent Cycle Life of Lithium-Metal Anodes in Lithium-Ion Batteries with Mussel-Inspired Polydopamine-Coated Separators, Adv. Energy Mater. 2(6) (2012) 645-650. [74] Y. Wang, L. Shi, H. Zhou, Z. Wang, R. Li, J. Zhu, Z. Qiu, Y. Zhao, M. Zhang, S. Yuan, Polyethylene separators modified by ultrathin hybrid films enhancing lithium ion transport performance and Li-metal anode stability, Electrochim. Acta 259 (2018) 386-394. [75] B. Wu, J. Lochala, T. Taverne, J. Xiao, The interplay between solid electrolyte interface (SEI) and dendritic lithium growth, Nano Energy 40 (2017) 34-41. [76] Y. Liu, Q. Liu, L. Xin, Y. Liu, F. Yang, E.A. Stach, J. Xie, Making Li-metal electrodes rechargeable by controlling the dendrite growth direction, Nat. Energy 2(7) (2017) 17083. [77] K. Yan, H.W. Lee, T. Gao, G. Zheng, H. Yao, H. Wang, Z. Lu, Y. Zhou, Z. Liang, Z. Liu, S. Chu, Y. Cui, Ultrathin two-dimensional atomic crystals as stable interfacial layer for improvement of lithium metal anode, Nano lett. 14(10) (2014) 6016-6022. [78] G. Zheng, S.W. Lee, Z. Liang, H.W. Lee, K. Yan, H. Yao, H. Wang, W. Li, S. Chu, Y. Cui, Interconnected hollow carbon nanospheres for stable lithium metal anodes, Nat. Nanotechnol. 9(8) (2014) 618-623. [79] X.-Q. Zhang, X.-B. Cheng, X. Chen, C. Yan, Q. Zhang, Fluoroethylene Carbonate Additives to Render Uniform Li Deposits in Lithium Metal Batteries, Adv. Funct. Mater. 27(10) (2017) 1605989-1605986. [80] Z. Zhang, Z. Peng, J. Zheng, S. Wang, Z. Liu, Y. Bi, Y. Chen, G. Wu, H. Li, P. Cui, Z. Wen, D. Wang, The long life-span of a Li-metal anode enabled by a protective layer based on the pyrolyzed N-doped binder network, J. Mater. Chem. A 5(19) (2017) 9339-9349. [81] X.Q. Zhang, X. Chen, R. Xu, X.B. Cheng, H.J. Peng, R. Zhang, J.Q. Huang, Q. Zhang, Columnar Lithium Metal Anodes, Angew. Chem. 56(45) (2017) 14207-14211. [82] B. Zhu, Y. Jin, X. Hu, Q. Zheng, S. Zhang, Q. Wang, J. Zhu, Poly(dimethylsiloxane) Thin Film as a Stable Interfacial Layer for High-Performance Lithium-Metal Battery Anodes, Adv. Mater. 29(2) (2017) 1603755-1603760. [83] W. Liu, W. Li, D. Zhuo, G. Zheng, Z. Lu, K. Liu, Y. Cui, Core-Shell Nanoparticle Coating as an Interfacial Layer for Dendrite-Free Lithium Metal Anodes, ACS Cent. Sci. 3(2) (2017) 135-140. [84] Z. Peng, N. Zhao, Z. Zhang, H. Wan, H. Lin, M. Liu, C. Shen, H. He, X. Guo, J.-G. Zhang, D. Wang, Stabilizing Li/electrolyte interface with a transplantable protective layer based on nanoscale LiF domains, Nano Energy 39 (2017) 662-672. [85] Peled, Emanuel. The electrochemical behavior of alkali and alkaline earth metals in nonaqueous

battery systems—the solid electrolyte interphase model. J. Electrochem. Soc. 126(12) (1979) 2047-2051. [86] Z. Li, J. Huang, B. Yann Liaw, V. Metzler, J. Zhang, A review of lithium deposition in lithium-ion and lithium metal secondary batteries, J. Power Sources 254 (2014) 168-182. [87] K. Xu, Electrolytes and Interphases in Li-Ion Batteries and Beyond, Chem. Rev. 114(23) (2014) 11503-11618. [88] W.Y. Li, H.B. Yao, K. Yan, G.Y. Zheng, Z. Liang, Y.M. Chiang, Y. Cui, The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth, Nat. Commun. 6 (2015) 8. [89] F. Ding, W. Xu, X. Chen, J. Zhang, M.H. Engelhard, Y. Zhang, B.R. Johnson, J.V. Crum, T.A. Blake, X. Liu, J.G. Zhang, Effects of Carbonate Solvents and Lithium Salts on Morphology and Coulombic Efficiency of Lithium Electrode, J. Electrochem. Soc. 160(10) (2013) A1894-A1901. [90] Chen, X., Shen, X., Li, B., Peng, H. J., Cheng, X. B., Li, B. Q., ... & Zhang, Q., Ion–Solvent Complexes Promote Gas Evolution from Electrolytes on a Sodium Metal Anode, Angew. Chem. 57(3) (2018) 734-737. [91] Y. Gofer, M. Ben-Zion, D. Aurbach, Solutions of LiAsF6 in 1, 3-dioxolane for secondary lithium batteries, J. power sources 39(2) (1992) 163-178. [92] R. Miao, J. Yang, Z. Xu, J. Wang, Y. Nuli, L. Sun, A new ether-based electrolyte for dendrite-free lithium-metal based rechargeable batteries, Sci. Rep. 6 (2016) 21771. [93] E. Markevich, G. Salitra, F. Chesneau, M. Schmidt, D. Aurbach, Very Stable Lithium Metal Stripping–Plating at a High Rate and High Areal Capacity in Fluoroethylene Carbonate-Based Organic Electrolyte Solution, ACS Energy Lett. 2(6) (2017) 1321-1326. [94] D. Aurbach, Review of selected electrode–solution interactions which determine the performance of Li and Li ion batteries, J. power sources 89(2) (2000) 206-218. [95] S.S. Zhang, An unique lithium salt for the improved electrolyte of Li-ion battery, Electrochem. Commun. 8(9) (2006) 1423-1428. [96] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chem. Rev. 104(10) (2004) 4303-4418. [97] R. Herr, Organic electrolytes for lithium cells, Electrochim. Acta 35(8) (1990) 1257-1265. [98] D. Aurbach, E. Zinigrad, Y. Cohen, H. Teller, A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions, Solid state ionics 148(3) (2002) 405-416. [99] K. Naoi, M. Mori, Y. Naruoka, W.M. Lamanna, R. Atanasoski, The Surface Film Formed on a Lithium Metal Electrode in a New Imide Electrolyte, Lithium Bis(perfluoroethylsulfonylimide)[LiN (C2F5SO2)2], J. Electrochem. Soc. 146(2) (1999) 462-469. [100] M. Odziemkowski, D. Irish, An Electrochemical Study of the Reactivity at the Lithium Electrolyte/Bare Lithium Metal Interface I. Purified Electrolytes, J. Electrochem. Soc. 139(11) (1992) 3063-3074. [101] M. Odziemkowski, D. Irish, An Electrochemical Study of the Reactivity at the Lithium Electrolyte/Bare Lithium Metal Interface II. Unpurified Solvents, J. Electrochem. Soc. 140(6) (1993) 1546-1555. [102] I.A. Shkrob, T.W. Marin, Y. Zhu, D.P. Abraham, Why bis (fluorosulfonyl) imide is a “magic anion” for electrochemistry, J. Phys. Chem. C 118(34) (2014) 19661-19671. [103] H. Kim, F. Wu, J.T. Lee, N. Nitta, H.-T. Lin, M. Oschatz, W.I. Cho, S. Kaskel, O. Borodin, G. Yushin, In Situ Formation of Protective Coatings on Sulfur Cathodes in Lithium Batteries with

LiFSI-Based Organic Electrolytes, Adv. Energy Mater. 5(6) (2015) 1401792-1401799. [104] J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, J.-G. Zhang, High rate and stable cycling of lithium metal anode, Nat. Commun. 6 (2015). [105] J. Hu, G. Long, S. Liu, G. Li, X. Gao, A LiFSI–LiTFSI binary-salt electrolyte to achieve high capacity and cycle stability for a Li–S battery, Chem. Commun. 50(93) (2014) 14647-14650. [106] D. Aurbach, E. Zinigrad, H. Teller, Y. Cohen, G. Salitra, H. Yamin, P. Dan, E. Elster, Attempts to improve the behavior of Li electrodes in rechargeable lithium batteries, Journal of The Electrochemical Society 149(10) (2002) A1267-A1277. [107] R. Miao, J. Yang, X. Feng, H. Jia, J. Wang, Y. Nuli, Novel dual-salts electrolyte solution for dendrite-free lithium-metal based rechargeable batteries with high cycle reversibility, J. Power Sources 271 (2014) 291-297. [108] C. Liao, K.S. Han, L. Baggetto, D.A. Hillesheim, R. Custelcean, E.S. Lee, B. Guo, Z. Bi, D.e. Jiang, G.M. Veith, Synthesis and Characterization of Lithium Bis (fluoromalonato) borate for Lithium-Ion Battery Applications, Advanced Energy Materials 4(6) (2014). [109] X.-G. Sun, C. Liao, L. Baggetto, B. Guo, R.R. Unocic, G.M. Veith, S. Dai, Bis (fluoromalonato) borate (BFMB) anion based ionic liquid as an additive for lithium-ion battery electrolytes, J. Mater. Chem. A 2(20) (2014) 7606-7614. [110] R. Younesi, G.M. Veith, P. Johansson, K. Edström, T. Vegge, Lithium salts for advanced lithium batteries: Li–metal, Li–O2, and Li–S, Energy Environ. Sci. 8(7) (2015) 1905-1922. [111] T. Schedlbauer, B. Hoffmann, S. Krüger, H.J. Gores, M. Winter, Results from a novel method for corrosion studies of electroplated lithium metal based on measurements with an impedance scanning electrochemical quartz crystal microbalance, Energies 6(7) (2013) 3481-3505. [112] Z. Xu, J. Wang, J. Yang, X. Miao, R. Chen, J. Qian, R. Miao, Enhanced Performance of a Lithium-Sulfur Battery Using a Carbonate-Based Electrolyte, Angew. Chem. 55(35) (2016) 10372-10375. [113] T. Jaumann, J. Balach, U. Langklotz, V. Sauchuk, M. Fritsch, A. Michaelis, V. Teltevskij, D. Mikhailova, S. Oswald, M. Klose, G. Stephani, R. Hauser, J. Eckert, L. Giebeler, Lifetime vs. rate capability: Understanding the role of FEC and VC in high-energy Li-ion batteries with nano-silicon anodes, Energy Storage Mater. 6 (2017) 26-35. [114] J. Zheng, M.H. Engelhard, D. Mei, S. Jiao, B.J. Polzin, J.-G. Zhang, W. Xu, Electrolyte additive enabled fast charging and stable cycling lithium metal batteries, Nat. Energy 2(3) (2017) 17012. [115] S. Choudhury, L.A. Archer, Lithium Fluoride Additives for Stable Cycling of Lithium Batteries at High Current Densities, Adv. Electronic Mater. 2(2) (2016) 1500246-1500252. [116] S.S. Zhang, A review on electrolyte additives for lithium-ion batteries, J. Power Sources 162(2) (2006) 1379-1394. [117] Q.C. Liu, J.J. Xu, S. Yuan, Z.W. Chang, D. Xu, Y.B. Yin, L. Li, H.X. Zhong, Y.S. Jiang, J.M. Yan, X.B. Zhang, Artificial Protection Film on Lithium Metal Anode toward Long-Cycle-Life Lithium-Oxygen Batteries, Adv. Mater. 27(35) (2015) 5241-5247. [118] X. Ren, Y. Zhang, M.H. Engelhard, Q. Li, J.-G. Zhang, W. Xu, Guided Lithium Metal Deposition and Improved Lithium Coulombic Efficiency through Synergistic Effects of LiAsF6 and Cyclic Carbonate Additives, ACS Energy Lett. 3 (2017) 14-19. [119] S.S. Zhang, Role of LiNO3 in rechargeable lithium/sulfur battery, Electrochim. Acta 70 (2012) 344-348. [120] S. Xiong, K. Xie, Y. Diao, X. Hong, Characterization of the solid electrolyte interphase on

lithium anode for preventing the shuttle mechanism in lithium–sulfur batteries, J. Power Sources 246 (2014) 840-845. [121] C.-Z. Zhao, X.-B. Cheng, R. Zhang, H.-J. Peng, J.-Q. Huang, R. Ran, Z.-H. Huang, F. Wei, Q. Zhang, Li 2 S 5 -based ternary-salt electrolyte for robust lithium metal anode, Energy Storage Mater. 3 (2016) 77-84. [122] C. Yan, X.-B. Cheng, C.-Z. Zhao, J.-Q. Huang, S.-T. Yang, Q. Zhang, Lithium metal protection through in-situ formed solid electrolyte interphase in lithium-sulfur batteries: The role of polysulfides on lithium anode, J. Power Sources 327 (2016) 212-220. [123] M. Agostini, B. Scrosati, J. Hassoun, An Advanced Lithium-Ion Sulfur Battery for High Energy Storage, Adv. Energy Mater. 5(16) (2015) 1500481-1500486. [124] C. Qu, Y. Chen, X. Yang, H. Zhang, X. Li, H. Zhang, LiNO 3 -free electrolyte for Li-S battery: A solvent of choice with low K sp of polysulfide and low dendrite of lithium, Nano Energy 39 (2017) 262-272. [125] A. Jozwiuk, B.B. Berkes, T. Weiß, H. Sommer, J. Janek, T. Brezesinski, The critical role of lithium nitrate in the gas evolution of lithium–sulfur batteries, Energy Environ. Sci. 9(8) (2016) 2603-2608. [126] G. Li, Y. Gao, X. He, Q. Huang, S. Chen, S.H. Kim, D. Wang, Organosulfide-plasticized solid-electrolyte interphase layer enables stable lithium metal anodes for long-cycle lithium-sulfur batteries, Nat. Commun. 8(1) (2017) 850. [127] J. Qian, W. Xu, P. Bhattacharya, M. Engelhard, W.A. Henderson, Y. Zhang, J.-G. Zhang, Dendrite-free Li deposition using trace-amounts of water as an electrolyte additive, Nano Energy 15 (2015) 135-144. [128] X.B. Cheng, M.Q. Zhao, C. Chen, A. Pentecost, K. Maleski, T. Mathis, X.Q. Zhang, Q. Zhang, J. Jiang, Y. Gogotsi, Nanodiamonds suppress the growth of lithium dendrites, Nat. Commun. 8(1) (2017) 336. [129] S. Choudhury, Z. Tu, S. Stalin, D. Vu, K. Fawole, D. Gunceler, R. Sundararaman, L.A. Archer, Electroless Formation of Hybrid Lithium Anodes for Fast Interfacial Ion Transport, Angew. Chem. 56(42) (2017) 13070-13077. [130] Q. Zhao, Z. Tu, S. Wei, K. Zhang, S. Choudhury, X. Liu, L.A. Archer, Building Organic/Inorganic Hybrid Interphases for Fast Interfacial Transport in Rechargeable Metal Batteries, Angew. Chem. 130(4) (2018) 1004-1008. [131] X.-B. Cheng, M.-Q. Zhao, C. Chen, A. Pentecost, K. Maleski, T. Mathis, X.-Q. Zhang, Q. Zhang, J. Jiang, Y. Gogotsi, Nanodiamonds suppress the growth of lithium dendrites, Nat. Commun. 8(1) (2017) 336. [132] V. Fleury, J.N. Chazalviel, M. Rosso, B. Sapoval, The role of the anions in the growth speed of fractal electrodeposits, J. Electroanal. Chem. Interfac. Electrochem. 290(1-2) (1990) 249-255. [133] S.-K. Jeong, H.-Y. Seo, D.-H. Kim, H.-K. Han, J.-G. Kim, Y.B. Lee, Y. Iriyama, T. Abe, Z. Ogumi, Suppression of dendritic lithium formation by using concentrated electrolyte solutions, Electrochem. Commun. 10(4) (2008) 635-638. [134] J. Qian, W.A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, J.G. Zhang, High rate and stable cycling of lithium metal anode, Nat. Commun. 6 (2015) 6362. [135] J. Qian, B.D. Adams, J. Zheng, W. Xu, W.A. Henderson, J. Wang, M.E. Bowden, S. Xu, J. Hu, J.-G. Zhang, Anode-Free Rechargeable Lithium Metal Batteries, Adv. Funct. Mater. 26(39) (2016) 7094-7102.

[136] Q. Ma, Z. Fang, P. Liu, J. Ma, X. Qi, W. Feng, J. Nie, Y.-S. Hu, H. Li, X. Huang, L. Chen, Z. Zhou, Improved Cycling Stability of Lithium-Metal Anode with Concentrated Electrolytes Based on Lithium (Fluorosulfonyl)(trifluoromethanesulfonyl)imide, ChemElectroChem 3(4) (2016) 531-536. [137] P. Liu, Q. Ma, Z. Fang, J. Ma, Y.-S. Hu, Z.-B. Zhou, H. Li, X.-J. Huang, L.-Q. Chen, Concentrated dual-salt electrolytes for improving the cycling stability of lithium metal anodes, Chin. Phys. B 25(7) (2016) 078203. [138] L. Suo, Y.S. Hu, H. Li, M. Armand, L. Chen, A new class of Solvent-in-Salt electrolyte for high-energy rechargeable metallic lithium batteries, Nat. Commun. 4 (2013) 1481. [139] Y.Z. Zhang, S. Liu, G.C. Li, G.R. Li, X.P. Gao, Sulfur/polyacrylonitrile/carbon multi-composites as cathode materials for lithium/sulfur battery in the concentrated electrolyte, J. Mater. Chem. A 2(13) (2014) 4652-4659. [140] R. Cao, J. Chen, K.S. Han, W. Xu, D. Mei, P. Bhattacharya, M.H. Engelhard, K.T. Mueller, J. Liu, J.-G. Zhang, Effect of the Anion Activity on the Stability of Li Metal Anodes in Lithium-Sulfur Batteries, Adv. Funct. Mater. 26(18) (2016) 3059-3066. [141] H. Kim, F. Wu, J.T. Lee, N. Nitta, H.-T. Lin, M. Oschatz, W.I. Cho, S. Kaskel, O. Borodin, G. Yushin, In Situ Formation of Protective Coatings on Sulfur Cathodes in Lithium Batteries with LiFSI-Based Organic Electrolytes, Adv. Energy Mater. 5(6) (2015) 1401792-1401799. [142] N. Togasaki, T. Momma, T. Osaka, Enhanced cycling performance of a Li metal anode in a dimethylsulfoxide-based electrolyte using highly concentrated lithium salt for a lithium−oxygen battery, J. Power Sources 307 (2016) 98-104. [143] F. Li, T. Zhang, Y. Yamada, A. Yamada, H. Zhou, Enhanced Cycling Performance of Li-O2 Batteries by the Optimized Electrolyte Concentration of LiTFSA in Glymes, Adv. Energy Mater. 3(4) (2013) 532-538. [144] Y. Liu, L. Suo, H. Lin, W. Yang, Y. Fang, X. Liu, D. Wang, Y.-S. Hu, W. Han, L. Chen, Novel approach for a high-energy-density Li–air battery: tri-dimensional growth of Li2O2 crystals tailored by electrolyte Li+ ion concentrations, J. Mater. Chem. A 2(24) (2014) 9020-9024. [145] B. Liu, W. Xu, P. Yan, X. Sun, M.E. Bowden, J. Read, J. Qian, D. Mei, C.-M. Wang, J.-G. Zhang, Enhanced Cycling Stability of Rechargeable Li-O2 Batteries Using High-Concentration Electrolytes, Adv. Funct. Mater. 26(4) (2016) 605-613. [146] X.-B. Cheng, C. Yan, J.-Q. Huang, P. Li, L. Zhu, L. Zhao, Y. Zhang, W. Zhu, S.-T. Yang, Q. Zhang, The gap between long lifespan Li-S coin and pouch cells: The importance of lithium metal anode protection, Energy Storage Mater. 6 (2017) 18-25. [147] C. Monroe, J. Newman, The Impact of Elastic Deformation on Deposition Kinetics at Lithium/Polymer Interfaces, J. Electrochem. Soc. 152(2) (2005) A396-A404. [148] R. Khurana, J.L. Schaefer, L.A. Archer, G.W. Coates, Suppression of lithium dendrite growth using cross-linked polyethylene/poly(ethylene oxide) electrolytes: a new approach for practical lithium-metal polymer batteries, J. Am. Chem. Soc. 136(20) (2014) 7395-7402. [149] M.B. Armand, M.J. Duclot, P. Rigaud, Polymer solid electrolytes Stability domain, Solid State Ionics 3(1981) 429-430. [150] A. Nishimoto, M. Watanabe, Y. Ikeda, S. Kohjiya, High ionic conductivity of new polymer electrolytes based on high molecular weight polyether comb polymers, Electrochim. Acta 43(10-11) (1998) 1177-1184. [151] H.J. Walls, J. Zhou, J.A. Yerian, P.S. Fedkiw, S.A. Khan, M.K. Stowe, G.L. Baker, Fumed silica-based composite polymer electrolytes -synthesis rheology and electrochemistry, J. Power Sources

(2000) 156–162. [152] Croce, F., Appetecchi, G. B., Persi, L., & Scrosati, B., Nanocomposite polymer electrolytes for lithium batteries. Nature 394(6692) (1998) 456-458. [153] L. Damen, J. Hassoun, M. Mastragostino, B. Scrosati, Solid-state, rechargeable Li/LiFePO4 polymer battery for electric vehicle application, J. Power Sources 195(19) (2010) 6902-6904. [154] S. Jafirin, I. Ahmad, A. Ahmad, Potential Use of Cellulose from Kenaf in Polymer Electrolytes Based on MG49 Rubber Composites, Bioresources 8(4) (2013) 5947-5964. [155] Q. Pan, D.M. Smith, H. Qi, S. Wang, C.Y. Li, Hybrid electrolytes with controlled network structures for lithium metal batteries, Adv. Mater. 27(39) (2015) 5995-6001. [156] D. Lin, W. Liu, Y. Liu, H.R. Lee, P.C. Hsu, K. Liu, Y. Cui, High Ionic Conductivity of Composite Solid Polymer Electrolyte via In Situ Synthesis of Monodispersed SiO2 Nanospheres in Poly(ethylene oxide), Nano Lett. 16(1) (2016) 459-65. [157] W. Zhou, S. Wang, Y. Li, S. Xin, A. Manthiram, J.B. Goodenough, Plating a Dendrite-Free Lithium Anode with a Polymer/Ceramic/Polymer Sandwich Electrolyte, J. Am. Chem. Soc. 138(30) (2016) 9385-9388. [158] S. Choudhury, R. Mangal, A. Agrawal, L.A. Archer, A highly reversible room-temperature lithium metal battery based on crosslinked hairy nanoparticles, Nat. Commun. 6 (2015) 10101. [159] I. Gurevitch, R. Buonsanti, A.A. Teran, B. Gludovatz, R.O. Ritchie, J. Cabana, N.P. Balsara, Nanocomposites of Titanium Dioxide and Polystyrene-Poly(ethylene oxide) Block Copolymer as Solid-State Electrolytes for Lithium Metal Batteries, J. Electrochem. Soc. 160(9) (2013) A1611-A1617. [160] R. Bouchet, S. Maria, R. Meziane, A. Aboulaich, L. Lienafa, J.P. Bonnet, T.N. Phan, D. Bertin, D. Gigmes, D. Devaux, R. Denoyel, M. Armand, Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries, Nat. Mater. 12(5) (2013) 452-457. [161] W. Luo, L. Zhou, K. Fu, Z. Yang, J. Wan, M. Manno, Y. Yao, H. Zhu, B. Yang, L. Hu, A Thermally Conductive Separator for Stable Li Metal Anodes, Nano lett. 15(9) (2015) 6149-6154. [162] W. Huang, Z. Zhu, L. Wang, S. Wang, H. Li, Z. Tao, J. Shi, L. Guan, J. Chen, Quasi-solid-state rechargeable lithium-ion batteries with a calix[4]quinone cathode and gel polymer electrolyte, Angew. Chem. 52(35) (2013) 9162-9166. [163] Q. Lu, Y.B. He, Q. Yu, B. Li, Y.V. Kaneti, Y. Yao, F. Kang, Q.H. Yang, Dendrite-Free, High-Rate, Long-Life Lithium Metal Batteries with a 3D Cross-Linked Network Polymer Electrolyte, Adv. Mater. 29(13) (2017) 1604460-1604467. [164] P. Zhang, M. Li, B. Yang, Y. Fang, X. Jiang, G.M. Veith, X.G. Sun, S. Dai, Polymerized Ionic Networks with High Charge Density: Quasi-Solid Electrolytes in Lithium-Metal Batteries, Adv. Mater. 27(48) (2015) 8088-8094. [165] K. Yin, Z. Zhang, X. Li, L. Yang, K. Tachibana, S.-i. Hirano, Polymer electrolytes based on dicationic polymeric ionic liquids: application in lithium metal batteries, J. Mater. Chem. A 3(1) (2015) 170-178. [166] C. Monroe, J. Newman, The impact of elastic deformation on deposition kinetics at lithium/polymer interfaces, J. Electrochem. Soc. 152(2) (2005) A396-A404. [167] W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.-G. Zhang, Lithium metal anodes for rechargeable batteries, Energy Environ. Sci. 7(2) (2014) 513-537. [168] E. Quartarone, P. Mustarelli, Electrolytes for solid-state lithium rechargeable batteries: recent advances and perspectives, Chem. Soc. Rev. 40(5) (2011) 2525-2540. [169] X.-X. Zeng, Y.-X. Yin, N.-W. Li, W.-C. Du, Y.-G. Guo, L.-J. Wan, Reshaping lithium

plating/stripping behavior via bifunctional polymer electrolyte for room-temperature solid Li metal batteries, J. Am. Chem. Soc. 138(49) (2016) 15825-15828. [170] Y.S. Jung, D.Y. Oh, Y.J. Nam, K.H. Park, Issues and Challenges for Bulk‐Type All‐Solid‐ State Rechargeable Lithium Batteries using Sulfide Solid Electrolytes, Israel J. Chem. 55(5) (2015) 472-485. [171] R. Chen, W. Qu, X. Guo, L. Li, F. Wu, The pursuit of solid-state electrolytes for lithium batteries: from comprehensive insight to emerging horizons, Mater. Horiz. 3(6) (2016) 487-516. [172] J.G. Kim, B. Son, S. Mukherjee, N. Schuppert, A. Bates, O. Kwon, M.J. Choi, H.Y. Chung, S. Park, A review of lithium and non-lithium based solid state batteries, J. Power Sources 282 (2015) 299-322. [173] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A lithium superionic conductor, Nat. Mater. 10(9) (2011) 682-686. [174] P. Knauth, Inorganic solid Li ion conductors: An overview, Solid State Ionics 180(14) (2009) 911-916. [175] J. Bates, N. Dudney, G. Gruzalski, R. Zuhr, A. Choudhury, C. Luck, J. Robertson, Electrical properties of amorphous lithium electrolyte thin films, Solid state ionics 53 (1992) 647-654. [176] W.D. Richards, L.J. Miara, Y. Wang, J.C. Kim, G. Ceder, Interface stability in solid-state batteries, Chem. Mater. 28(1) (2015) 266-273. [177] Y. Zhu, X. He, Y. Mo, Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first-principles calculations, ACS Appl. Mater. & Inter. 7(42) (2015) 23685-23693. [178] M. Rawlence, I. Garbayo, S. Buecheler, J. Rupp, On the chemical stability of post-lithiated garnet Al-stabilized Li7La3Zr2O12 solid state electrolyte thin films, Nanoscale 8(31) (2016) 14746-14753. [179] S. Ohta, T. Kobayashi, T. Asaoka, High lithium ionic conductivity in the garnet-type oxide Li7− X La3(Zr2− X, NbX) O12 (X= 0–2), J. Power Sources 196(6) (2011) 3342-3345. [180] H. Buschmann, J. Dölle, S. Berendts, A. Kuhn, P. Bottke, M. Wilkening, P. Heitjans, A. Senyshyn, H. Ehrenberg, A. Lotnyk, Structure and dynamics of the fast lithium ion conductor “Li7La3Zr2O12”, Phys. Chem. Chem. Phys. 13(43) (2011) 19378-19392. [181] C.L. Tsai, V. Roddatis, C.V. Chandran, Q. Ma, S. Uhlenbruck, M. Bram, P. Heitjans, O. Guillon, Li7La3Zr2O12 Interface Modification for Li Dendrite Prevention, ACS Appl. Mater. Inter. 8(16) (2016) 10617-10626. [182] Z. Liang, G. Zheng, C. Liu, N. Liu, W. Li, K. Yan, H. Yao, P.-C. Hsu, S. Chu, Y. Cui, Polymer nanofiber-guided uniform lithium deposition for battery electrodes, Nano Lett. 15(5) (2015) 2910-2916. [183] A. Sharafi, H.M. Meyer, J. Nanda, J. Wolfenstine, J. Sakamoto, Characterizing the Li–Li7La3Zr 2O12

interface stability and kinetics as a function of temperature and current density, J. Power Sources

302 (2016) 135-139. [184] R. Sudo, Y. Nakata, K. Ishiguro, M. Matsui, A. Hirano, Y. Takeda, O. Yamamoto, N. Imanishi, Interface behavior between garnet-type lithium-conducting solid electrolyte and lithium metal, Solid State Ionics 262 (2014) 151-154. [185] W. Luo, Y. Gong, Y. Zhu, K.K. Fu, J. Dai, S.D. Lacey, C. Wang, B. Liu, X. Han, Y. Mo, E.D. Wachsman, L. Hu, Transition from Superlithiophobicity to Superlithiophilicity of Garnet Solid-State Electrolyte, J. Am. Chem. Soc. 138(37) (2016) 12258-12262.

[186] C. Wang, Y. Gong, B. Liu, K. Fu, Y. Yao, E. Hitz, Y. Li, J. Dai, S. Xu, W. Luo, Conformal, Nanoscale ZnO Surface Modification of Garnet-Based Solid-State Electrolyte for Lithium Metal Anodes, Nano Lett. 17(1) (2016) 565-571. [187] X. Han, Y. Gong, K.K. Fu, X. He, G.T. Hitz, J. Dai, A. Pearse, B. Liu, H. Wang, G. Rubloff, Negating interfacial impedance in garnet-based solid-state Li metal batteries, Nat. Mater. 16(5) (2017) 572-579. [188] B. Xu, W. Li, H. Duan, H. Wang, Y. Guo, H. Li, H. Liu, Li3PO4-added garnet-type Li6.5La3Zr1.5Ta0.5O12 for Li-dendrite suppression, J. Power Sources 354 (2017) 68-73. [189] J. Liu, D.G.D. Galpaya, L. Yan, M. Sun, Z. Lin, C. Yan, C. Liang, S. Zhang, Exploiting a robust biopolymer network binder for an ultrahigh-areal-capacity Li–S battery, Energy Environ. Sci. 10(3) (2017) 750-755. [190] K. Fu, Y. Gong, J. Dai, A. Gong, X. Han, Y. Yao, C. Wang, Y. Wang, Y. Chen, C. Yan, Y. Li, E.D. Wachsman, L. Hu, Flexible, solid-state, ion-conducting membrane with 3D garnet nanofiber networks for lithium batteries, Proc. Natl. Acad. Sci. USA 113(26) (2016) 7094-7099. [191] K. Fu, Y. Gong, G.T. Hitz, D.W. McOwen, Y. Li, S. Xu, Y. Wen, L. Zhang, C. Wang, G. Pastel, J. Dai, B. Liu, H. Xie, Y. Yao, E.D. Wachsman, L. Hu, Three-dimensional bilayer garnet solid electrolyte based high energy density lithium metal–sulfur batteries, Energy Environ. Sci. 10(7) (2017) 1568-1575. [192] C.-Z. Zhao, X.-Q. Zhang, X.-B. Cheng, R. Zhang, R. Xu, P.-Y. Chen, H.-J. Peng, J.-Q. Huang, Q. Zhang, An anion-immobilized composite electrolyte for dendrite-free lithium metal anodes, Proc. Natl. Acad. Sci. USA 114(42) (2017) 11069-11074. [193] A. Sharafi, E. Kazyak, A.L. Davis, S. Yu, T. Thompson, D.J. Siegel, N.P. Dasgupta, J. Sakamoto, Surface Chemistry Mechanism of Ultra-Low Interfacial Resistance in the Solid-State Electrolyte Li7La3Zr2O12, Chem. Mater. 29(18) (2017) 7961-7968. [194] C. Wang, Y. Gong, J. Dai, L. Zhang, H. Xie, G. Pastel, B. Liu, E. Wachsman, H. Wang, L. Hu, In Situ Neutron Depth Profiling of Lithium Metal-Garnet Interfaces for Solid State Batteries, J. Am. Chem. Soc. 139(40) (2017) 14257-14264. [195] W. Xu, J.L. Wang, F. Ding, X.L. Chen, E. Nasybutin, Y.H. Zhang, J.G. Zhang, Lithium metal anodes for rechargeable batteries, Energy Environ. Sci. 7(2) (2014) 513-537. [196] P. Verma, P. Maire, P. Novak, A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries, Electrochim. Acta 55(22) (2010) 6332-6341. [197] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, A. Subramanian, H.Y. Fan, L.A. Qi, A. Kushima, J. Li, In Situ Observation of the Electrochemical Lithiation of a Single SnO2 Nanowire Electrode, Science 330(6010) (2010) 1515-1520. [198] X.H. Liu, H. Zheng, L. Zhong, S. Huan, K. Karki, L.Q. Zhang, Y. Liu, A. Kushima, W.T. Liang, J.W. Wang, J.H. Cho, E. Epstein, S.A. Dayeh, S.T. Picraux, T. Zhu, J. Li, J.P. Sullivan, J. Cumings, C.S. Wang, S.X. Mao, Z.Z. Ye, S.L. Zhang, J.Y. Huang, Anisotropic Swelling and Fracture of Silicon Nanowires during Lithiation, Nano lett. 11(8) (2011) 3312-3318. [199] X.H. Liu, L. Zhong, L.Q. Zhang, A. Kushima, S.X. Mao, J. Li, Z.Z. Ye, J.P. Sullivan, J.Y. Huang, Lithium fiber growth on the anode in a nanowire lithium ion battery during charging, Appl. Phys. Lett. 98(18) (2011) 3. [200] F. Sagane, R. Shimokawa, H. Sano, H. Sakaebe, Y. Iriyama, In-situ scanning electron microscopy observations of Li plating and stripping reactions at the lithium phosphorus oxynitride glass electrolyte/Cu interface, J. Power Sources 225 (2013) 245-250. [201] K. Nishikawa, T. Mori, T. Nishida, Y. Fukunaka, M. Rosso, T. Homma, In Situ Observation of

Dendrite Growth of Electrodeposited Li Metal, J. Electrochem. Soc. 157(11) (2010) A1212-A1217. [202] J. Steiger, D. Kramer, R. Monig, Mechanisms of dendritic growth investigated by in situ light microscopy during electrodeposition and dissolution of lithium, J. Power Sources 261 (2014) 112-119. [203] R.R. Unocic, X.G. Sun, R.L. Sacci, L.A. Adamczyk, D.H. Alsem, S. Dai, N.J. Dudney, K.L. More, Direct Visualization of Solid Electrolyte Interphase Formation in Lithium-Ion Batteries with In Situ Electrochemical Transmission Electron Microscopy, Microsc. Microanal. 20(4) (2014) 1029-1037. [204] H. Sano, H. Sakaebe, H. Matsumoto, Observation of electrodeposited lithium by optical microscope in room temperature ionic liquid-based electrolyte, J. Power Sources 196(16) (2011) 6663-6669. [205] R.L. Sacci, J.M. Black, N. Balke, N.J. Dudney, K.L. More, R.R. Unocic, Nanoscale Imaging of Fundamental Li Battery Chemistry: Solid-Electrolyte Interphase Formation and Preferential Growth of Lithium Metal Nanoclusters, Nano lett. 15(3) (2015) 2011-2018. [206] G. Rong, X. Zhang, W. Zhao, Y. Qiu, M. Liu, F. Ye, Y. Xu, J. Chen, Y. Hou, W. Li, W. Duan, Y. Zhang, Liquid-Phase Electrochemical Scanning Electron Microscopy for In Situ Investigation of Lithium Dendrite Growth and Dissolution, Adv. Mater. 29(13) (2017) 1606187-1606193. [207] A.J. Leenheer, K.L. Jungjohann, K.R. Zavadil, J.P. Sullivan, C.T. Harris, Lithium Electrodeposition Dynamics in Aprotic Electrolyte Observed in Situ via Transmission Electron Microscopy, ACS Nano 9(4) (2015) 4379-4389. [208] R.L. Sacci, N.J. Dudney, K.L. More, L.R. Parent, I. Arslan, N.D. Browning, R.R. Unocic, Direct visualization of initial SEI morphology and growth kinetics during lithium deposition by in situ electrochemical transmission electron microscopy, Chem. Commun. 50(17) (2014) 2104-2107. [209] L.Y. Lim, S.F. Fan, H.H. Hng, M.F. Toney, Storage Capacity and Cycling Stability in Ge Anodes: Relationship of Anode Structure and Cycling Rate, Adv. Energy Mater. 5(15) (2015) 1500599-1500566. [210] Y.Y. Li, F. El Gabaly, T.R. Ferguson, R.B. Smith, N.C. Bartelt, J.D. Sugar, K.R. Fenton, D.A. Cogswell, A.L.D. Kilcoyne, T. Tyliszczak, M.Z. Bazant, W.C. Chueh, Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes, Nat. Mater. 13(12) (2014) 1149-1156. [211] S. Chandrashekar, N.M. Trease, H.J. Chang, L.S. Du, C.P. Grey, A. Jerschow, Li-7 MRI of Li batteries reveals location of microstructural lithium, Nat. Mater. 11(4) (2012) 311-315. [212] R. Bhattacharyya, B. Key, H.L. Chen, A.S. Best, A.F. Hollenkamp, C.P. Grey, In situ NMR observation of the formation of metallic lithium microstructures in lithium batteries, Nat. Mater. 9(6) (2010) 504-510. [213] J. Wandt, C. Marino, H.A. Gasteiger, P. Jakes, R.A. Eichel, J. Granwehr, Operando electron paramagnetic resonance spectroscopy-formation of mossy lithium on lithium anodes during charge-discharge cycling, Energy Environ. Sci. 8(4) (2015) 1358-1367. [214] Y. Li, Y. Li, A. Pei, K. Yan, Y. Sun, C.-L. Wu, L.-M. Joubert, R. Chin, A.L. Koh, Y. Yu, J. Perrino, B. Butz, S. Chu, Y. Cui, Atomic structure of sensitive battery materials and Interfaces revealed by cryo-electron microscopy, Science 358(6362) (2017) 506-510.