Si anode for next-generation lithium-ion battery

Journal Pre-proof Si Anode for Next Generation Lithium-ion Battery Wen-Feng Ren, Yao Zhou, Jun-Tao Li, Ling Huang, Shi-Gang Sun PII:

S2451-9103(19)30087-0

DOI:

https://doi.org/10.1016/j.coelec.2019.09.006

Reference:

COELEC 457

To appear in:

Current Opinion in Electrochemistry

Received Date: 5 July 2019 Revised Date:

3 September 2019

Accepted Date: 16 September 2019

Please cite this article as: Ren W-F, Zhou Y, Li J-T, Huang L, Sun S-G, Si Anode for Next Generation Lithium-ion Battery, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2019.09.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Si Anode for Next Generation Lithium-ion Battery

Wen-Feng Ren1, Yao Zhou2, Jun-Tao Li*2, Ling Huang1, and Shi-Gang Sun*1,2

1

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative

Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China 2

College of Energy, Xiamen University, Xiamen 361005, China

∗Corresponding

author: [email protected]; [email protected]

Abstract Si-based materials with high theoretical storage capacity and low working potential are one of the ideal anode materials for next generation lithium ion batteries, but their large volume change and low conductivity obstruct the commercial application. This article presents a brief overview of insights of charge-discharge mechanism and the main challenges of Si-based anodes in the past few years, and outlines typical solving strategies, new mechanism, advanced characterization technology and future directions.

Introduction Lithium-ion batteries are considered to be an ideal energy storage device owing to their great merits of high energy density, long cycle life, low self-discharge rate, wide working potential, thus extensively applied in portable electronic devices, large-scale energy storage, and electric vehicles[1]. However, traditional lithium ion batteries cannot meet the high energy density demands from the current applications. Therefore, developing high energy density electrode materials is imperative for the next generation lithium-ion batteries. 1

Silicon (Si) is a kind of very promising anode material for lithium-ion batteries as Si possesses a theoretical specific capacity almost tenfold of that of the conventional graphite material[2,3]. However, its commercial application is hindered by the poor cycling stability due to large volume change of Si (>300%) during repeated Li-Si alloying/de-alloying processes. In the current article, we review recent insights of Si-based anodes for lithium ion batteries. We first illuminate the charge-discharge mechanism and the main challenges of Si-based anodes. We then focus on the typical solving strategies to improve the electrochemical performance of Si-based anodes. At last, we review briefly new mechanism and advanced characterization technology in the past few years and outlines future directions.

Charge-discharge mechanism and challenges of Si-based anodes The bare Si powders (10 µm) as lithium ion battery anodes (Figure 1A) exhibited flat lithiation/delithiation voltage plateau (0.1-0.2 V / 0.4-0.5 V), large irreversible capacity (2090 mAh g-1) in the first cycle and poor cycling performance (only 10 cycles)[4]. Besides, in-situ XRD confirmed that crystalline Si (c-Si) is transformed to the amorphous Si (a-Si) and c-Li15Si4 (~60 mV) during the first lithiation, as well as the following cycles. Such transformation corresponds to the phase change between a-LixSi and a-LizSi + c-Li15Si4, as shown in Figure 1B[5]. The experimental and theoretical volume variation ((VLi-Si-VSi)/VSi) verified that the volume change of Si columns presents a nearly linear relationship at different lithiated states (Figure 1C), indicating that a fully lithium insertion leads to 300% volume expansion[6]. The large volume variation causes cracking and pulverization of the Si active materials, confirmed by in-situ TEM analysis (Figure 1D)[7]. The large volume variation of Si (~300%) during repeated Li-Si alloying/dealloying processes would cause the following problems (Figure 1E): (1) Gradual loss of electric contact for Si with conductive network and current collector; (2) Continuous formation of solid electrolyte interphase (SEI) layer, which results in large irreversible capacity and poor cycling performance; (3) Cracking and pulverization of Si active materials[8]. 2

3

Figure 1

(A) Galvanostatic charge-discharge voltage profiles of Si powders (10 µm)[4]. (B) Phase diagram during the charge-discharge cycling of a Li/Si electrochemical cell[5]. (C) Volume variation of crystalline Li-Si alloys during lithiation/delithiation processes[6]. (D) In-situ TEM images of Si nanoparticles (620 nm) during chemical lithiation in one minute[7]. (E) Three representative failure mechanisms of Si-based anodes: pulverization, delamination, and unstable SEI layer[8]. (F) Schematic illustration of Si nanostructures and morphology change before and after cycling[3]. (G) Classification of polymers in terms of polymer architecture[8]. (H) Possible molecular fragments observed in the FEC/VC decomposition products[9]. (I) Schematic illustration of conductive additives used in Si-based anodes.

Typical solving strategies 1. Active materials of Si-based anodes (1) Particle size and morphology control Nanostructured Si materials with particle size below 150 nm can hold entirety of 4

deformation stress derived from the large volume change of Si during alloying and dealloying processes. Besides, the void space within porous Si can also accommodate the large volume change. Therefore, various nanostructured Si (Figure 1F)[3], including 0D Si nanoparticles, 1D Si nanowires, nanorods and nanotubes, 2D thin films, and 3D porous structure, are designed to enhance the cycling performance of Si-based anodes[10-13]. (2) Composites The composite matrix with high mechanical strength and high conductivity can alleviate deformation stress and increase the electronic conductivity of the Si-based electrode, thereby preventing the capacity from fading. In addition, the composite structure can significantly reduce the contact area between active materials and electrolyte, which promotes the formation of stable SEI layer. Various matrix, including graphitic and amorphous carbon[14-16], metal and oxides[17,18], and other conductive materials (MXene)[19,20], are developed for the improvement of electrochemical performance of Si-based anodes. Besides, the organic functional groups modification on Si surface can bond with the binder, which assists the formation of stable interface between electrode and electrolyte and reinforces the electrode integrity to accommodate the volume changes of Si[21]. Furthermore, in comparison with c-Si, the SiOx has low volume change during the cycling processes and exhibits relatively stable cycling performance. The reaction mechanism of SiOx has been unraveled via in-situ solid-state NMR spectroscopy[22], but the large initial irreversible capacity in 1st cycle originated from the formation of Li2O and lithium silicates hinders its wide application. Among the reported composite strategies, graphite-Si composites are considered as the most promising anode materials for commercialization. In addition to the high capacity provided by Si, the graphite in the composite electrodes can act as active materials to provide extra capacity; the graphite can also serve as the conductive medium to improve the electrical conductivity of the whole electrode, and as the buffer matrix to alleviate the volume change of Si as the void framework among 5

graphite particles could provide space for Si swelling[23]. Ma et al. prepared macropore-coordinated graphite-silicon composite which exhibited high specific capacity (527 mAh g-1) and initial coulombic efficiency (93%) in half-cell[24]. Importantly, a variety of technological parameters including the size of Si and graphite, the mass ratio of Si and graphite, the distribution of Si, the void within the electrodes and the preparation method can influence the electrochemical performance of graphite-Si composite anodes and need to be controlled reasonably. (3) Structure design Structure designs of Si-based active materials are based on incorporation and utilization of voids within composite structure, which can efficiently relieve the deformable stress from a dramatic and substantial volumetric expansion. Moreover, it can prevent also the formation of electrical isolation and maintain consequently the integrity of electrode. Recently, solid Si core with hollow shell[25], porous shell[14], and solid shell[14], porous Si core with solid shell[26], nanostructured Si embedded in porous and flaky matrix[27] are designed and investigated. In addition, prelithiation (chemical lithiation and electrochemical lithiation) of Si-based active materials or Si-based electrode disk has been often used to reduce the consumption of Li ion in initial cycle from SEI formation and irreversible reactions.

2. Other components of Si-based anodes (1) Binder Binder plays a crucial role in the cohesion of active materials and conductive additives together onto current collector. PVDF binder with weak Van der Waals force cannot maintain of electrode integrity of Si-based anodes. Various binders have been developed for the improvement of the cycling stability of Si-based anodes, as summarized in Figure 1G[8]. We have reviewed water soluble binders for high energy density electrode materials[28]. Besides, a series of nature gum[29], hydrogel alginate (M-alg, M = Al, Ba, Mn, Zn)[30], ion and electron conducting polymer[31], and interweaving 3D network[32] binders have been reported. Basically, the optimal binder 6

for Si-based anodes should possess the following merits: high chemical and electrochemical stability, suitable adhesiveness, strong ion/electron conductivity, functional group for self-healing, cost effectiveness and environmental friendliness. (2) Electrolyte additives Apart from the above-mentioned strategies, robust and stable SEI films are essential for achieving excellent electrochemical performance for Si-based anodes. Large volume variation breaks SEI layer formed on Si surface and causes the continuous formation of SEI layer. Morphology and composition of the SEI layer are greatly influenced by the components of electrolyte, which can be optimized through selecting appropriate electrolyte additives and electrolyte salts to replace the commonly used carbonate-based electrolytes. Among these methods, improving electrolyte additives is regarded as an effective route to promote the formation of stable SEI layer, thereby enhancing capacity retention. Electrolyte additives, including vinylene carbonate (VC) and fluoroethylene carbonate (FEC), are often used to address cycling stability and coulombic efficiency problems of Si-based anodes, because these electrolyte additives undergo reductive reaction on the surface of Si-based active materials ahead of the reductive deposition of ethylene carbonate (EC). Studies of solution and solid-state NMR techniques have confirmed that VC and FEC additives can contribute to the formation of cross-linked PEO-type polymers and some of the organic SEI components can covalently bond to Si surface (Figure 1H), thereby enhancing cycling stability and improving capacity retention[9,33]. Therefore, an optimal electrolyte additive for Si-based anodes should possess higher reductive potential than the conventional solvent, high chemical compatibility with other electrolyte components, and good film forming characteristics, which thus supports the formation of robust and stable SEI films. (3) Conductive additives Conductive additives are one of the essential elements in lithium ion battery electrode. Many types of conductive carbons (Figure 1I), such as carbon black nanoparticles, carbon nanotubes, carbon nanowires, and graphene nanosheets. In 7

addition, construction of 3D conductive framework network within Si-based anodes is effective for improving the electron transportation kinetics, as well as prevents the nanostructured active materials from agglomeration within Si-based anodes. Moreover, the deformable stress derived from volume changes of Si-based materials can be efficiently accommodated using stable and flexible conductive additives, thus maintaining the integrity of the whole Si-based anodes. Thus, the optimal conductive additives for Si-based anodes should possess the advantages of excellent electrical conductivity, high chemical and electrochemical stability, high strength, good flexibility and low cost.

3. Architecture design of Si-based anodes Although high reversible capacity and long cycling life of Si-based anodes could be obtained through optimizing electrode components, the low areal mass loading and areal capacity should be addressed for their commercial application. These issues can be solved through structural design of 2D/3D network active materials, 3D conductive framework, and 3D current collector. Firstly, designing 2D/3D structure of active materials with binder free on Cu current collector can promote the electron and ion transportation in the whole electrode, strengthen the contact between the Si particles and Cu current collector, and avoid the individually detachment of Si particles from current collector. As examples, a Si/C multilayer film electrode (Figure 2A) with well-controlled Si-C interfaces via a magnetron sputtering method[34] and an electrode that had a network of interconnected carbon-coated porous Si nanowires as a binder-free anode (Figure 2B)[35] were reported. Secondly, constructing 3D conductive framework within Si-based electrode disks can increase mass loading density, enhance mechanical property and electron migration, retard structural degradation during alloying/dealloying processes. Recently, Batmaz et al. designed 3D conductive matrixed Si-based anodes (Figure 2C) using fluid-induced fracture and in-situ thermolysis of polymer binder[36]. We have 8

developed a robust Si nanoparticles@conductive carbon framework@polymer composite electrodes (Figure 2D)[37]. Li et al. reported an in-situ construction of 3D all-carbon conductive and mechanical networks through the growth of ultrathin graphdiyne nanosheets on Si-covered Cu foils, as shown in Figure 2E[38]. Thirdly, 3D current collector has been developed to support and bind Si nanoparticles, increase Si loading, improve electron migration, and relieve structural degradation during lithiation/delithiation processes. Shang et al. realized the formation of the mechanical and conductive graphdiyne networks on Si-covered 3D Cu nanowires current collector, as shown in Figure 2F[39]. The representative demonstration was reported by Harpak et al. to fabricate large-scale self-catalyzed spongelike Si nanostructure networks on 3D nanoporous stainless steel fibers surfaces (Figure 2G) at low growth temperatures[40]. Last but not the least, appropriate positive electrodes for lithium ion full cells with Si-based anodes are also important and have been extensively investigated, indicating Li(Ni0.6Mn0.2Co0.2)O2 could stabilize the Si alloy in the negative electrode via a CO2 release mechanism[41], since Si alloy reacted with CO2 during cycling and CO2 could be used as a powerful additive leading to an effective SEI layer on Si alloys[42].

9

Figure 2

(A) Schematic illustration of Si/C multilayer blocks formed by stress[34]. (B) Schematic illustration and SEM image of the interconnected carbon coated porous Si nanowires

[35]

. (C) The electrode fabrication process of Si microparticles and their

SEM images before and after fluid-induced fracture[36]. (D) Si@CNF@polymer composite electrode structures during lithiation/delithiation processes and its pristine SEM image[37]. (E) Preparation of Si-covered Cu foils coated by graphdiyne nanosheets and its SEM image[38]. (F) Preparation of Si-covered 3D Cu nanowires coated by graphdiyne networks and its SEM image[39]. (G) Preparation of spongelike Si nanostructure networks on 3D nanoporous stainless steel fibers and its SEM image[40].

New mechanism and advanced characterization technology (1) Morphological changes and lithiation mechanism of porous Si Porous Si materials have been demonstrated as one of the most promising anode materials due to their excellent electrochemical performance and the wide investigation of their performance enhancement mechanism. Firstly, microscopic morphological changes of carbon-coated porous Si (Si/C) composites (Figure 3A) during lithiation and delithiation are successfully recorded through in-situ nanobattery configuration and TEM instrument, which reveals that the self-volume inward expansion mechanism of porous Si/C effectively mitigates electrochemically-induced 10

mechanical degradation of the porous Si/C electrode during cycling[43]. Secondly, the critical fracture diameter of porous Si particles reaches up to 1.52 µm through in-situ TEM observation, larger than that of c-Si (150 nm) and a-Si (870 nm)[44]. Thirdly, after full lithiation, nanostructured porous Si transforms to amorphous LixSi phase without formation of crystalline Li15Si4 phase (Figure 3B) which leads to large volume change and deteriorate cycling performance. Such transformation was confirmed by in-situ TEM nanobattery, first-principle molecular dynamic simulation, and in-situ solid-state 7Li NMR spectroscopy[44,45]. At last, the spatial ratio and location of the space for alleviating volume expansion (SAV) and external pressure of Si-based with porous structure for accommodating volume expansion are investigated via a combined electrochemical and computational methods (Figure 3C), revealing that locally distributed void space could effectively alleviate the volume expansion pressure during Li insertion process, thus preventing Si pulverization derived from the space for alleviating volume expansion[46]. (2) Structural control and lithiation/delithiation mechanism of Graphite-Si composites Graphite-Si composites are considered as the most feasible alternatives for next-generation anodes, due to their high gravimetric capacity, superior cyclability, and high tap density. On the one hand, the Si-blocking in mesopores of graphite leads to the continuous formation of SEI layer, the cracking of graphite flake, and the fracture of Si, resulting from the high deformable stress, while Si-layer in internal macropores of graphite exhibit low stress, thin SEI layer, and good electrical contact. Therefore, the macropore-coordinated graphite-Si (MGS) hybrid (Figure 3D) without Si-layers located on the mesopores because of carbon pre-filling (carbon-blocking) demonstrates a minimized electrode swelling ratio and excellent cycling stability comparable to that of conventional graphite[24]. On the other hand, lithiation and delithiation of graphite and Si component in graphite-Si composites electrode during electrochemical cycling can be directly observed via operando energy dispersive X-ray diffraction technique in 2032-type coin cells (Figure 3E), which can help to optimize the graphite-Si ratio in blended electrodes and to improve the cycling 11

performance and long-term stability by avoiding the excessive volume expansion of Si particles[47].

Figure 3

(A) In-situ TEM nanobattery and images during different lithiation times of carbon-coated porous Si[43]. (B) In-situ TEM nanobattery, lithiation manners of ball-milled Si and porous Si nanoparticles, and SAED patterns of porous Si particles[44]. (C) The charging and discharging processes of various Si materials and the corresponding properties and SEM images[46]. (D) Schematic model of Si-layer-coated G and MGS, cross-sectional schematic illustration and SEM image of MGS[24]. (E) Operando energy dispersive X-ray diffraction, and capacity and specific capacity of the graphite and Si components during lithiation and delithiation processes[47].

(3) Formation mechanism of SEI layer on Si-based anodes SEI layer is a passivation layer naturally formed on electrode surface, which avoids further reaction between electrodes and electrolytes, while its formation mechanism and component are not well-understood yet. Recently, the formation processes of inorganic SEI layers on native oxide-terminated Si wafer anodes were investigated via in-situ synchrotron X-ray reflectivity, linear sweep voltammetry, ex-situ X-ray 12

photoelectron spectroscopy, and first principles calculations, and proposed two well-defined inorganic SEI layers next to the Si anode: a bottom-SEI layer (adjacent to the electrode) formed via lithiation of the native oxide; a top-SEI layer mainly consisting of the electrolyte decomposition product LiF, as shown in Figure 4A[48]. Besides, a non-destructive operando pressure measurements technique (Figure 4B) for physically detecting the growth of SEI films during cycling of Si-containing lithium ion pouch cells was established for a correlation between irreversible volume expansion and capacity loss caused by a continually thickening SEI[49]. (4) Failure mechanism and optimal surface oxide layer of Si-based anodes Although microscopic morphological changes of single Si particle can be observed by in-situ TEM nanobattery, study of electrode structure change of real battery system is required for unraveling the failure mechanism of Si-based anodes. Recently, synchrotron X-ray nano-tomography (Figure 4C) was used to reveal the failure mechanism and the morphological evolution in different capacity cycling modes in nanoporous Si-based anodes, demonstrating that the failure mechanism of the nanoporous Si electrodes results from a mesoscopic to macroscopic deformation and that the shorter cycling life in higher-capacity cycling mode stems from particle agglomeration[50]. In addition, the presence of surface oxide layer on Si nanoparticles is inevitable because of its high activity in air. Experiment and chemomechanical modeling confirmed an optimal surface oxide layer of about 5 nm thick on Si@SiOx/C nanocomposite (Figure 4D), which exhibited a combination of high capacity and cycle stability[51].

13

Figure 4

(A) Experimental setup for a multi-modal approach and schematic illustration of the potential-dependent SEI growth mechanism[48]. (B) Experimental setup for operando pressure measurements, capacity and average discharge pressure versus cycle number[49]. (C) Experimental setup for X-ray nano-tomography and the morphological evolution of the nanoporous-Si electrodes at different cycles in constant capacity mode[50]. (D) Thickness of the oxide layers under different oxidation conditions and an optimal thickness regime (indicated in green)[51].

Concluding remarks Si-based materials with high theoretical specific capacity are promising anode of next generation lithium ion batteries for satisfying the demand in high energy density of fast development of electric vehicles, large scale energy storage, artificial intelligence and 5G technology. In this article, we review the progresses in improvements of the coulombic efficiency and cycling stability of Si-based anodes through the control and design of Si-based active materials, the optimization of other electrode components (binder, electrolyte additives, and conductive additives), as well as fabrication of architecture of Si-based anodes. In addition, investigation of the electrochemical mechanism of Si-based anodes via advanced characterization technology is overviewed. 14

Future directions towards the realization of practical application for Si-based anodes may focus on the following aspects: (1) Design of architectural Si-based anodes can

incorporate the advantage of each components of Si-based anodes, and

significantly alleviate the volume change during the alloying/de-alloying processes; (2) Modification of Si active materials by surface functional group and exploring new binder, conductive additives and current collector can improve the bonding interaction during the discharge/charge processes which benefits to maintain the structural integrality for Si-based anodes; (3) Construction of stable and robust SEI layers can increase the coulombic efficiency and avoid the formation of electrical isolation within electrode through the surface physical barrier and chemical reaction between Si active materials and electrolyte. (4) Development of advanced in-situ time- and space-resolution characterization technologies for real battery system can help understanding the electrochemical and material chemistry mechanisms of Si-based anodes during the lithiation/delithiation processes. It is expected that the continuous development of these key aspects will accelerate practical application of Si-based anodes in the next generation lithium ion batteries.

Acknowledgments This work is supported by grants from China Postdoctoral Science Foundation (2018M632575), National Natural Science Foundation of China (21621091, 21875197),

and

National

Key

Research

and

Development

of

China

(2016YFB0100202).

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https://onlinelibrary.wiley.com/doi/full/10.1002/aenm.201801718 ·An optimal surface oxide layer on Si surface was proposed by experiment and theoretical calculation.

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Highlights: (1) Charge-discharge mechanism and technical barriers of Si-based anodes are illuminated. (2) Typical solving strategies for improving Si-based anodes performance are presented. (3) New mechanism and advanced characterization technology are detailed introduced. (4) The perspectives and suggestions of Si-based anodes for future study are proposed.

Conflict of Interest: There is no conflict of interest.