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Low volume change composite lithium metal anodes
Accepted Manuscript Low volume change composite lithium metal anodes Zi-Jian Zheng, Qi Su, Qiao Zhang, Xin-Cheng Hu, Ya-Xia Yin, Rui Wen, Huan Ye, Zheng-Bang Wang, Yu-Guo Guo PII:
S2211-2855(19)30617-2
DOI:
https://doi.org/10.1016/j.nanoen.2019.103910
Article Number: 103910 Reference:
NANOEN 103910
To appear in:
Nano Energy
Received Date: 29 May 2019 Revised Date:
2 July 2019
Accepted Date: 15 July 2019
Please cite this article as: Z.-J. Zheng, Q. Su, Q. Zhang, X.-C. Hu, Y.-X. Yin, R. Wen, H. Ye, Z.-B. Wang, Y.-G. Guo, Low volume change composite lithium metal anodes, Nano Energy (2019), doi: https:// doi.org/10.1016/j.nanoen.2019.103910. 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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT Low volume change composite lithium metal anodes Zi-Jian Zhenga, Qi Sua, Qiao Zhanga, Xin-Cheng Hub,d, Ya-Xia Yinb,d, Rui Wenb,d, Huan Yeb,c*, Zheng-Bang Wanga*, Yu-Guo Guob,d*
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Keywords: Lithium metal batteries, Lithium metal anodes, Hybrid hosts, Low volume change, Long lifespan
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TOC figure
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A low volume change composite lithium metal anode with dendrite free is constructed by integrating a 3D conducting scaffold with a metal-organic frameworks (MOFs)
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coating.
ACCEPTED MANUSCRIPT Low volume change composite lithium metal anodes Zi-Jian Zhenga, Qi Sua, Qiao Zhanga, Xin-Cheng Hub,d, Ya-Xia Yinb,d, Rui Wenb,d, Huan Yeb,c*, Zheng-Bang Wanga*, Yu-Guo Guob,d* a
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Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China b CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China c College of Science, Huazhong Agricultural University, Wuhan 430070, China d University of Chinese Academy of Sciences, Beijing 100049, China
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* E-mail: [email protected]; [email protected]; [email protected]
Abstract
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The ramified growth of lithium dendrites and their enormous volume change upon cycling bear the primary responsibility for the poor cyclability and serious safety of lithium metal batteries. Herein, a low volume change composite lithium metal anode
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is realized by encaging Li into a hybrid host featuring a 3D conducting scaffold with a
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metal-organic frameworks (MOFs) coating. The scaffold with high pore volume is favorable to accommodate Li with high areal capacity and alleviate the variation in electrode dimensions. The MOFs layer with abundant interconnected micropores serving as “ion sieve” can boost uniform distribution of Li ions while its high Young’s modulus (>32 GPa) can arrest dendrite propagation. The thus-formed anode displays a negligible dimension variation (<5%), enabling stable cycling in both symmetric cells (>1000 h) and full cells (>200 cycle-life). This study may open up new directions to
ACCEPTED MANUSCRIPT construct safe Li metal batteries through an industry-adoptable technology. Keywords: Lithium metal batteries, Lithium metal anodes, Hybrid hosts, Low volume change, Long lifespan
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1. Introduction The ever-increasing demand for portable electronic devices, electrical vehicles, and grid-scale energy storage drives the technological progress in high energy density
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as well as high security rechargeable batteries [1-5]. Among various candidates,
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lithium metal batteries (LMBs) are recognized as one of the most promising electrochemical energy storage devices due to their extremely high theoretical energy density [6-9]. Li metal might be the ultimate option for the anode in a Li battery because of its intrinsic advantages, in terms of high specific capacity (3860 mA h g−1),
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low density (0.534 g cm−3), and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode) [10,11]. The practical application of Li metal anode has long been held back, however, by its severe safety hazards and poor
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cyclability, which mainly roots in the ramified growth of Li dendrites and
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significantly dimensional change upon cycling [12,13]. Targeted at above obsessions of Li metal anodes, tremendous effective strategies
have been employed to improve the electrochemical performance of Li anodes, such as optimizing organic electrolytes [14], creating protective interface layers [15,16], engineering 3D host structures for Li metal [17-23], and developing solid electrolytes [24-28]. The protective interface layers have attracted considerable attention because they are effective in regulating Li deposition, homogenizing the Li-ion flux, reducing
ACCEPTED MANUSCRIPT the local current density, and suppressing the growth of Li dendrites. Until now, various materials, range from pure polymer layers [16], three-dimensional inorganic materials [29-31], to polymer nanocomposite layers [32], are employed as coatings on
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Li/Cu foil surfaces. These coatings are effective to some extent in dendrite formation, especially under static conditions or at the initial stage of cycling, however, they still wear off after prolonged battery operations due to the relatively low modulus and
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poor deformation capacity. Considering the hostless nature of metallic Li and the
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limited pore volume of Cu foil, the Li anode might experience enormous volume expansion/shrinkage during the plating/striping process, bringing severe safety concerns. Therefore, one can firmly deduce that only scrapes a protective interface layer onto the Cu or Li surface is inefficient to address the security of Li anode. A 3D
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Li host that features a high ion conductivity interface layer is probably the most ideal type of anode configuration, which are expected to address the problems of dendritic growth and enormous volume variation.
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Metal organic Frameworks (MOFs) comprising of organic ligands and metal ions,
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a representative of recently developed porous materials, have been developed continuously, with a focus on selective gas separation [33,34], catalyze [35], and energy storage [36,37]. The many extraordinary characteristics of MOF, including structural diversity, strong mechanical strength, tunable porosity, easy access for ion transport, and electrically insulating, make it an appealing candidate as separators and coating in batteries [38,39]. Herein, we construct a smart Li metal anode configuration to suppress dendrite propagation and accommodate huge electrode
ACCEPTED MANUSCRIPT dimension variation on cycling, prepared by decorating a 3D conducting scaffold with a MOFs coating (Fig. 1). The bottom 3D conducting scaffold with a porous structure facilitates the Li deposition with high areal capacity. The upper Zr-based MOF layer
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with uniform microporous channels can serve as “ionic sieves” [40] for Li to distribute incoming Li+ and realize uniform Li+ transport, leading to dendrite-free morphology, wherein their high Young’s modulus (~32 GPa) and naturally insulating
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property help guide the densely growth of Li deposits into the 3D conducting scaffold
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in a given cycling capacity. As a result, a composite Li metal anode with the characteristics of high electrochemical activity, dendrite-free morphology, negligible electrode volume variation (less than 5% for 20 mA h cm−2), and ultralong cycle-life is achieved. Symmetric Li|Li cells can be stably cycled for over 1000 h at a current
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density of 1 mA cm−2, and for over 500 h at a current density of 2 mA cm−2. Furthermore, a 300 cycle-life for a full cell paired with a LiFePO4 cathode with ultrahigh areal capacity (2.3 mA h cm−2) demonstrates its practicability.
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2. Experimental section
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2.1. Synthesis of MOF-808
0.5 mmol Trimesic acid (Aladdin, with a purity of >98%) and 0.5 mmol
ZrOCl2·8H2O (Aldrich, with a purity of >99.5%) were added into N, N-dimethylformamide (DMF)/formic acid (20 mL/20 mL) to form a homogeneous solution. The solution was sealed in a 100 mL Teflon autoclave, which was then heated at 135 °C for 2 days. A white precipitate was collected by filtration and washed three times with 20 mL of fresh DMF. Then the as-obtained MOF-808 was immersed
ACCEPTED MANUSCRIPT into 10 mL of anhydrous DMF for 3 days, during which time the DMF was replaced 3 times per day. Then water- and acetone-exchange process follow the same as the DMF-exchanged process for another 3 days. The acetone-exchanged product was then
2.2. Preparation of MOF-HCF/Cu/Li
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respectively dried at 25 °C and at 150 °C for 24 h to yield activated sample.
Firstly, free-standing hollow carbon fibers (HCF) were prepared by carbonizing
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the commercial cotton. Secondly, a mixture of MOF and polyvinylidene fluoride at a
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weight ratio of 3:1 was dissolved into N-methylpyrrolidone to form slurry. Thirdly, plating the MOF/PVDF coating on the HCF skeleton via electrospinning technique. The voltage, flow rate and tip collector distance were fixed at 10 kV, 0.5 mL h–1 and 10 cm, respectively. A mixture of MOF and polyvinylidene fluoride at a weight ratio
electrodes.
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of 9:1 was pasted on a Cu foil and Li foil to prepare the MOF-Cu and MOF-Li
2.3. Electrochemical measurements
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Electrochemical measurements were carried out by using CR2032-type coin cells
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assembled in an argon-filled glove box. Metal Li foil was used as a counter electrode, Cu, MOF-Cu and MOF-HCF as the working electrode. Celgard microporous polypropylene/polyethylene/polypropylene composite film was used as the separator. An ether electrolyte of 1 M bis(trifluoromethane) sulfonamide lithium salt (LiTFSI) in 1,3-dioxolane (DOL) and dimethoxymethane (DME) containing LiNO3 (1%) was used.
ACCEPTED MANUSCRIPT For the electrochemical cycling performance, the coin cells were tested with a capacity of 2, 4, 6, 8, 10, 20 mA h cm–2 under the current density of 0.5, 1 and 2 mA cm–2. For symmetric batteries, 8 mA h cm–2 of metallic Li were pre-deposited onto
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bare Cu, MOF-Cu and MOF-HCF electrodes to form three Cu@Li, MOF-Cu@Li and MOF-HCF@Li electrodes, respectively, and then the three electrodes were disassembled in a glovebox and reassembled into Li|Li symmetric cells to evaluate the
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cycling stability of the Li anodes. Cyclic voltammetry (CV) measurements were
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performed on an Autolab PG302N with a scan rate of 0.1 mV s–1 in the potential range of -0.2–5 V (vs. Li+/Li). Because ether electrolyte cannot resist the oxidization over 4 V, the electrolyte employed for the electrochemical stability measurement was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate
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(DEC) (1:1:1 in wt%) with 2% VC as the additive. Electrochemical impedance spectra measurement was conducted on the Autolab with frequency range from 100 kHz to 0.01 Hz.
pairing
a
LiFePO4
cathode
with
Cu@Li,
MOF-Cu@Li,
and
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assembled
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To prove its availability of MOF coating in a Li battery, a full cell was
MOF-HCF@Li anodes. Prior to assemble the full cell, Cu, MOF-Cu and MOF-HCF were first pre-deposited 8 mA h cm−2 Li in half cells. The LiFePO4 cathode was prepared by mixing the active material LiFePO4, super carbon, and polyvinlidene fluoride at a weight ratio of 8:1:1. The loading mass of LiFePO4 is ~10-15 mg cm−2. Full cells were tested at a rate of 0.2 and 1 C. 2.4. Structure characterizations
ACCEPTED MANUSCRIPT The SEM (SU-6701, operating at 20 kV), ex situ SEM and EDX elemental mapping (Tecnai F20) were performed to visualize the morphological characteristics, microstructures of the MOF, and the deposition morphology of metallic Li on bare Cu,
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MOF-Cu and MOF-HCF electrodes. Nitrogen adsorption and desorption isotherm at 77.3 K was carried out using a Nova 2000e surface area–pore size analyzer to characterize the pore size distribution of MOF-808 nanoparticles. XRD measurements
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were conducted using a Philips PW3710 with filtered Cu Kα radiation (Rigaku
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D/max-2500, λ=1.5405 A) to explore the MOF and the MOF-Li-Cu structures. AFM was performed with Bruker Multimode 8 with a Nanoscope V controller in an Ar-filled glove box to observe the morphology and the Young's modulus of the MOF coating.
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3. Results and Discussions
A zirconium-based MOF, MOF-808, was prepared and used in this study due to its high chemical and physical stability. As shown in Fig. 2a, MOF-808 is constructed Zr6O4(OH)4(−CO2)6
secondary
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from
building
unit
(SBU)
linked
by
six
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1,3,5-benzenetricarboxylate (BTC) organic linkers to form tetrahedral and adamantane-shaped cages with an internal pore diameter of 4.8 Å and 18 Å, respectively [41], which can be confirmed by the nitrogen adsorption-desorption isotherm test (Fig. S1). The interconnected microporous structure is of interest for use of MOF-808 as “ion sieve” to distribute ions and prevent their accumulation. Besides, the small micropores in the MOF are less than the critical size of stable Li metal dendrite, further decreasing the possibility of dendrite penetration [42]. X-ray
ACCEPTED MANUSCRIPT diffraction (XRD) pattern of the as-prepared MOF-808 agrees well with the simulated one, suggesting the successful formation of target microporous structure with high crystallinity (Fig. 2b). Scanning electron microscope (SEM) demonstrates that the
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prepared MOF-808 particles possess an octahedral shape with an average diameter of 370 nm (Fig. 2c). Investigation of the mechanical property and chemical stability were executed by coating MOF-808 powder crystals on copper foil (defined as
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MOF-Cu). As shown by SEM images in Fig. S2, a continuous and dense MOF
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coating with a thickness of about 22 µm was formed on the Cu foil. Atomic force microscopy (AFM) measurement shows that the MOF coating exhibits a relatively smooth surface (Fig. 2d). For performing the mechanical property measurement, the peak force tapping (PFT) mode of the AFM was used and the value of modulus for
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MOF coating is up to 32 GPa (Fig. 2e), which is more than five times higher than that of previous reported artificial layer (6 GPa), a value that is supposed high enough to suppress Li dendrites growth [43]. Cyclic voltammetry was applied to evaluate the
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electrochemical stability of the MOF coating. As shown in Fig. S3, no cathodic
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oxidation current is observed, indicating the stability of MOF against Li metal. The high electrochemical stability of the MOF coating is essential to reduce side reaction upon cycling, and thus can ensure high Coulombic efficiency and cyclability. The wettability of MOF coating toward molten Li metal and electrolyte was characterized via contact angle measurement. The results displayed in Fig. S4 and S5 show that the MOF coating exhibits poor wetting to molten Li metal, but good wetting to electrolyte,
ACCEPTED MANUSCRIPT indicating the dual function of MOF coating, with excellent Li ion conductivity via electrolyte in MOFs and Li dendrite resistance. To elucidate the effect of MOFs coating on uniformizing Li+ flux and preventing
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the growth of Li dendrites, we firstly investigated the deposition behaviour of metallic Li by separately depositing 2, 4 and 8 mA h cm−2 of Li metal on bare Cu and MOF-Cu at the same current density of 0.5 mA cm−2. After depositing 2 mA h cm−2 of
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Li on bare Cu electrode, the top surface of the as-grown Li exhibits dendritic
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morphology with large amounts of visible filament-shaped Li dendrites (Fig. S6a). The wire-like large dendrites have a diameter of around 1-2 µm and a length of over 100 µm, which might penetrate the separator, inducing short circuit of a working cell. A fluffy Li metal layer with the thickness of 25.3 µm was observed, as shown in the
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cross-sectional SEM images (Fig. 3a). With increasing the deposition areal capacity to 4 and 8 mA h cm−2, the thickness of Li metal layer increases to 42.5 and 73.3 µm (Fig. 3b, c), respectively, which corresponds to a remarkably high electrode volume
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expansion of ~170% and ~290% (Fig. 3h).
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For the MOF-Cu electrodes, the top view SEM image (Fig. S6b) shows no typically ragged growth of wire-like Li deposits. The cross-sectional SEM images shown in Fig. 3d-f clearly display almost all Li deposits are well confined between the MOF coating and Cu foil at various deposition capacity, forming a MOF-Li-Cu sandwich structure. The thus-formed sandwich structure may be interpreted as MOF is of ionic conductivity but electrically insulating nature, which is ideal for the growth of Li from the bottom Cu foil. During further Li deposition, the front of the Li metal
ACCEPTED MANUSCRIPT grows continuously and elevates the MOF coating, resulting in the Li deposits encaged between the protective layer and the Cu foil. This sandwich structure is further certified by XRD (Fig. S7) and elemental mappings through the good matches
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in elemental mappings between Zr, O, and Cu in the composite anode (Fig. S8). Li metal of 2, 4 and 8 mA h cm−2 exhibits a respective thickness of 12.8, 24.9 and 51.3 µm, which is very close to a theoretical Li deposition thickness of 12.4, 24.7 and 49.4
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µm based on the theoretical specific capacity (3860 mA h g−1), and the density of
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metallic Li (0.534 g cm−3), as well as the surface area of Cu foil (Φ10, 0.785 cm−2), and is significantly smaller than the Li metal layer on Cu foil, as clearly shown in Fig. 3g. Even depositing 8 mA h cm−2 of Li metal into MOF-Cu at a higher current density of 1 mA cm−2, a dense structure and an equal thickness for the deposited Li was also
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observed (Fig. S9). In contrast, Li metal on bare Cu demonstrated a porous structure and enlarged thickness (Fig. S10).
Despite considerable improvement in homogenizing Li+ flux and arresting the
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growth of Li dendrites for the MOF-Cu@Li anodes, their poor cycling performance
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and the serious safety issues originated from the relatively electrode volume variation are restricting their practical utilization. Limited by the effective pore volume of Cu foil, the Li deposits sandwiched by the Cu foil and MOF layer still lead to large expansion/shrinkage of the whole electrode during Li plating/stripping process, as shown in Fig. 3h. Therefore, to achieve high-energy Li-metal batteries with enhanced safety, it is imperative to conceive an efficient strategy to construct a desired structure while ensuring long life-cycle for the Li metal anode. 3D porous current collector has
ACCEPTED MANUSCRIPT been considered as a feasible route to alleviate the huge volume changes of Li metal during cycling due to its porous structure, which ensures ample space to accommodate Li, and thus limits the growth of Li dendrites. The SEI on 3D
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nanostructured Li anode shows undesirable durability in the long-term cycling, however, which might lead to the formation of Li dendrites on their surface during prolonged battery operation. Herein, we develop a composite Li-metal anode that
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combines hollow carbon fibers (HCF) as the 3D conducting scaffold with MOF
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coating as a high ion conductivity interface layer (Fig. 4a). Benefit from the synergetic effect of the 3D porous scaffold and MOFs coating, Li deposits are supposed to accommodate well into the framework, free from dendrites and large volume changes. Through applying electrospinning technique, the 3D HCFs were
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coated with a thin MOF layer, generating a 3D MOF-HCF hybrid structure, as displayed in Fig. S11 and S12a. The MOF-HCF electrode size is on average ~122 µm, wherein, the thickness of the MOF coating is around 17 µm (as shown in the inset of
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Fig. 4b). After plating 6, 10, 15 and 20 mA h cm−2 of Li at 1 mA cm−2, since all Li
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deposits are well wrapped into the 3D conducting HCF scaffold, the MOF-HCF@Li composite anodes exhibit a respective electrode thickness of ~124 (Fig. 4c), ~126 (Fig. 4d), ~127 (Fig. S13) and ~127 µm (Fig. 4e), corresponding to only ~1.6%, ~3.3%, ~4.1% and ~4.1% electrode volume change rate (Fig. 4g). No dendritic Li can be observed on the surface of the MOF-HCF electrode (Fig. S12b). On the contrary, due to the superior electrical conductivity of the HCF scaffold, numerous wire-like and filament-like Li grew up on the HCF surface at high areal capacity, resulting from the
ACCEPTED MANUSCRIPT heterogeneous and rough surface of the HCF (Fig. S14). The Li metal can also be stripped reversibly from the 3D MOF-HCF current collector. As shown in Fig. 4f, the 3D MOF-HCF scaffolds remain structurally stable (~123 µm) after Li stripping.
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Further, the thickness of the cycled whole MOF-HCF@Li electrode remained unchanged (~128 µm) and the HCF remained an intact structure, indicating its highly stability (Fig. S15).
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The Li deposition behavior has a remarkable impact on its electrochemical
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performance. Fig. 5a compares the voltage profiles of the three types of symmetrical cells (Cu@Li, MOF-Cu@Li, and MOF-HCF@Li) at 1 mA cm−2 for 1 mA h cm−2. Utilizing the bare Cu foil as the current collector, the Li deposits tend to form long wire-like dendrites, which might facilitate severe electrolyte decomposition and
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formation of “dead Li”, and thus deteriorate the cyclability of the Li anode. This is proved by a necking plot of Cu@Li symmetrical cell. The MOF-Cu@Li symmetrical cell shows a much lower voltage fluctuation and a smaller overpotential,
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demonstrating a uniform Li deposition associated with a stable SEI, causing rapid
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charge transfer kinetics. Because Li deposits are well restricted between the MOF coating and Cu substrates, where they are free from contact with electrolyte and short circuit, the MOF coated electrodes display an enhanced cycle life. However, the cell encounters failure after cycling for approaching 700 h, which mainly attributes to the elevated electrode thickness, aggravating the reaction between Li and electrolyte. In contrast, the MOF-HCF @Li symmetrical cell displays stable voltage profile with a small overpotential of 20 mV and longer cycle-life of ~1050 h. When increasing the
ACCEPTED MANUSCRIPT current density to 2 mA cm−2, the MOF-Cu@Li symmetrical cell behaves worse with a large voltage fluctuation and a relatively higher overpotential of 50 mV as well as a much shorter lifespan of 80 h. On the contrary, the MOF-HCF@Li symmetrical cell
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shows small overpotential of 20 mV for over 500 h (Fig. S16). Being plated/stripped at a higher current density of 3 mA cm−2, the MOF-HCF@Li symmetrical cell shows overpotential of ~100 mV for over 170 h (Fig. S17), indicating its superior rate
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capability of the MOF-HCF@Li anode. To confirm the better cyclic stability of the
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MOF layer, coulombic efficiency of Li plating/stripping on MOF-Cu, bare Cu and MOF-HCF is performed. The Li anode on the MOF-Cu exhibits a significantly improved Coulombic efficiency of 98% over 70 cycles (Fig. S18), compared with that of the Li anode on bare Cu (≈90%). Impressively, with the increase of the areal
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capacity to 8 mA h cm−2, the Li anode on the MOF-HCF could still maintain fairly high and stable Coulombic efficiency of ~ 99% and 98.5% at a higher current density of 1 mA cm−2 and 2 mA cm−2 (Fig. S19 and S20). Short-circuit time is a valuable
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parameter to evaluate the effective dendrite suppression. Thus, Li was continuously
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deposited onto Cu foil, Li foil and MOF-Cu substrates at 0.5 mA cm−2 until short circuit. Results show that MOF coating on Cu foil significantly suppresses or delays dendritic Li growth by delivering a long short-circuit time of over 340 h, which is better than the planar Li and Cu foils (Fig. S21). Interfacial stability and charge transfer behavior can be further explored by the electrochemical impedance spectroscopy (EIS) analysis conducted on symmetric cells prior to and after cycling (Fig. S22). It is noted that the semicircle at the high
ACCEPTED MANUSCRIPT frequency range can be attributed to the interfacial resistance at SEI and the charge-transfer resistance at the electrode surface. The MOF-Li anodes show a slightly higher interfacial resistance (45 Ω) than that of bare Li foils (25 Ω) before
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cycling, which originates from the MOF coating covered on the electrodes. After 5 cycles, the augmentation in the interfacial resistance for the Li foils is a good indicator of the consumption of electrolyte and the formation of the SEI layer. In
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contrast, the MOF-Li anodes show significant decrease in resistances, which is
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possibly due to the stable and uniform SEI layer. In addition, it is found that the resistance of the MOF-Li symmetric cell is lower than that of the bare Li counterpart, which indicates that the MOF coating affords fewer active Li surface and easy access of Li ions across the electrode surface. These facts agree well with the
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electrochemical performance from cycling tests that the MOF interface layer ensured localized Li deposition, more stable SEI layer, and thus a better cyclability. Aiming at further exploring the potential of the designed hybrid structure for
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practical application, full cells were assembled using Cu@Li (8 mA h cm−2),
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MOF-Cu@Li (8 mA h cm−2), and MOF-HCF@Li (8 mA h cm−2) anodes against commercial LiFePO4 (LFP) cathodes (designated as Cu@Li|LFP, MOF-Cu@Li|LFP, and MOF-HCF@Li|LFP respectively). As shown in Fig. 5b, with a high ion-conductivity MOF coating on the anodes, much lower voltage polarization for LFP can be achieved, indicating the remarkably enhanced kinetics. It is worth noting that the MOF-HCF@Li|LFP cell exhibits the optimal cycling stability with ≈91.3% of the reversible capacity retained after 200 cycles (Fig. 5c). Although the Cu@Li|LFP
ACCEPTED MANUSCRIPT cell shows a comparable reversible capacity (138 mA h g−1) when compared with the MOF-Cu@Li|LFP cell counterpart (142 mA h g−1), the Cu@Li|LFP cell presents relative inferior cycling stability with a capacity retention of 78% after 200 cycles
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(Fig. 5c). The mainly reasons for the rapid capacity decay in Cu@Li|LFP cells can be attributed to the depletion of Li metal and electrolyte through continuous SEI and ‘‘dead Li’’ formation during cycling. To satisfy the high energy density requirement of
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a battery, full cells pairing with LFP cathode with high areal capacity of ~ 2.3 mA h
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cm−2 and MOF-HCF@Li anode were assembled. The MOF-HCF@Li|LFP cell demonstrates fairly stable cycling with a high output capacity from 160 mA h g−1 at the initial stage to 149 mA h g−1 after 200 cycles, corresponding to 93% capacity retention (Fig. 5d). Even at a high rate of 1 C, the MOF-HCF@Li|LFP cell can still
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run up to 300 cycles with a capacity retention of 88% (Fig. 5e), indicating high Li utilization of the MOF-HCF@Li anode. 4. Conclusions
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In summary, a stably composite lithium metal anode with negligible electrode
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dimension variation has been realized via integrating a 3D conducting scaffold with a MOF coating. The porous 3D conducting scaffold enables high areal capacity of Li and low electrode volume change. The specially engineered MOF coating layer, with properly tunable pore sizes, weak binding to the current collector, naturally insulating property, and high electrochemical stability, enables Li deposition into the conducting scaffold to decrease the depletion of Li metal and electrolyte. Their very high mechanical modulus (~32 GPa) effectively restricted dendrite propagation and
ACCEPTED MANUSCRIPT realized the densely growth of lithium deposits. Therefore, a Li metal anode with superior electrochemical performance, in terms of negligible volume change, long-term cycling stability, and improved safety is achieved. Benefit from these
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advantages, a full cell configuration in combination of a protected anode with a high areal LiFePO4 cathode demonstrate its practicability by showcasing 200 cycle-life corresponding to a high capacity retention of 93%. By completely localizing the dense
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lithium deposits, the engineered Li anode demonstrates tremendous potential to
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construct safe Li metal batteries through an industry-adoptable technology. Acknowledgements
This work was supported by the Basic Science Center Project of the National Natural Science Foundation of China (Grant no. 51788104), the National Key R&D
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Program of China (Grant no. 2016YFA0202500), the National Natural Science Foundation of China (Grant nos. 21773264, 21805105, 51703052, 21802036), the Fundamental Research Funds for the Central Universities of China (2662017QD028),
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and the Science and Technology Department of Hubei Province (2018FB238,
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2018CFB110).
Conflict of interest
The authors declare no competing financial interest.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version.
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Fig. 1. Schematic illustration of the Li plating/stripping process on MOF-HCF current
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collector.
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Fig. 2. Structural and morphology characterization of MOF. (a) The metal clusters (Zr6O4(OH)4(−CO2)6), organic likers (1,3,5-benzenetricarboxylate), and structure of MOF-808 with internal pore diameter of 4.8 Å (yellow ball) and 18 Å (pink ball). (b) XRD pattern of the MOF-808 particles. (c) SEM image of the MOF-808 particles. (d)
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AFM image of MOF-808 and (e) corresponding Young’s modulus mapping.
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Fig. 3. Morphology of Li-metal anode on Cu and MOF-Cu during plating/stripping.
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Side-SEM image of planar Cu after plating (a) 2 mA h cm−2, (b) 4 mA h cm−2 and (c) 8 mA h cm−2 of Li metal on Cu foil at 0.5 mA cm−2. Side-SEM image of MOF-Cu
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after plating (d) 2 mA h cm−2, (e) 4 mA h cm−2 and (f) 8 mA h cm−2 of Li metal on MOF-Cu foil at 0.5 mA cm−2. (g) The electrode thickness comparison of Cu, MOF-Cu, and theoretical value at different deposition capacities. (h) The volume expansion rate of Cu and MOF-Cu at different deposition capacities.
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Fig. 4. Morphology of Li-metal anode on MOF-HCF during plating/stripping. (a) The model of MOF-HCF@Li anode in cycling. (b) Side-SEM image of pristine MOF-HCF without Li metal. The inset shows the high-magnification image of the
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MOF coating. Side-SEM images of electrode after plating (c) 6 mA h cm−2, (d) 10 mA h cm−2, (e) 20 mA h cm−2 of Li metal into MOF-HCF at 1 mA cm−2. (f) Side-SEM
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image of MOF-HCF electrode after stripping 10 mA h cm−2 from the Li anode. (g)
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Electrode volume change rate of MOF-HCF@Li anode after plating.
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Fig. 5. Electrochemical characterizations of the electrodes for Li plating/stripping. (a)
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Voltage profiles of metallic Li plating/stripping in Cu@Li, MOF-Cu@Li, and MOF-HCF@Li symmetric cells at 1 mA cm−2 for 1 mA h cm−2. (b) The charge/discharge profiles of full cells with LiFePO4 as the cathode and Cu@Li, MOF-Cu@Li, MOF-HCF@Li as the anode at 0.2 C and (c) corresponding cycling performance. Cycling performance of full cells pairing with LFP cathode with high areal capacity of ~ 2.3 mA h cm−2 and MOF-HCF@Li anode: (d) at 0.2 C and (e) at 1 C.
ACCEPTED MANUSCRIPT Research Highlights A low volume change composite lithium metal anode is constructed by encaging Li into a hybrid host. The host, with a dual-layer structure, can alleviate electrode volume variation and
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suppress dendritic Li growth.
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The thus-formed Li anode works stably in both symmetric cells and full cells.
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Zi-jian Zheng is an associate Professor at Hubei University. He obtained his Ph. D in Beijing University of Chemical Technology. His research focuses on designing and fabricating novel materials through computer simulation and experimental tools.
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Qi Su is currently a Postgraduate at Hubei University. Her research interests focus on lithium metal batteries and sodium-ion batteries.
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Qiao Zhang is currently a master student under the supervision of Prof. Zheng-Bang Wang. His research focuses on sodium ion batteries.
Xin-Cheng Hu is currently a Ph.D. candidate under the supervisionof Prof. Rui Wen at Institute of Chemistry, Chinese Academy of Sciences (ICCAS). His research interests focus on in situ AFM study on anode processes in Mg S batteries. Ya-Xia Yin is a Professor at ICCAS. She received her Ph.D. in Beijing University of Chemical Technology. Her research focuses on nanostructured electrode materials for advanced Li-ion and Li-S batteries, and sodium-ion batteries.
Rui Wen received her Ph.D. in Physical Chemistry from ICCAS in 2008. She worked at the WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University in Japan as a Research Assistant from 2008 to 2011. Then she moved to Institute of Physical and Chemical Research (RIKEN) in Japan as a postdoctoral fellow. In 2013 she received
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Humboldt postdoctoral research foundation for working at Kiel University in Germany from 2013 to 2015. In 2015 she joined ICCAS as a Professor of Physical Chemistry. Her research interests focus on interfacial electrochemistry, scanning probe microscopy and it's application in energy storage with batteries.
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Huan Ye is an associate Professor at Huazhong Agricultural University. She received her Ph.D. at Institute of Chemistry, Chinese Academy of Sciences (ICCAS). Her research focuses on advanced nanocomposite cathode materials and lithium metal anode for rechargeable lithium batteries.
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Zheng-Bang Wang is currently a professor at Hubei University. He received his PhD from Karlsruhe Institute of Technology (KIT) in 2015 under supervision of Prof. Christof Wöll. Afterward, he continued working together with Prof. Wöll as a postdoctoral researcher at KIT. In 2017, he moved back to China and was appointed as Chutian Professor at Hubei University. His research interests include metal-organic framework thin films, new style porous polymer thin films, and their applications in the fields of energy and environment.
Yu-Guo Guo is a Professor of Chemistry at ICCAS. He received his Ph.D. in Physical Chemistry from ICCAS in 2004. He worked at the Max Planck Institute for Solid State Research at Stuttgart (Germany) first as a Guest Scientist and then a Staff Scientist from 2004 to 2007. He joined ICCAS as a full professor in 2007. His research focuses on nanostructured energy materials and electrochemical energy storage devices, such as Li-ion, Li-S and solid lithium batteries.
S2211-2855(19)30617-2
DOI:
https://doi.org/10.1016/j.nanoen.2019.103910
Article Number: 103910 Reference:
NANOEN 103910
To appear in:
Nano Energy
Received Date: 29 May 2019 Revised Date:
2 July 2019
Accepted Date: 15 July 2019
Please cite this article as: Z.-J. Zheng, Q. Su, Q. Zhang, X.-C. Hu, Y.-X. Yin, R. Wen, H. Ye, Z.-B. Wang, Y.-G. Guo, Low volume change composite lithium metal anodes, Nano Energy (2019), doi: https:// doi.org/10.1016/j.nanoen.2019.103910. 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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT Low volume change composite lithium metal anodes Zi-Jian Zhenga, Qi Sua, Qiao Zhanga, Xin-Cheng Hub,d, Ya-Xia Yinb,d, Rui Wenb,d, Huan Yeb,c*, Zheng-Bang Wanga*, Yu-Guo Guob,d*
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Keywords: Lithium metal batteries, Lithium metal anodes, Hybrid hosts, Low volume change, Long lifespan
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TOC figure
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A low volume change composite lithium metal anode with dendrite free is constructed by integrating a 3D conducting scaffold with a metal-organic frameworks (MOFs)
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coating.
ACCEPTED MANUSCRIPT Low volume change composite lithium metal anodes Zi-Jian Zhenga, Qi Sua, Qiao Zhanga, Xin-Cheng Hub,d, Ya-Xia Yinb,d, Rui Wenb,d, Huan Yeb,c*, Zheng-Bang Wanga*, Yu-Guo Guob,d* a
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Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China b CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China c College of Science, Huazhong Agricultural University, Wuhan 430070, China d University of Chinese Academy of Sciences, Beijing 100049, China
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* E-mail: [email protected]; [email protected]; [email protected]
Abstract
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The ramified growth of lithium dendrites and their enormous volume change upon cycling bear the primary responsibility for the poor cyclability and serious safety of lithium metal batteries. Herein, a low volume change composite lithium metal anode
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is realized by encaging Li into a hybrid host featuring a 3D conducting scaffold with a
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metal-organic frameworks (MOFs) coating. The scaffold with high pore volume is favorable to accommodate Li with high areal capacity and alleviate the variation in electrode dimensions. The MOFs layer with abundant interconnected micropores serving as “ion sieve” can boost uniform distribution of Li ions while its high Young’s modulus (>32 GPa) can arrest dendrite propagation. The thus-formed anode displays a negligible dimension variation (<5%), enabling stable cycling in both symmetric cells (>1000 h) and full cells (>200 cycle-life). This study may open up new directions to
ACCEPTED MANUSCRIPT construct safe Li metal batteries through an industry-adoptable technology. Keywords: Lithium metal batteries, Lithium metal anodes, Hybrid hosts, Low volume change, Long lifespan
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1. Introduction The ever-increasing demand for portable electronic devices, electrical vehicles, and grid-scale energy storage drives the technological progress in high energy density
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as well as high security rechargeable batteries [1-5]. Among various candidates,
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lithium metal batteries (LMBs) are recognized as one of the most promising electrochemical energy storage devices due to their extremely high theoretical energy density [6-9]. Li metal might be the ultimate option for the anode in a Li battery because of its intrinsic advantages, in terms of high specific capacity (3860 mA h g−1),
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low density (0.534 g cm−3), and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode) [10,11]. The practical application of Li metal anode has long been held back, however, by its severe safety hazards and poor
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cyclability, which mainly roots in the ramified growth of Li dendrites and
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significantly dimensional change upon cycling [12,13]. Targeted at above obsessions of Li metal anodes, tremendous effective strategies
have been employed to improve the electrochemical performance of Li anodes, such as optimizing organic electrolytes [14], creating protective interface layers [15,16], engineering 3D host structures for Li metal [17-23], and developing solid electrolytes [24-28]. The protective interface layers have attracted considerable attention because they are effective in regulating Li deposition, homogenizing the Li-ion flux, reducing
ACCEPTED MANUSCRIPT the local current density, and suppressing the growth of Li dendrites. Until now, various materials, range from pure polymer layers [16], three-dimensional inorganic materials [29-31], to polymer nanocomposite layers [32], are employed as coatings on
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Li/Cu foil surfaces. These coatings are effective to some extent in dendrite formation, especially under static conditions or at the initial stage of cycling, however, they still wear off after prolonged battery operations due to the relatively low modulus and
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poor deformation capacity. Considering the hostless nature of metallic Li and the
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limited pore volume of Cu foil, the Li anode might experience enormous volume expansion/shrinkage during the plating/striping process, bringing severe safety concerns. Therefore, one can firmly deduce that only scrapes a protective interface layer onto the Cu or Li surface is inefficient to address the security of Li anode. A 3D
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Li host that features a high ion conductivity interface layer is probably the most ideal type of anode configuration, which are expected to address the problems of dendritic growth and enormous volume variation.
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Metal organic Frameworks (MOFs) comprising of organic ligands and metal ions,
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a representative of recently developed porous materials, have been developed continuously, with a focus on selective gas separation [33,34], catalyze [35], and energy storage [36,37]. The many extraordinary characteristics of MOF, including structural diversity, strong mechanical strength, tunable porosity, easy access for ion transport, and electrically insulating, make it an appealing candidate as separators and coating in batteries [38,39]. Herein, we construct a smart Li metal anode configuration to suppress dendrite propagation and accommodate huge electrode
ACCEPTED MANUSCRIPT dimension variation on cycling, prepared by decorating a 3D conducting scaffold with a MOFs coating (Fig. 1). The bottom 3D conducting scaffold with a porous structure facilitates the Li deposition with high areal capacity. The upper Zr-based MOF layer
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with uniform microporous channels can serve as “ionic sieves” [40] for Li to distribute incoming Li+ and realize uniform Li+ transport, leading to dendrite-free morphology, wherein their high Young’s modulus (~32 GPa) and naturally insulating
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property help guide the densely growth of Li deposits into the 3D conducting scaffold
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in a given cycling capacity. As a result, a composite Li metal anode with the characteristics of high electrochemical activity, dendrite-free morphology, negligible electrode volume variation (less than 5% for 20 mA h cm−2), and ultralong cycle-life is achieved. Symmetric Li|Li cells can be stably cycled for over 1000 h at a current
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density of 1 mA cm−2, and for over 500 h at a current density of 2 mA cm−2. Furthermore, a 300 cycle-life for a full cell paired with a LiFePO4 cathode with ultrahigh areal capacity (2.3 mA h cm−2) demonstrates its practicability.
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2. Experimental section
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2.1. Synthesis of MOF-808
0.5 mmol Trimesic acid (Aladdin, with a purity of >98%) and 0.5 mmol
ZrOCl2·8H2O (Aldrich, with a purity of >99.5%) were added into N, N-dimethylformamide (DMF)/formic acid (20 mL/20 mL) to form a homogeneous solution. The solution was sealed in a 100 mL Teflon autoclave, which was then heated at 135 °C for 2 days. A white precipitate was collected by filtration and washed three times with 20 mL of fresh DMF. Then the as-obtained MOF-808 was immersed
ACCEPTED MANUSCRIPT into 10 mL of anhydrous DMF for 3 days, during which time the DMF was replaced 3 times per day. Then water- and acetone-exchange process follow the same as the DMF-exchanged process for another 3 days. The acetone-exchanged product was then
2.2. Preparation of MOF-HCF/Cu/Li
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respectively dried at 25 °C and at 150 °C for 24 h to yield activated sample.
Firstly, free-standing hollow carbon fibers (HCF) were prepared by carbonizing
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the commercial cotton. Secondly, a mixture of MOF and polyvinylidene fluoride at a
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weight ratio of 3:1 was dissolved into N-methylpyrrolidone to form slurry. Thirdly, plating the MOF/PVDF coating on the HCF skeleton via electrospinning technique. The voltage, flow rate and tip collector distance were fixed at 10 kV, 0.5 mL h–1 and 10 cm, respectively. A mixture of MOF and polyvinylidene fluoride at a weight ratio
electrodes.
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of 9:1 was pasted on a Cu foil and Li foil to prepare the MOF-Cu and MOF-Li
2.3. Electrochemical measurements
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Electrochemical measurements were carried out by using CR2032-type coin cells
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assembled in an argon-filled glove box. Metal Li foil was used as a counter electrode, Cu, MOF-Cu and MOF-HCF as the working electrode. Celgard microporous polypropylene/polyethylene/polypropylene composite film was used as the separator. An ether electrolyte of 1 M bis(trifluoromethane) sulfonamide lithium salt (LiTFSI) in 1,3-dioxolane (DOL) and dimethoxymethane (DME) containing LiNO3 (1%) was used.
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bare Cu, MOF-Cu and MOF-HCF electrodes to form three Cu@Li, MOF-Cu@Li and MOF-HCF@Li electrodes, respectively, and then the three electrodes were disassembled in a glovebox and reassembled into Li|Li symmetric cells to evaluate the
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cycling stability of the Li anodes. Cyclic voltammetry (CV) measurements were
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performed on an Autolab PG302N with a scan rate of 0.1 mV s–1 in the potential range of -0.2–5 V (vs. Li+/Li). Because ether electrolyte cannot resist the oxidization over 4 V, the electrolyte employed for the electrochemical stability measurement was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate
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(DEC) (1:1:1 in wt%) with 2% VC as the additive. Electrochemical impedance spectra measurement was conducted on the Autolab with frequency range from 100 kHz to 0.01 Hz.
pairing
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To prove its availability of MOF coating in a Li battery, a full cell was
MOF-HCF@Li anodes. Prior to assemble the full cell, Cu, MOF-Cu and MOF-HCF were first pre-deposited 8 mA h cm−2 Li in half cells. The LiFePO4 cathode was prepared by mixing the active material LiFePO4, super carbon, and polyvinlidene fluoride at a weight ratio of 8:1:1. The loading mass of LiFePO4 is ~10-15 mg cm−2. Full cells were tested at a rate of 0.2 and 1 C. 2.4. Structure characterizations
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MOF-Cu and MOF-HCF electrodes. Nitrogen adsorption and desorption isotherm at 77.3 K was carried out using a Nova 2000e surface area–pore size analyzer to characterize the pore size distribution of MOF-808 nanoparticles. XRD measurements
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were conducted using a Philips PW3710 with filtered Cu Kα radiation (Rigaku
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D/max-2500, λ=1.5405 A) to explore the MOF and the MOF-Li-Cu structures. AFM was performed with Bruker Multimode 8 with a Nanoscope V controller in an Ar-filled glove box to observe the morphology and the Young's modulus of the MOF coating.
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3. Results and Discussions
A zirconium-based MOF, MOF-808, was prepared and used in this study due to its high chemical and physical stability. As shown in Fig. 2a, MOF-808 is constructed Zr6O4(OH)4(−CO2)6
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1,3,5-benzenetricarboxylate (BTC) organic linkers to form tetrahedral and adamantane-shaped cages with an internal pore diameter of 4.8 Å and 18 Å, respectively [41], which can be confirmed by the nitrogen adsorption-desorption isotherm test (Fig. S1). The interconnected microporous structure is of interest for use of MOF-808 as “ion sieve” to distribute ions and prevent their accumulation. Besides, the small micropores in the MOF are less than the critical size of stable Li metal dendrite, further decreasing the possibility of dendrite penetration [42]. X-ray
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prepared MOF-808 particles possess an octahedral shape with an average diameter of 370 nm (Fig. 2c). Investigation of the mechanical property and chemical stability were executed by coating MOF-808 powder crystals on copper foil (defined as
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MOF-Cu). As shown by SEM images in Fig. S2, a continuous and dense MOF
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coating with a thickness of about 22 µm was formed on the Cu foil. Atomic force microscopy (AFM) measurement shows that the MOF coating exhibits a relatively smooth surface (Fig. 2d). For performing the mechanical property measurement, the peak force tapping (PFT) mode of the AFM was used and the value of modulus for
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MOF coating is up to 32 GPa (Fig. 2e), which is more than five times higher than that of previous reported artificial layer (6 GPa), a value that is supposed high enough to suppress Li dendrites growth [43]. Cyclic voltammetry was applied to evaluate the
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electrochemical stability of the MOF coating. As shown in Fig. S3, no cathodic
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oxidation current is observed, indicating the stability of MOF against Li metal. The high electrochemical stability of the MOF coating is essential to reduce side reaction upon cycling, and thus can ensure high Coulombic efficiency and cyclability. The wettability of MOF coating toward molten Li metal and electrolyte was characterized via contact angle measurement. The results displayed in Fig. S4 and S5 show that the MOF coating exhibits poor wetting to molten Li metal, but good wetting to electrolyte,
ACCEPTED MANUSCRIPT indicating the dual function of MOF coating, with excellent Li ion conductivity via electrolyte in MOFs and Li dendrite resistance. To elucidate the effect of MOFs coating on uniformizing Li+ flux and preventing
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the growth of Li dendrites, we firstly investigated the deposition behaviour of metallic Li by separately depositing 2, 4 and 8 mA h cm−2 of Li metal on bare Cu and MOF-Cu at the same current density of 0.5 mA cm−2. After depositing 2 mA h cm−2 of
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Li on bare Cu electrode, the top surface of the as-grown Li exhibits dendritic
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morphology with large amounts of visible filament-shaped Li dendrites (Fig. S6a). The wire-like large dendrites have a diameter of around 1-2 µm and a length of over 100 µm, which might penetrate the separator, inducing short circuit of a working cell. A fluffy Li metal layer with the thickness of 25.3 µm was observed, as shown in the
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cross-sectional SEM images (Fig. 3a). With increasing the deposition areal capacity to 4 and 8 mA h cm−2, the thickness of Li metal layer increases to 42.5 and 73.3 µm (Fig. 3b, c), respectively, which corresponds to a remarkably high electrode volume
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expansion of ~170% and ~290% (Fig. 3h).
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For the MOF-Cu electrodes, the top view SEM image (Fig. S6b) shows no typically ragged growth of wire-like Li deposits. The cross-sectional SEM images shown in Fig. 3d-f clearly display almost all Li deposits are well confined between the MOF coating and Cu foil at various deposition capacity, forming a MOF-Li-Cu sandwich structure. The thus-formed sandwich structure may be interpreted as MOF is of ionic conductivity but electrically insulating nature, which is ideal for the growth of Li from the bottom Cu foil. During further Li deposition, the front of the Li metal
ACCEPTED MANUSCRIPT grows continuously and elevates the MOF coating, resulting in the Li deposits encaged between the protective layer and the Cu foil. This sandwich structure is further certified by XRD (Fig. S7) and elemental mappings through the good matches
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in elemental mappings between Zr, O, and Cu in the composite anode (Fig. S8). Li metal of 2, 4 and 8 mA h cm−2 exhibits a respective thickness of 12.8, 24.9 and 51.3 µm, which is very close to a theoretical Li deposition thickness of 12.4, 24.7 and 49.4
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µm based on the theoretical specific capacity (3860 mA h g−1), and the density of
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metallic Li (0.534 g cm−3), as well as the surface area of Cu foil (Φ10, 0.785 cm−2), and is significantly smaller than the Li metal layer on Cu foil, as clearly shown in Fig. 3g. Even depositing 8 mA h cm−2 of Li metal into MOF-Cu at a higher current density of 1 mA cm−2, a dense structure and an equal thickness for the deposited Li was also
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observed (Fig. S9). In contrast, Li metal on bare Cu demonstrated a porous structure and enlarged thickness (Fig. S10).
Despite considerable improvement in homogenizing Li+ flux and arresting the
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growth of Li dendrites for the MOF-Cu@Li anodes, their poor cycling performance
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and the serious safety issues originated from the relatively electrode volume variation are restricting their practical utilization. Limited by the effective pore volume of Cu foil, the Li deposits sandwiched by the Cu foil and MOF layer still lead to large expansion/shrinkage of the whole electrode during Li plating/stripping process, as shown in Fig. 3h. Therefore, to achieve high-energy Li-metal batteries with enhanced safety, it is imperative to conceive an efficient strategy to construct a desired structure while ensuring long life-cycle for the Li metal anode. 3D porous current collector has
ACCEPTED MANUSCRIPT been considered as a feasible route to alleviate the huge volume changes of Li metal during cycling due to its porous structure, which ensures ample space to accommodate Li, and thus limits the growth of Li dendrites. The SEI on 3D
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nanostructured Li anode shows undesirable durability in the long-term cycling, however, which might lead to the formation of Li dendrites on their surface during prolonged battery operation. Herein, we develop a composite Li-metal anode that
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combines hollow carbon fibers (HCF) as the 3D conducting scaffold with MOF
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coating as a high ion conductivity interface layer (Fig. 4a). Benefit from the synergetic effect of the 3D porous scaffold and MOFs coating, Li deposits are supposed to accommodate well into the framework, free from dendrites and large volume changes. Through applying electrospinning technique, the 3D HCFs were
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coated with a thin MOF layer, generating a 3D MOF-HCF hybrid structure, as displayed in Fig. S11 and S12a. The MOF-HCF electrode size is on average ~122 µm, wherein, the thickness of the MOF coating is around 17 µm (as shown in the inset of
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Fig. 4b). After plating 6, 10, 15 and 20 mA h cm−2 of Li at 1 mA cm−2, since all Li
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deposits are well wrapped into the 3D conducting HCF scaffold, the MOF-HCF@Li composite anodes exhibit a respective electrode thickness of ~124 (Fig. 4c), ~126 (Fig. 4d), ~127 (Fig. S13) and ~127 µm (Fig. 4e), corresponding to only ~1.6%, ~3.3%, ~4.1% and ~4.1% electrode volume change rate (Fig. 4g). No dendritic Li can be observed on the surface of the MOF-HCF electrode (Fig. S12b). On the contrary, due to the superior electrical conductivity of the HCF scaffold, numerous wire-like and filament-like Li grew up on the HCF surface at high areal capacity, resulting from the
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Further, the thickness of the cycled whole MOF-HCF@Li electrode remained unchanged (~128 µm) and the HCF remained an intact structure, indicating its highly stability (Fig. S15).
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The Li deposition behavior has a remarkable impact on its electrochemical
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performance. Fig. 5a compares the voltage profiles of the three types of symmetrical cells (Cu@Li, MOF-Cu@Li, and MOF-HCF@Li) at 1 mA cm−2 for 1 mA h cm−2. Utilizing the bare Cu foil as the current collector, the Li deposits tend to form long wire-like dendrites, which might facilitate severe electrolyte decomposition and
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formation of “dead Li”, and thus deteriorate the cyclability of the Li anode. This is proved by a necking plot of Cu@Li symmetrical cell. The MOF-Cu@Li symmetrical cell shows a much lower voltage fluctuation and a smaller overpotential,
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demonstrating a uniform Li deposition associated with a stable SEI, causing rapid
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charge transfer kinetics. Because Li deposits are well restricted between the MOF coating and Cu substrates, where they are free from contact with electrolyte and short circuit, the MOF coated electrodes display an enhanced cycle life. However, the cell encounters failure after cycling for approaching 700 h, which mainly attributes to the elevated electrode thickness, aggravating the reaction between Li and electrolyte. In contrast, the MOF-HCF @Li symmetrical cell displays stable voltage profile with a small overpotential of 20 mV and longer cycle-life of ~1050 h. When increasing the
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shows small overpotential of 20 mV for over 500 h (Fig. S16). Being plated/stripped at a higher current density of 3 mA cm−2, the MOF-HCF@Li symmetrical cell shows overpotential of ~100 mV for over 170 h (Fig. S17), indicating its superior rate
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capability of the MOF-HCF@Li anode. To confirm the better cyclic stability of the
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MOF layer, coulombic efficiency of Li plating/stripping on MOF-Cu, bare Cu and MOF-HCF is performed. The Li anode on the MOF-Cu exhibits a significantly improved Coulombic efficiency of 98% over 70 cycles (Fig. S18), compared with that of the Li anode on bare Cu (≈90%). Impressively, with the increase of the areal
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capacity to 8 mA h cm−2, the Li anode on the MOF-HCF could still maintain fairly high and stable Coulombic efficiency of ~ 99% and 98.5% at a higher current density of 1 mA cm−2 and 2 mA cm−2 (Fig. S19 and S20). Short-circuit time is a valuable
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parameter to evaluate the effective dendrite suppression. Thus, Li was continuously
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deposited onto Cu foil, Li foil and MOF-Cu substrates at 0.5 mA cm−2 until short circuit. Results show that MOF coating on Cu foil significantly suppresses or delays dendritic Li growth by delivering a long short-circuit time of over 340 h, which is better than the planar Li and Cu foils (Fig. S21). Interfacial stability and charge transfer behavior can be further explored by the electrochemical impedance spectroscopy (EIS) analysis conducted on symmetric cells prior to and after cycling (Fig. S22). It is noted that the semicircle at the high
ACCEPTED MANUSCRIPT frequency range can be attributed to the interfacial resistance at SEI and the charge-transfer resistance at the electrode surface. The MOF-Li anodes show a slightly higher interfacial resistance (45 Ω) than that of bare Li foils (25 Ω) before
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cycling, which originates from the MOF coating covered on the electrodes. After 5 cycles, the augmentation in the interfacial resistance for the Li foils is a good indicator of the consumption of electrolyte and the formation of the SEI layer. In
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contrast, the MOF-Li anodes show significant decrease in resistances, which is
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possibly due to the stable and uniform SEI layer. In addition, it is found that the resistance of the MOF-Li symmetric cell is lower than that of the bare Li counterpart, which indicates that the MOF coating affords fewer active Li surface and easy access of Li ions across the electrode surface. These facts agree well with the
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electrochemical performance from cycling tests that the MOF interface layer ensured localized Li deposition, more stable SEI layer, and thus a better cyclability. Aiming at further exploring the potential of the designed hybrid structure for
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practical application, full cells were assembled using Cu@Li (8 mA h cm−2),
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MOF-Cu@Li (8 mA h cm−2), and MOF-HCF@Li (8 mA h cm−2) anodes against commercial LiFePO4 (LFP) cathodes (designated as Cu@Li|LFP, MOF-Cu@Li|LFP, and MOF-HCF@Li|LFP respectively). As shown in Fig. 5b, with a high ion-conductivity MOF coating on the anodes, much lower voltage polarization for LFP can be achieved, indicating the remarkably enhanced kinetics. It is worth noting that the MOF-HCF@Li|LFP cell exhibits the optimal cycling stability with ≈91.3% of the reversible capacity retained after 200 cycles (Fig. 5c). Although the Cu@Li|LFP
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(Fig. 5c). The mainly reasons for the rapid capacity decay in Cu@Li|LFP cells can be attributed to the depletion of Li metal and electrolyte through continuous SEI and ‘‘dead Li’’ formation during cycling. To satisfy the high energy density requirement of
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a battery, full cells pairing with LFP cathode with high areal capacity of ~ 2.3 mA h
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cm−2 and MOF-HCF@Li anode were assembled. The MOF-HCF@Li|LFP cell demonstrates fairly stable cycling with a high output capacity from 160 mA h g−1 at the initial stage to 149 mA h g−1 after 200 cycles, corresponding to 93% capacity retention (Fig. 5d). Even at a high rate of 1 C, the MOF-HCF@Li|LFP cell can still
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run up to 300 cycles with a capacity retention of 88% (Fig. 5e), indicating high Li utilization of the MOF-HCF@Li anode. 4. Conclusions
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In summary, a stably composite lithium metal anode with negligible electrode
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dimension variation has been realized via integrating a 3D conducting scaffold with a MOF coating. The porous 3D conducting scaffold enables high areal capacity of Li and low electrode volume change. The specially engineered MOF coating layer, with properly tunable pore sizes, weak binding to the current collector, naturally insulating property, and high electrochemical stability, enables Li deposition into the conducting scaffold to decrease the depletion of Li metal and electrolyte. Their very high mechanical modulus (~32 GPa) effectively restricted dendrite propagation and
ACCEPTED MANUSCRIPT realized the densely growth of lithium deposits. Therefore, a Li metal anode with superior electrochemical performance, in terms of negligible volume change, long-term cycling stability, and improved safety is achieved. Benefit from these
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advantages, a full cell configuration in combination of a protected anode with a high areal LiFePO4 cathode demonstrate its practicability by showcasing 200 cycle-life corresponding to a high capacity retention of 93%. By completely localizing the dense
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lithium deposits, the engineered Li anode demonstrates tremendous potential to
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construct safe Li metal batteries through an industry-adoptable technology. Acknowledgements
This work was supported by the Basic Science Center Project of the National Natural Science Foundation of China (Grant no. 51788104), the National Key R&D
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Program of China (Grant no. 2016YFA0202500), the National Natural Science Foundation of China (Grant nos. 21773264, 21805105, 51703052, 21802036), the Fundamental Research Funds for the Central Universities of China (2662017QD028),
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and the Science and Technology Department of Hubei Province (2018FB238,
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2018CFB110).
Conflict of interest
The authors declare no competing financial interest.
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version.
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Fig. 1. Schematic illustration of the Li plating/stripping process on MOF-HCF current
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collector.
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Fig. 2. Structural and morphology characterization of MOF. (a) The metal clusters (Zr6O4(OH)4(−CO2)6), organic likers (1,3,5-benzenetricarboxylate), and structure of MOF-808 with internal pore diameter of 4.8 Å (yellow ball) and 18 Å (pink ball). (b) XRD pattern of the MOF-808 particles. (c) SEM image of the MOF-808 particles. (d)
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AFM image of MOF-808 and (e) corresponding Young’s modulus mapping.
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Fig. 3. Morphology of Li-metal anode on Cu and MOF-Cu during plating/stripping.
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Side-SEM image of planar Cu after plating (a) 2 mA h cm−2, (b) 4 mA h cm−2 and (c) 8 mA h cm−2 of Li metal on Cu foil at 0.5 mA cm−2. Side-SEM image of MOF-Cu
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after plating (d) 2 mA h cm−2, (e) 4 mA h cm−2 and (f) 8 mA h cm−2 of Li metal on MOF-Cu foil at 0.5 mA cm−2. (g) The electrode thickness comparison of Cu, MOF-Cu, and theoretical value at different deposition capacities. (h) The volume expansion rate of Cu and MOF-Cu at different deposition capacities.
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Fig. 4. Morphology of Li-metal anode on MOF-HCF during plating/stripping. (a) The model of MOF-HCF@Li anode in cycling. (b) Side-SEM image of pristine MOF-HCF without Li metal. The inset shows the high-magnification image of the
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MOF coating. Side-SEM images of electrode after plating (c) 6 mA h cm−2, (d) 10 mA h cm−2, (e) 20 mA h cm−2 of Li metal into MOF-HCF at 1 mA cm−2. (f) Side-SEM
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image of MOF-HCF electrode after stripping 10 mA h cm−2 from the Li anode. (g)
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Electrode volume change rate of MOF-HCF@Li anode after plating.
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Fig. 5. Electrochemical characterizations of the electrodes for Li plating/stripping. (a)
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Voltage profiles of metallic Li plating/stripping in Cu@Li, MOF-Cu@Li, and MOF-HCF@Li symmetric cells at 1 mA cm−2 for 1 mA h cm−2. (b) The charge/discharge profiles of full cells with LiFePO4 as the cathode and Cu@Li, MOF-Cu@Li, MOF-HCF@Li as the anode at 0.2 C and (c) corresponding cycling performance. Cycling performance of full cells pairing with LFP cathode with high areal capacity of ~ 2.3 mA h cm−2 and MOF-HCF@Li anode: (d) at 0.2 C and (e) at 1 C.
ACCEPTED MANUSCRIPT Research Highlights A low volume change composite lithium metal anode is constructed by encaging Li into a hybrid host. The host, with a dual-layer structure, can alleviate electrode volume variation and
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suppress dendritic Li growth.
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The thus-formed Li anode works stably in both symmetric cells and full cells.
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Zi-jian Zheng is an associate Professor at Hubei University. He obtained his Ph. D in Beijing University of Chemical Technology. His research focuses on designing and fabricating novel materials through computer simulation and experimental tools.
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Qi Su is currently a Postgraduate at Hubei University. Her research interests focus on lithium metal batteries and sodium-ion batteries.
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Qiao Zhang is currently a master student under the supervision of Prof. Zheng-Bang Wang. His research focuses on sodium ion batteries.
Xin-Cheng Hu is currently a Ph.D. candidate under the supervisionof Prof. Rui Wen at Institute of Chemistry, Chinese Academy of Sciences (ICCAS). His research interests focus on in situ AFM study on anode processes in Mg S batteries. Ya-Xia Yin is a Professor at ICCAS. She received her Ph.D. in Beijing University of Chemical Technology. Her research focuses on nanostructured electrode materials for advanced Li-ion and Li-S batteries, and sodium-ion batteries.
Rui Wen received her Ph.D. in Physical Chemistry from ICCAS in 2008. She worked at the WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University in Japan as a Research Assistant from 2008 to 2011. Then she moved to Institute of Physical and Chemical Research (RIKEN) in Japan as a postdoctoral fellow. In 2013 she received
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Humboldt postdoctoral research foundation for working at Kiel University in Germany from 2013 to 2015. In 2015 she joined ICCAS as a Professor of Physical Chemistry. Her research interests focus on interfacial electrochemistry, scanning probe microscopy and it's application in energy storage with batteries.
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Huan Ye is an associate Professor at Huazhong Agricultural University. She received her Ph.D. at Institute of Chemistry, Chinese Academy of Sciences (ICCAS). Her research focuses on advanced nanocomposite cathode materials and lithium metal anode for rechargeable lithium batteries.
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Zheng-Bang Wang is currently a professor at Hubei University. He received his PhD from Karlsruhe Institute of Technology (KIT) in 2015 under supervision of Prof. Christof Wöll. Afterward, he continued working together with Prof. Wöll as a postdoctoral researcher at KIT. In 2017, he moved back to China and was appointed as Chutian Professor at Hubei University. His research interests include metal-organic framework thin films, new style porous polymer thin films, and their applications in the fields of energy and environment.
Yu-Guo Guo is a Professor of Chemistry at ICCAS. He received his Ph.D. in Physical Chemistry from ICCAS in 2004. He worked at the Max Planck Institute for Solid State Research at Stuttgart (Germany) first as a Guest Scientist and then a Staff Scientist from 2004 to 2007. He joined ICCAS as a full professor in 2007. His research focuses on nanostructured energy materials and electrochemical energy storage devices, such as Li-ion, Li-S and solid lithium batteries.