Research Progress on the Structural Design and Optimization of Silicon Anodes for Lithium-Ion Batteries: A Mini-Review

Yu, Zhi; Cui, Lijiang; Zhong, Bo; Qu, Guoxing

doi:10.3390/coatings13091502

Open AccessReview

Research Progress on the Structural Design and Optimization of Silicon Anodes for Lithium-Ion Batteries: A Mini-Review

¹

School of Physics and Materials Science, Nanchang University, Nanchang 330031, China

²

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China

^*

Author to whom correspondence should be addressed.

Coatings 2023, 13(9), 1502; https://doi.org/10.3390/coatings13091502

Submission received: 30 June 2023 / Revised: 31 July 2023 / Accepted: 21 August 2023 / Published: 25 August 2023

(This article belongs to the Special Issue Advanced Electrode Coatings for Energy Conversion and Storage)

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Abstract

:

Silicon anodes have been considered one of the most promising anode candidates for the next generation of high-energy density lithium-ion batteries due to the high theoretical specific capacity (4200 mAh g⁻¹) of Si. However, high lithiation capacity endows silicon anodes with severe volume expansion effects during the charge/discharge cycling. The repeated volume expansions not only lead to the pulverization of silicon particles and the separation of electrode materials from the current collector, but also bring rupture/formation of solid electrolyte interface (SEI) and continuous electrolyte consumption, which seriously hinders the commercial application of silicon anodes. Structural design and optimization are the key to improving the electrochemical performances of silicon anodes, which has attracted wide attention and research in recent years. This paper mainly summarizes and compares the latest research progress for the structural design and optimization of silicon anodes.

Keywords:

lithium-ion batteries; silicon anode; structural optimization; surface structural; artificial SEI

1. Introduction

With information and communication technology upgrading rapidly, digital products are developing in an increasingly convenient direction, and the demand for the performance of energy storage devices is increasing. Lithium-ion batteries occupy a wide electronics market due to their advantages of high energy density, high output voltage, low self-discharge, no memory effect, long cycle life, and environmental friendliness [1]. Nevertheless, with the popularization and rapid development of energy-intensive mobile electronics, such as electric vehicles, smartphones, and drones, the energy density of traditional lithium-ion batteries is no longer meeting the requirement of long endurance [2].

The specific capacity of electrode materials is a crucial factor in determining the energy density of batteries. Currently, the commercial anode for lithium-ion batteries is generally graphite with a theoretical specific capacity of 372 mAh g⁻¹. The practical specific capacity of the graphite anode in the market has been pushed to 365 mAh g⁻¹, approaching its theoretical limit [3], while people’s requirements for the energy density of lithium-ion batteries are continuously increasing. It is imperative to develop new anode materials with higher specific capacities. As an environmentally friendly material, Si has an ultrahigh theoretical specific capacity of 4200 mAh g⁻¹, which is 10 times that of graphite. With the addition of the advantage of high earth abundance, Si has been regarded as one of the most promising anode materials for next-generation lithium-ion batteries [4].

However, the commercial application of silicon anodes is still hindered by its serious volume expansion. Square cells and soft pack cells are extremely sensitive to expansion, so these two types of batteries are difficult to apply to silicon-based materials. At present, the commercial silicon anode is mainly used in large cylinder batteries, which have high strength and can adapt to expansion and release more capacity. The repeated huge volume changes during cycling not only give rise to the pulverization of silicon particles and the separation of electrode materials from the current collector, but also lead to the rupture of the surface SEI, which results in continuous electrolyte consumption and a thickening of the SEI film. Significant efforts have been made to overcome these shortcomings, and various structural design and optimization strategies have been put forward. Silicon anodes have attracted extensive research interest in recent years. Kong et al. [5] and Zhang et al. [6] have broadly summarized silicon-based material modification strategies, advanced characterization techniques, and the mechanisms for capacity decay.

This review systematically analyzes the critical problems existing in silicon anodes and summarizes the latest research progress on the structural design and optimization of silicon anodes. The future development direction of silicon anodes is also prospected. Different from other review articles, this work mainly focuses on the modification of the physical structure of the silicon anode. From the physical structure modification of silicon particles itself to the overall structural modification of silicon anodes. The structural design and optimization of silicon anodes from different aspects are included. Different structures of silicon anodes designed by different preparation methods and their electrochemical performances are compared and discussed. Some novel structure and preparation methods of silicon anodes are summarized and analyzed, which may provide some new ideas for the research of silicon anodes.

2. Problems with Silicon Anodes

Although silicon anode has many advantages, its commercialization process is still trapped by some problems, mainly in the following three aspects (Figure 1).

Firstly, silicon particles contract and expand in the process of the cycle, and the volume expansion rate can reach 300%, which is the most critical problem in silicon anodes [7]. The huge volume shrinkage and expansion cause silicon anodes to pulverize or rupture and active silicon particles to detach from current collectors, breaking away from electrical contact [8,9]. Secondly, during the charging and discharging of the first cycle of silicon anodes, a solid electrolyte film will be formed between the electrode surface and electrolyte, also known as SEI. Due to the formation of SEI, a part of the lithium ions from positive electrodes is consumed, and this consumption is irreversible, resulting in lower Coulombic efficiency and specific capacity in the first cycle. The expansion effect of silicon anodes in the cycle process also leads to rupture and regeneration of SEI, which seriously affects cycling performance [10]. All these consequences caused by volume expansion result in a severe capacity fade. Thirdly, silicon is a semiconductor material with poor conductivity (1 × 10⁻³ S cm⁻¹). As electrode materials, poor conductivity affects the transport of electrons, making the rate performance of lithium-ion batteries poor [11,12].

The above are the main problems of silicon anodes in lithium-ion batteries, and finding efficient solutions is still a challenge.

Figure 1. The schematic failure modes of silicon anodes during repeated lithiation/delithiation process, including the pulverization and cracking of active Si particles, solid-electrolyte interphase (SEI) formation on the surfaces of Si, and fracture of electrodes. Reproduced with permission [13]. Copyright 2016, Nature Energy.

3. Silicon Anode Structure Optimization

To cope with the expansion effect of silicon particles, many researchers optimize the structure of silicon anodes by changing the source of electrode materials and electrode design. The common methods to optimize the structure of silicon anodes mainly include reducing the size of silicon particles, preparing porous silicon, and designing silicon anodes without a current collector or binder. In addition to these, silicon carbon composite can form a hierarchical structure which is an effective strategy. Silicon has poor electrical conductivity, so most of the references are compounded with carbon. There are also some examples of hierarchical structures in the article. Because the content intersects, it is not described in detail in a separate form.

3.1. Size Reduction

Most commercial Si materials use micron-sized because the cost of micron-sized Si is low. However, micron-sized Si has serious volume expansion during the cycle process, resulting in particle pulverization or rupture. Reducing the particle size of Si particles can decrease stress concentration. Meanwhile, the surface contact area with the electrolyte is increased and the electron migration distance is shortened, facilitating both the ionic and electronic conductivity improvement of silicon anodes [14]. Many studies (reported by Demirkan [15], Lee [16], Chen [17], Park [18], Nogroho [19], and Zhao [20] et al.) have prepared nanoscale Si materials with different shapes or structures, all of which exhibit excellent electrochemical performances.

Bang et al. [21] prepared uniformly sized porous silicon nanowire anodes by block copolymer lithography. Porous silicon nanowires, due to their special linear structure, have a larger surface area and are more conducive to the occurrence of reactions. Combined with the porous structure to buffer volume expansion, the diameter of the silicon nanowires can remain almost unchanged after cycling. Whereas the diameter of non-porous silicon nanowires increases and the Si materials transform into a sheet structure after several cycles. To prepare high-performance silicon nanowire anodes, Chan’s group [22] grow silicon whiskers vertically on stainless steel foil by VLS (gas-liquid-solid growth) method (Figure 2A) using gold as a catalyst. The initial discharge-specific capacity of the silicon nanowire anode reached the maximum theoretical value of 4200 mAh g⁻¹ and the discharge-specific capacity was maintained as high as 3500 mAh g⁻¹ after 20 cycles at 0.2 C. At a high rate of 1 C, there was still a discharge-specific capacity of 2100 mAh g⁻¹. The diameter of the silicon nanowires before cycling was about 89 nm and mildly grew to 141 nm after cycling. There was no distinct rupture during cycling. In addition to reducing the size of silicon material themselves, silicon nanowires can form two-dimensional linear materials, which could avoid multidimensional crush caused by volume expansion during the cycle. Moreover, nanowire structures can adapt to large volume strain, provide good electrical contact, and shorten the ion migration distance.

Except for silicon nanowires, two-dimensional nano-silicon has also been extensively studied. Tamirat et al. [23] prepared ultra-thin silicon nanolayers and implanted the silicon nanolayers into Ni_xSi/Ni nanoparticles (Si@NixSi/Ni). The Ni substrate in Si@NixSi/Ni provides mechanical support for active materials, maintaining electrode integrity during battery operation. Sakabe et al. [24] successfully prepared porous amorphous silicon film anodes. Wang et al. [25] synthesized multilayer silicon films by radio frequency magnetron sputtering. Chen et al. [26] prepared a silicon film anode by sputtering silicon on copper foil via a magnetron sputtering method. The as-prepared silicon film could retain a reversible capacity of 1300 mAh g⁻¹ after 500 cycles at 0.5 C. Compared with the traditional vapor deposition method, this method makes the contact between active materials and the current collector more close and forms a Si-Cu layer, preventing the separation of active silicon from the current collector to a certain extent. There are different methods for the design of ultra-thin nano-silicon layers. The thinner the better in electrochemical performance as a thin film silicon anode has small volume change and strong adhesion to the current collector. A two-dimensional structure design can also shorten ion and electron migration distance and could help enhance the energy density of batteries compared to the one-dimensional structure.

In terms of size reduction, it is still a great challenge to achieve sub-nanometer (<1 nm) size silicon particles. Sung et al. [27] used ethylene as a growth inhibitor to prevent silicon particles from continuously growing after chemical vapor nucleation, successfully preparing sub-nanometer-size silicon particles. Sub-nanometer-size silicon anodes enhance cycle stability, with a coulomb efficiency of 98.5% after 99 cycles. In a 110 Ah full battery, a high capacity retention of 91% after 2875 cycles was obtained. Such high performances should be mainly attributed to the remarkable size decrease in silicon particles. The small particle size could increase the specific surface area of the active materials and the contact area with the electrolyte; thus, shortening ion or electron migration distance. More importantly, sub-nanometer-size silicon could reduce the stress of volume expansion, resist to pulverize, and improve charge and discharge efficiency to a large extent.

Karuppiah et al. [28] used a simple and scalable method to grow silicon nanowires on graphite to obtain Gt-SiNM composite materials through using diphenyl silane as a silicon source and gold as a catalyst (Figure 2B). The uniform distribution of SiNM and graphite flakes prevents electrode pulverization and alleviates the volume expansion of Si. The capacity retention rate reached 87% after 250 cycles at a 2 C rate. This preparation method is simple, low-cost, and scaleable. The specific capacity can be adjusted according to the actual requirements. The addition of graphite facilitates electron transfer and adapts to silicon expansion.

Compared with a solid nanowire structure, a hollow nanotube structure has more advantages in resisting volume expansion. Wen et al. [29] prepared silicon nanotubes (Figure 2C) by magnesium thermal reduction, which showed much improved electrochemical performances. Hollow silicon nanotubes may provide a larger electrochemically active area for lithium ions bedding, because both the inner and outer walls of silicon nanotubes can be accessible to the electrolyte. Park et al. [30] used alumina as a template to reduce and decompose silicon precursor, and then etched the template to obtain Si nanotubes. Finally, the surface of the Si nanotubes was coated with depositing carbon. The initial outer diameter of Si nanotubes is 200–250 nm, and the wall thickness is 40 nm. During the cycle process, Si nanotubes act as pathways for lithium ions and increase ion migration speed. At 1 C rate, the Si nanotubes released a reversible specific capacity of 3247 mAh g⁻¹ and a capacity retention rate of 89% after 200 cycles. The thickness of Si nanotubes increased to 300 nm after 200 cycles, without an observed crack in the Si nanotubes.

Many studies have shown that hollow structures combining nanominiaturization are more effective in mitigating the volume expansion of Si. For instance, Ashuri et al. [31] obtained carbon-coated hollow silicon nanospheres (HSi@C), which showed better cycle stability than micron size. Liang et al. [32] designed porous silicon nanospheres with using silica sol as feed materials by hydrothermal method. The silicon nanospheres are prepared by hydrothermal course alone and, without any carbon coating, are different from common silicon anode material designs. Pu et al. [33] prepared silicon nanospheres by a simple and expandable spray drying method, using fulvic acid (FA) as a carbon source to prepare carbon shells coated on the silicon spheres (Figure 2D). There is some space between the carbon shells and the silicon spheres to buffer volume expansion. Fulvic acid has wealthy functional groups that bond with silicon particles to form secondary particles. Due to the nanosphere structure of silicon carbon composite electrodes (Si@FA), Si@FA showed significantly better electrochemical performance than pure silicon electrodes. After 100 cycles at a current density of 0.1 A g⁻¹, Si@FA had a reversible specific capacity of 917.28 mAh g⁻¹.

Table 1 summarizes and compares the above electrochemical performance of silicon anode materials modified by size reduction.

Figure 2. (A) SEM image of silicon nanowires. Reproduced with permission [22]. Copyright 2008, Nature Nanotechnology. (B) The scheme represents the synthesis of SiNWs on graphite. Reproduced with permission [28]. Copyright 2020, ACS Nano. (C) Schematic illustration for the synthesis of Si nanotubes. Reproduced with permission [29]. Copyright 2013, Electrochem Commun. (D) Schematic of the preparation procedure of Si@FA-X. Reproduced with permission [33]. Copyright 2022, Chemical Physics Letters.

Table 1. Electrochemical performance of silicon anode materials modified by size reduction.

Sample	Synthesis Method	Cycling Stability
Sample	Synthesis Method	Discharge Capacity [mA h g⁻¹]	After nth Cycle	Current Density	Ref.
porous silicon nanowire	block copolymer lithography	1450	50	0.1 C	[21]
silicon whiskers	VLS	3500	20	0.2 C	[22]
Si@NixSi/Ni	ball milling and anneal	521.5	5000	500 mA g⁻¹	[23]
Porous amorphous si film	radio-frequency magnetron sputtering	300	100	0.1 mA cm⁻²	[24]
multilayer-like Si thin films	magnetron sputtering	1905.05	100	0.2 C	[25]
silicon film	magnetron sputtering	1316.7	500	0.5 C	[26]
Gt-SiNM	liquid phase deposition and heating	900	300	0.2 C	[28]
silicon nanotubes	in situ growth and magnesiothermic thermal reduction	1000	90	0.5 C	[29]
Si nanotubes	chemical deposition	2800	200	1 C	[30]
HSi@C	sol-gel reaction and magnesiothermic and hydrogen reductions	700	100	2 A g⁻¹	[31]
spherical porous silicon	hydrothermal	960	500	3.6 A g⁻¹	[32]
Si@FA	spray drying	917.28	100	0.1 A g⁻¹	[33]

3.2. Porous Structure

In addition to reducing the dimensions of silicon particles, creating space for the expansion of silicon particles can also somewhat buffer the volume expansion. Space is created by porous particles in the silicon itself. During the cycle, the expansion will preferentially grow towards the pores. To some extent, it inhibits the volume expansion of the overall electrode, effectively prevents electrode pulverization or rupture, and improves the electrochemical performance of the battery.

Tesfaye et al. [34] prepared porous Si nanotubes on stainless steel substrates by sacrificing the ZnO nanowire template method. The silicon nanotubes are arranged vertically on the stainless steel foil and are not stacked on top of each other. This arrangement improves the reaction activity of Si materials to a greater extent, embedding more lithium ions and thus, releasing more capacity. Saager et al. [35] deposited silicon and zinc under a vacuum, then annealed to remove the zinc, resulting in a porous silicon film (Figure 3A). The Zn in the film can be thoroughly removed (Figure 3B), leaving a pore size of approximately nanometers to micrometers. Compared with dense silicon thin films, the electrochemical performance has improved, but the cyclic stability is still insufficient for practical applications. The electrode with a Si loading mass of 0.56 mg cm⁻² showed a reversible specific capacity of 1300 mAh g⁻¹ after 30 cycles at a rate of 0.1 C. When the loading mass of Si is 0.46 mg cm⁻², the capacity can remain stable for over 150 cycles. The porous silicon film could be formed using Zn as a sacrificial template, and the pore size and pore rate can be adjusted according to the content of Zn. Furthermore, Zn can be almost completely removed, which has great significance for the regulation of material porosity. As a sacrifice material, Zn is cheap and even recyclable. Therefore, this strategy does not cause a large amount of resource waste.

Jin et al. [36] easily synthesized porous silicons by a magnesium thermal reduction and further coated the TiSi₂ layer on the porous silicons to obtain the final sample P-Si@TiSi₂. Li et al. [37] prepared a large mesoporous silicon sponge (MSS) with a pore diameter of approximately 50 nm by an electrochemical etching method (Figure 3C). This mesoporous structure accommodates most volume changes and limits the volume expansion to 30%. Compared to the 300% volume expansion of traditional silicon electrodes, it greatly reduces the volume expansion effect and does not cause electrode pulverization even after 1000 cycles. The MSS showed a specific capacity of 750 mAh g⁻¹ based on the entire electrode mass and with a capacity retention rate of 80%, after 1000 cycles at a current density of 1 A g⁻¹.

Yan et al. [38] designed porous silicon nanoparticles (N-PoSi@C) by a simple alkaline etching method (Figure 3D), which can adapt to the large volume expansion of silicon. This is a low-cost, scalable, high-yield preparation method, and the structure can effectively relieve the stress caused by expansion. Ge et al. [39] synthesized porous boron-doped silicon nanowires by direct etching boron-doped silicon wafers. There are also studies on preparing silicon into a spherical, hollow, and porous structure. The effect of large pore size and porosity is manifested in excellent electrochemical performance, and high specific capacity is still available at high current density. At the current density of 18 A g⁻¹, the specific capacity can be maintained above 1100 mAh g⁻¹ after 250 cycles, which is very rare. Xiao et al. [40] synthesized SiO₂ by condensation and hydrolysis of tetraethoxysilane and octadecyl trimethoxysilane, then converted SiO₂ into Si by Mg steam, and finally removing MgO through acidification to prepare porous hollow silicon spheres (hp-SiNSs). The unique structure converts the volume expansion inward, and thus the volume expansion of the silicon anode is greatly relieved. Under 0.05 C rate, the specific capacity was 1800 mAh g⁻¹ after 200 cycles. In common nano-silicon spheres, there is a rare sealed shell on the outside. Uniform holes in the shell and internal hollow structures have sufficient advantages to cope with the stress generated by volume expansion.

An et al. [41] prepared an ant nest-like block porous silicon anode. Mg was alloyed with Si powder to generate Mg₂Si, and the byproduct Mg₃N₂ was removed through N₂ nitridation followed by acidification to separate ant nest-like porous silicon (AMPSi). Finally, a carbon coating was applied to prepare AMPSi@C (Figure 3E). The silicon anode of three-dimensional interconnected nanoribbons with continuous nanopores showed excellent resistance to pulverization and volume expansion during cycling. The AMPSi@C released a capacity of 1271 mAh g⁻¹ after 1000 cycles at 2100 mA g⁻¹, with a capacity retention rate of 90%. At a high areal-specific capacity of 5.1 mAh cm⁻², the expansion rate was only 17.8%. The pores in the ant nest structure are interconnected, which can accommodate the expansion of silicon particles in any direction to maintain the structure integrity of the silicon anode.

Similar porous composite silicon anodes have been widely reported. Dong et al. [42] synthesized Si@C porous composites in situ to achieve low expansion rates. The space between the silicon and the carbon layer buffers the silicon expansion stress and helps to form a stable SEI. Hu et al. [43] prepared silicon/carbon composite porous nano-anodes to achieve high cycle stability. Jia et al. [44] obtained CNT@Si yarn-like spheres by in situ aluminothermic reduction of CNT@SiO₂ core-shell tubular microspheres, and coated the spheres with a carbon layer, ultimately obtaining CNT@Si@C microspheres (Figure 3F). The Si in CNT@Si@C was obtained by the reduction of SiO₂, bringing a porous structure. The volume expansion of CNT@Si and CNT@Si@C after lithiation was only 10% and 40%, respectively. At a current density of 1 mA cm⁻¹, the reversible specific capacity of CNT@Si@C was about 1500 mAh g⁻¹, and the capacity retention rate was 87% after 1500 cycles. Although porous structures can adapt to the expansion of silicon volume, most of the porous structures have poor mechanical properties. CNT@Si@C has good mechanical properties (>200 MPa) and only a low apparent volume expansion of 40% when fully lithiated. Good mechanical properties should also be an important factor that must be considered in the design of silicon anodes.

Table 2 summarizes and compares the above electrochemical performance of silicon anode material modified by porous structure.

Figure 3. (A) Scheme of forming separated grains of zinc (red) and silicon (blue) after deposition. (B) Scheme of porous silicon film after expelling zinc by thermal annealing. Reproduced with permission [35]. Copyright 2019, Surface and Coatings Technology. (C) A schematic model of the MSS particle. Reproduced with permission [37]. Copyright 2014, Nat Commun. (D) Schematic illustration of the synthesis of the N-PoSi and the porous formation mechanism. Reproduced with permission [38]. Copyright 2022, Materials Today Nano. (E) Schematic diagram of the preparation process of AMPSi and AMPSi@C. Reproduced with permission [41]. Copyright 2019, Nat Commun. (F) Schematic figure showing the synthesis of CNT@Si@C microspheres. Reproduced with permission [44]. Copyright 2020, Nat Commun.

Table 2. Electrochemical performance of silicon anode material modified by porous structure.

Sample	Synthesis Method	Cycling Stability
Sample	Synthesis Method	Discharge Capacity [mA h g⁻¹]	After nth Cycle	Current Density	Ref.
SiNTs	template-directed	1670	30	0.05 C	[34]
porous silicon thin films	the vapor deposition and annealing	1300	30	0.1 C	[35]
P-Si@TiSi₂	magnesium thermal reduction and etching	1180	50	400 mA g⁻¹	[36]
MSS	electrochemical etching	640	1000	1 A g⁻¹	[37]
N-PoSi@C	etching and CVD	1170.7	100	4 A g⁻¹	[38]
porous doped silicon nanowires	chemical etching	1100	250	18 A g⁻¹	[39]
hp-SiNSs	magnesium thermal reduction	1800	200	0.05 C	[40]
AMPSi@C	thermal nitridation and etching	1271	1000	2100 mA g⁻¹	[41]
porous Si@C	ball grinding and calcination	630	100	0.2 C	[42]
Porous C/Si nanocomposite	template method	450	50	150 mA g⁻¹	[43]
CNT@Si@C	in situ aluminothermic reduction	1500	1500	1 mA cm⁻²	[44]

3.3. Binder/Current Collector Free Silicon Anode

During the cycle process, the expansion and pulverization of silicon particles lead to the detachment of active substances from the collector, silicon particles loss of electrical contact, and thus severe capacity fading. The traditional preparation method involves mixing active substances, conductive carbon black, and binders in a ratio of 8:1:1 and coating them on copper foil. Some existing studies have prepared a silicon anode without binders or current collectors. Compared with traditional electrode preparation methods, the advantage is preventing active substances from peeling off the current collector during the shrinkage and expansion process, reducing the electrode load and saving costs.

Guo et al. [45] synthesized a carbon scaffold structure silicon anode to alleviate the expansion of silicon volume and eliminate the binder. The presence of a carbon skeleton prevents cracked silicon particles from falling off and maintains good electrical contact. Dong et al. [46] added a small amount of Sn to the slurry to carbonize the original binder into a conductive skeleton. Sn was melted and extruded by hot pressing and was cooled to bond Si particles. During the process, Cu₃Si and Cu₃Sn interlayers were formed on the surface of the current collector, causing the active substance to bind closely to the current collector. The electrode was hot pressed to half of its original state, but still exhibited excellent electrochemical performance, maintaining a capacity of 932 mAh g⁻¹ after 500 cycles at a current density of 1500 mA g⁻¹. The rate performance had also been enhanced, with a specific discharge capacity of 1075 and 773 mAh g⁻¹ at 3000 and 4500 mA g⁻¹ current densities. This electrode type has high conductivity and density, which can achieve high volume capacity. Through a simple hot pressing process, active substances and the current collector can be closely connected, and the Si-Cu layer formed between active substances and the current collector is a best proof of close contact. A tight transition layer could help prevent silicon particles from losing electrical contact. However, this method is not suitable for large-scale production at present.

Favors et al. [47] synthesized nanofiber silicon paper electrodes (SiNFs) through magnesium thermal reduction and electrospinning, then used C₂H₂ as a carbon source to coat the silicon paper electrodes with carbon (Figure 4A). The independent carbon-coated silicon nanofiber anode has neither a binder nor a current collector, reducing electrode weight and increasing the proportion of active substances. In this electrode, the proportion of silicon exceeded 80%. The SiNFs displayed a specific capacity of 802 mAh g⁻¹ after 659 cycles at a rate of 0.1 C, with a Coulombic efficiency of 99.9%. Electrospinning can produce binder-free and collector-free electrodes, but some of the pulverized silicon particles may fall out from the gaps between the filaments, thus losing electrical contact.

Zhu et al. [48] connected the three-dimensional interconnected nitrogen/carbon network hollow carbon nanospheres and silicon nanodots, and finally obtained a binder-free SHCM/NCF anode. Zhang et al. [49] encapsulated silicon particles in multilayer graphene to prepare a RGO/Si anode without a binder. The porous structure of the RGO layer can promote the penetration of the electrolyte and improve the rate performance of the battery. Liu et al. [50] prepared silicon nanowires by a CVD method, and uniformly distributed silicon nanowires on carbon textiles through a spray-coating operation to prepare a binder-free silicon anode (Figure 4B). The close contact between silicon and carbon could increase the overall electrical conductivity of silicon anodes. After 200 cycles at 0.2 C, the binder-free silicon nanowire anode showed a specific capacity of 2950 mAh g⁻¹ and a capacity retention rate of 92%.

Wang et al. [51] prepared a silicon anode (VASCNT) by directly arranging silicon/carbon nanotubes vertically on an Inconel current collector, avoiding polymer binders. Haro et al. [52] prepared a special binder-free silicon anode, which was supported by the vapor deposition of Ta NPs (Figure 4C). The columnar amorphous silicon film was deposited on the Ta NPs support to form an arched structure and annealed. When the diameter of columnar silicon continues to grow, the tips come into contact with each other. Then the anode surface is encapsulated, and stable SEI can be formed on the surface. This structure causes its volume to expand towards the inner cavity at 0.5 C, the initial capacity reached 3230 mAh g⁻¹, and the capacity retention rate was 88% after 100 cycles (2832 mAh g⁻¹). The arch nanostructure provides a new idea for the structural design of silicon anode. Sealing the surface can form a stable SEI and reduce the stress of expansion.

Wang et al. [53] synthesized a novel self-supporting binder-free silicon anode with overlapping graphene and reduced graphene oxide-wrapped silicon nanowires (SiNW@G@RGO). Li et al. [54] prepared Si/C composites for electrospinning. Si adhered to the outside of the carbon tube, and Si/C was used as the core. Then carbon coated the structure to obtain the final Si/C–C material (Figure 4D). Si/C–C as an independent fiber conductive film without the addition of conductive agents and binders, can be directly used as an electrode. The connection between the silicon and carbon shell cushions the stress generated by the volume expansion of silicon during the cycling process, making the electrode have good cycling stability. At a current density of 50 mA g⁻¹, the specific capacity was 886.5 mAh g⁻¹, and the capacity retention rate was 72.4% after 50 cycles.

Shao et al. [55] prepared a binder-free anode (GFF/Si) by wrapping silicon particles in a three-dimensional conductive network of flexible graphene-fiber-fabric (GFF). The silicon particles are dispersed in the graphene fibers. Wrinkled graphene fibers have enough space to accommodate the changes in silicon volume. Liu et al. [56] also designed a hollow structure binder-free Si–Ni–C silicon anode by electrospinning; there was no sacrificial material in this process. The addition of Ni improved the conductivity of the composite material. This composite material maintained a specific capacity of 622 mAh g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. NiO was reduced to Ni by calcination and, at the same time, formed a hollow structure. It is necessary to build a material with a certain space. A suitable oxide was chosen as a template and reduced to form the desired space.

Most binder-free or collector-free electrodes are prepared by electrospinning in combination with other methods. Osaka et al. [57] crafted a binder-free Si–O–C composite film showing excellent cycle durability by electrodeposition. As we know, the capacity of the battery can be improved by increasing the thickness of the electrode to some extent. Electrodeposition technology, as a common technology, can increase the thickness of the electrode to improve the capacity of the battery at a low cost without any additives. This technology can be used for other lithium alloying materials. Yang et al. [58] prepared a binder-free silicon anode in an organic solvent by one-step electrophoretic deposition (EPD). Under the action of electrophoretic deposition, nano silicon and acetylene black (AB) form interconnected 3D deposition films (Figure 4E). The close contact between Si and AB shortens the ion migration distance and improves the charge transfer efficiency. The structure of the 3D network provides buffer space for silicon expansion. It was found that the electrode prepared under 5S EPD deposition performs best. Even at a high rate of 1 C and 2 C, the electrode showed a specific capacity of 1516 mAh g⁻¹ and 1231 mAh g⁻¹ after 200 cycles, respectively.

Table 3 summarizes and compares the above electrochemical performance of silicon anode material modified by binder/current collector free.

Figure 4. (A) Schematic diagram of the electrospinning process and subsequent reduction process. Reproduced with permission [47]. Copyright 2015, Scientific Reports. (B) Schematic illustration of the formation of hierarchical silicon-carbon textiles matrix. Reproduced with permission [50]. Copyright 2013, Scientific Reports. (C) Schematic breakdown of growth process into three steps: deposition of TaNS, columnar growth of silicon amorphous film exploiting the shadowing effect of the TaNS, and thermal annealing at 150 °C enhancing the mobility and consequent annihilation of voids at open surfaces. Reproduced with permission [52]. Copyright 2021, Commun Mater. (D) Schematic of Si/C composite nanofibers and Si/C–C core–shell composite nanofibers. Reproduced with permission [54]. Copyright 2014, Solid State Ionics. (E) Schematic of the process for fabrication of EPD electrode. Reproduced with permission [58]. Copyright 2015, ACS Applied Materials & Interfaces.

Table 3. Electrochemical performance of silicon anode material modified by binder/current collector free.

Sample	Synthesis Method	Cycling Stability
Sample	Synthesis Method	Discharge Capacity [mA h g⁻¹]	After nth Cycle	Current Density	Ref.
carbon scaffold Si anode	the slurry spray technique and carbonize	1792	123	0.05 C	[45]
Si–Sn/Cu₃Si@C–P₂₆₀	heat pressing	1532	100	300 mA g⁻¹	[46]
C-coated SiNFs	magnesium thermal reduction and electrospinning	802	659	0.1 C	[47]
SHCM/NCF	magnesium thermal reduction and electrospinning	1442	800	1 A g⁻¹	[48]
RGO/Si	homogenous dispersion and asting	835	100	10 A g⁻¹	[49]
Si NWs-carbon textiles matrix	CVD and spray-coating operation and thermal treatment.	2950	200	0.2 C	[50]
VASCNT	CVD	1950	20	100 mA g⁻¹	[51]
TaNS-Si	gas-phase deposition and magnetron sputtering	2832	100	0.5 C	[52]
SiNW@G@RGO	CVD and thermal reduction	1600	100	2.1 A g⁻¹	[53]
Si/C–C	electrospinning	886.5	50	50 mA g⁻¹	[54]
GFF/Si–37.5%	Electrospinning and two-step reduction	920	100	0.4 mA cm⁻²	[55]
Si–Ni–C	electrospinning and thermal treatment	622	100	100 mA g⁻¹	[56]
Si–O–C	electrodeposition	831	7000	250 μA cm⁻²	[57]
EPD-5s	electrophoretic deposition	1516	200	1 C	[58]

4. Optimization of Surface Structure of Silicon Anode

Reducing the dimension of silicon particles is a strategy for mitigating the volume expansion effect from materials source. Structural optimization can also be carried out externally to the silicon particles to leave space for the volume expansion of the silicon particles or provide stress to prevent the silicon particles from continuing to expand. The main methods for optimizing the external structure of silicon particles are to prepare shell-cores structures and sandwich structures.

4.1. Coating Structure

There are many studies on the shell core structure in the coating structure, generally using silicon as the core or forming a shell in the outer layer, leaving a particular space between the two. During the cycle process, the expansion of silicon occurs in the space between the shell and core. If the degree of expansion is too large, the shell will have a certain degree of stress that hinders the continuous growth of silicon particles. The shell structure limits the degree of silicon expansion, restrains the pulverization of silicon particles, and prevents direct contact between silicon particles and the electrolyte. At present, there are research on the modification of silicon anode by carbon coating, metal oxide coating, conductive polymer coating, non-metallic oxide coating, etc. This paper focuses on carbon coating.

Coating structures could not only weaken volume expansion effects, but also improve conductivity. Different coating structures have been designed to obtain high-performance silicon anodes. Song et al. [59] prepared a controllable spongy porous nanocarbon-coated silicon (sCCSi). The structure of the sponge porous coating can be adjusted based on the stirring temperature and pre-oxidation heating rate of the precursor, and a suitable sponge porous structure can greatly improve the conductivity. Tan et al. [60] used bitumen as a carbon source to prepare a homogeneous carbon-coated silicon anode. The carbonization temperature has a significant impact on the mechanical properties and conductivity of carbon coatings, so it is necessary to find the optimal carbonization temperature to enable silicon-carbon composite materials to have good electrochemical performance. Guan et al. [61] prepared a PSi/C electrode composed of Si/C nanobeads by spray drying and pyrolysis treatment (Figure 5A). In the PSi/C electrode, Si/C nanoparticles with an egg yolk structure are uniformly dispersed and interconnected to form a porous three-dimensional framework. There is sufficient space in the egg yolk structure Si/C structure and a porous structure between the carbon frameworks. The shell core structure and the porous structure are placed in one system. Both structures have sufficient space to cope with Si shrinkage and expansion, greatly improving the stability of the battery. The connection of carbon frameworks also contributes to the improvement of conductivity. The mass fraction of active substance Si in the PSi/C electrode was only 15.4%, but it released a specific capacity of 1357.43 mAh g⁻¹ at a current density of 100 mA g⁻¹. After 100 cycles, it still had a reversible capacity of 933.62 mAh g⁻¹. Even at 10 times the current density (1000 mA g⁻¹), a specific discharge capacity of 610.38 mAh g⁻¹ remained after 3000 cycles.

Bionics has been providing new ideas for scientific research. Ma et al. [62] found that the heartbeat has a similar mechanism to the contraction and expansion of silicon particles. Lithium ions flow to silicon when silicon particles expand, as if the heart relaxes. When the silicon particles contract, lithium ions flow out of the silicon particles, similar to the heart pumping blood. So they have prepared the silicon particles into independent heart structures (Figure 5B). Using the CVD method, sheet graphene is cross-linked to form a framework, which is uniformly filled with silicon particles to mimic the “heart”. Finally, carbon nanofibers are coated on the outer surface of this structure using electrospinning technology to replace the external arteries and veins of the heart, resulting in the “heart” structure of the G/Si@CFs anode. The coating of carbon nanofibers enhances electron transport, improves conductivity, and prevents the pulverization or rupture of silicon particles under the effect of coating, significantly improving the cycling performance and capacity of lithium-ion batteries. At a current density of 100 mA g⁻¹, the G/Si@CFs electrode had a specific capacity of 896.8 mAh g⁻¹ and a capacity retention rate of 86.5% after 200 cycles. This design of a biomimetic heart demonstrates the potential of biomimetic applications in the field of electrochemistry.

Most carbon shells are coated on particles by solvent gel or physical vapor deposition method, which is costly and requires high experimental conditions. Li et al. [63] prepared ball-milled silicon@carbon/reduced graphene oxide composites (bmSi@C/rGO) by electrostatic assembly. The carbon layer and graphite oxide layer provide a double-layer protection for silicon particles. They are not only a buffer layer for the volume expansion of silicon particles, preventing the breakage of silicon particles, but also provide good conductivity. The electrostatic self-assembly method provides an effective way to prepare graphene-coated materials. Because tannic acid can spontaneously polymerize outside the particles, Feng’s team [64] mixed the Si particles with mechanically milled Si/G precursors and tannic acid, then carbonized them to coat the particles with a carbon layer, obtained Si@TA and Si@TA-G samples. The addition of tannic acid coating acts as a barrier between Si and the electrolyte, preventing direct contact between the two and improving conductivity. After 150 cycles at 100 mA g⁻¹, the Si@TA showed a specific capacity of 927.4 mAh g⁻¹ and a capacity retention rate of 87.1%. Under the same conditions, Si@TA-G had a capacity of 1249.8 mAh g⁻¹ and a capacity retention rate of 93.6%.

Chen et al. [65] successfully realized a copper-coated silicon nanowire anode by chemical vapor deposition and magnetron sputtering. The addition of Cu coating can further improve the cycle stability of the battery, but it reduces the capacity. So we need to find the best copper coating thickness. Baek et al. [66] synthesized Ag-coated silicon nanowires via an isomorphic redox reaction. In this study, a metal-assisted chemical etching method is proposed, which is simple and cost-effective and can be used to prepare high-performance lithium-ion batteries. Chan et al. [67] synthesized silicon nanowires using the supercritical fluid-liquid-solid (SFLS) method and mixed them with multi-walled carbon nanotubes, and then carbon coated them through sucrose pyrolysis. The coating layer of sucrose pyrolysis was found to provide good electrical contact for the material. The carbon-coated silicon nanowire material maintains a capacity of 1500 mAh g⁻¹ after 30 cycles at 0.2 C. Compared with other silicon-carbon composites, this material is simple to synthesize and can achieve good performance using a simple mix.

Wu et al. [68] synthesized silicon nitride-coated silicon anodes by two-step DC sputtering on a copper microcone array (CMA). Silicon is sandwiched between CMA and SiNx, and the amount of SiNx greatly affects the capacity. He et al. [69] coated an ultra-thin alumina layer on a patterned silicon electrode by atomic layer deposition (ALD). The patterned silicon anode obtained by RIE is relatively rare in the preparation of a silicon anode. Al₂O₃ coating obtained by the ALD process enhances the fracture resistance of silicon particles. Zhang et al. [70] coated the surface of the porous silicon sphere after magnesium thermal reduction with a nitrogenous carbon layer. The coral-like silicon anode formed through high-temperature carbonization is cross-connected. This morphology is conducive to electron transport, providing a new idea for the preparation of a highly conductive silicon anode. Kong et al. [71] used PAN as a carbon and nitrogen source to prepare necklace-shaped silicon nanospheres with nitrogen-doped carbon coating (NL-Si@C) through electrospinning and magnesium thermal reduction (Figure 5C). Silicon nanospheres come from etched SiO₂. After being converted into Si, there is room left in the carbon shell for volume expansion. The content of SiO₂ can control the proportion of silicon in the material. In nitrogen-doped carbon shells, nanospheres are connected by the carbon shell, shortening ions’ migration distance, and improving the charge transfer efficiency. At the same time, the obstruction of the carbon layer was also conducive to the formation of stable SEI. The sample of [email protected] exhibited the best performance, with a specific capacity of 710 mAh g⁻¹ after 500 cycles at a current density of 200 mA g⁻¹. The chain design connected the coated silicon in series to improve the electrical conductivity, ensured the rapid migration of ions and electrons, and promoted the accessibility between electrolyte and electrode.

Song et al. [72] reported on synthesizing micro-sized silicon-carbon (Si–C) composites with primary sub-10 nm silicon particles and secondary microsize aggregates coated with carbon. This material has three advantages. Firstly, the subnano silicon particles can relieve the lattice stress of the expansion process and inhibit the fragmentation of silicon particles. Secondly, the conductive carbon coating can buffer the volume expansion of silicon and form a stable SEI. Finally, HF-HCl can etch SiO₂ to prevent lithium ions from reacting with SiO₂. Li et al. [73] used Ni NP as a template to prepare CNS, and treated Ni NP with HCl etching. Then, Si and Al₂O₃ were deposited on the CNS to obtain a hollow CNS/Si/Al₂O₃ shell core membrane structure (Figure 5D). The function of Al₂O₃ thin film is to reduce the formation of SEI, provide a good electron transfer channel and improve conductivity by the CNS contacts the surface of silicon particles. This hollow structure serves as a framework, allowing the expansion direction of silicon to move inward, providing buffer space for the volume expansion of silicon during the cycling process. Therefore, the electrode performance exhibits a good specific capacity and a capacity retention rate: after 100 cycles at a current density of 1 A g⁻¹, the specific capacity was 1560 mAh g⁻¹, and the capacity retention rate reached 85%.

In addition to these carbon coatings, researchers also tried to coat silicon particles with hydrogel. Wu et al. [74] used phytic acid as a gelling agent and prepared PAni from aniline monomer. The nitrogen group on Polyaniline was connected with an aniline monomer to form a cross-linked network hydrogel. Then SiNPs were added to mix and coated on copper foil to prepare a hydrogel silicon anode (Figure 5E). The hydrogel can be in situ polymerized on the surface of silicon particles and has good conductivity. Multiple pores in the cross-linked network allow the silicon to expand, resulting in an excellent electrochemical performance of the electrodes. At a high current density of 6 A g⁻¹, the hydrogel silicon anode showed a specific capacity of about 550 mAh g⁻¹ and a capacity retention rate of 90% after 5000 cycles. The method is scalable, compatible with current slurry coating technologies, and can be extended to the field of manufacturing high-performance lithium-ion battery.

Table 4 summarizes and compares the above electrochemical performance of silicon anode material modified by coating structure.

Figure 5. (A) PSi/C electrode structure and cyclic performance diagram. Reproduced with permission [61]. Copyright 2018, American Chemical Society. (B) Biomimetic heart electrode preparation flow chart. Reproduced with permission [62]. Copyright 2017, Scientific Reports. (C) Schematic illustration of the synthesis process for a necklace-like Si@C network. Reproduced with permission [71]. Copyright 2019, Science Bulletin. (D) Schematic of hollow CNS/Si/Al₂O₃ core-shell film fabrication processes. Reproduced with permission [73]. Copyright 2015, Scientific Reports. (E) Schematic illustration of 3D porous SiNP/conductive polymer hydrogel composite electrodes. Each SiNP is encapsulated within a conductive polymer surface coating and is further connected to the highly porous hydrogel framework. SiNPs have been conformally coated with a polymer layer either through interactions between surface –OH groups and the phosphonic acids in the crosslinker phytic acid molecules (right column), or the electrostatic interaction between negatively charged –OH groups and positively charged PAni due to phytic acid doping. Reproduced with permission [74]. Copyright 2013, Nat Commun.

Table 4. Electrochemical performance of silicon anode material modified by coating structure.

Sample	Synthesis Method	Cycling Stability
Sample	Synthesis Method	Discharge Capacity [mA h g⁻¹]	After nth Cycle	Current Density	Ref.
sCCSi	thermal annealing	736	300	0.5 A g⁻¹	[59]
Si@C#480	thermal annealing	898	480	0.5 C	[60]
PSi/C	spray-drying technique	933.6	100	0.1 A g⁻¹	[61]
G/Si@CFs	CVD	896.8	200	0.1 A g⁻¹	[62]
bmSi@C/rGO	thermal annealing	935.77	100	0.2 A g⁻¹	[63]
Si@TA-G	etching and CVD	1249.8	150	0.1 A g⁻¹	[64]
carbon-coated Si NWs	vapor–liquid–solid growth method	1540	40	0.5 C	[65]
Ag-coated SiNW	MACE	707	30	0.5 C	[66]
SFLS SiNW	gold-seeded SFLS growth	1500	30	0.2 C	[67]
N1-Si/CMAs	Cu Micro-cone Arrays (CMAs)	1121	200	0.2 C	[68]
20-ALD-Si	photolithography and reactive-ion etching (RIE)	1100	100	0.175 A g⁻¹	[69]
PSi@C (N)	low temperature magnesium thermal reduction technology and oxidation control	803.92	300	0.3 A g⁻¹	[70]
[email protected]	electrospinning and magnesium thermal reduction	710	500	0.2 A g⁻¹	[71]
Si–C composite	mild sol-gel process and subsequent thermal disproportionation	1570	150	0.4 A g⁻¹	[72]
CNS/Si/Al₂O₃	etching and CVD	1560	100	1 A g⁻¹	[73]
SiNP-PANi	in situ polymerization	550	5000	6 A g⁻¹	[74]

4.2. Sandwich Structure

The principle of a sandwich structure is similar to that of a coating structure. Silicon acts as an intermediate layer of the sandwich structure and does not directly come into contact with the electrolyte. The outer layer of silicon serves as a buffer layer, accommodating the volume expansion of silicon, thereby improving the cycling stability and specific capacity of silicon anodes.

Xu et al. [75] added a carbon layer between the current collector and the active material silicon to prepare a sandwiched silicon anode. The electrochemical performance of a pure silicon anode can be optimized using the most traditional and simple method to prepare sandwich structures. Zhao et al. [76] used the most traditional electrode preparation process by coating micrometer-level silicon slurry on copper foil, waiting for drying, and then coating graphene slurry on the silicon layer. The electrode was compacted through a rolling process to prepare a “graphene-silicon-copper” sandwich structure electrode (Figure 6A). It solves the problem of pulverization of silicon particles, ensures close contact between electrodes, and maintains good electrical contact. Compared to pure silicon and graphene, the initial specific capacity of the sandwich structure was 1700 mAh g⁻¹, and its specific capacity decreased to 878 mAh g⁻¹ after 30 cycles. After 1200 cycles, it had a reversible capacity of 466 mAh g⁻¹, and the performance was the best among the three. This performance advantage comes from the special sandwich structure, which buffers the volume expansion during charging and discharging, and prevents electrode pulverization or rupture, thereby ensuring effective electrical contact.

Zhang et al. [77] prepared sandwiched silicon/Ti₃C₂T_x MXene composites by electrostatic self-assembly. Tian et al. [78] synthesized a flexible and binder-free Si/MXene composite paper anode through valence anchoring and vacuum filtration (Figure 6B). The layered structure design allows silicon nanospheres to be evenly dispersed between interlayers of MXenes. It prevents MXenes from reptile up, promotes lithium ion transport efficiently, and adapts to severe Si volume expansion. Adding MXenes improves the conductivity of the electrode, and thus enhances electrochemical performance. At a current density of 200 mA g⁻¹, the specific discharge capacity remains at 2118 mAh g⁻¹ after 100 cycles. At a current density of 1000 mA g⁻¹, it showed a specific discharge capacity of 1672 mAh g⁻¹ after 200 cycles. In the rate performance test, the Si/MXene composite paper anode offered a reversible capacity of 890 mAh g⁻¹ at a high current density of 5000 mA g⁻¹. Si/MXene composite paper has two advantages. On the one hand, MXene has a certain degree of flexibility and space to adapt to the expansion of silicon. On the other hand, the presence of silicon particles prevents the stacking of MXene.

Sun et al. [79] synthesized a sandwiched graphite-silicon metal@C (MS-G@C) composite, which showed good electrochemical performance. The graphite network and carbon coating not only improve the electrical conductivity, but also provide a buffer for the stress generated by the silicon expansion. Hassan et al. [80] prepared SG by the Hummer method, and added SiNPs, GO, and PAN to mix well and sintered. The layered structure of SG–Si–SG was obtained, and the silicon particles were coated with c-PAN and graphene (Figure 6C). The network structure of graphene and cyclized PAN promote electric charge transfer and improve conductivity. A covalent bond connects Si–S, and the interaction between them also enhances the stability of the long-term cycle. At a current density of 2 A g⁻¹, the specific capacity exceeded 1000 mAh g⁻¹ after 2275 cycles. At the same time, this layered structure separated the electrolyte and SiNPs, stabilizing the formation of SEI, with a Coulombic efficiency of up to 99%. Due to the covalent interaction between silicon and sulfur atoms, the bond between silicon and SG is stronger than that of G. In this structure, SEI films are preferentially formed at the surface defects of graphene, and more stable SEI is formed on the Si surface.

Figure 6. (A) Schematic diagram of a sandwich silicon anode structure. Reproduced with permission [76]. Copyright 2016, Nanoscale. (B) Simulation diagram and cycle curve of Si/MXene composite paper. Reproduced with permission [78]. Copyright 2019, American Chemical Society. (C). Schematic of the preparation process and structure of the SG. Reproduced with permission [80]. Copyright 2015, Nat Commun.

Similar sandwich structure silicon anodes have been widely noted and studied. Huang et al. [81] prepared graphene/carbon nanotubes/silicon (G/CNT/Si) sandwich structure anodes without binder. The G/CNT/Si composite sandwich structure provides a new strategy for paper-based electrodes. Zhang et al. [82] prepared parallelly oriented graphene-sandwiched layered silicon/graphene hybrid microparticles. Combined with the simple filtration-grinding-filtration method, it provides a new idea for the improvement of graphene in silicon anode. Huang et al. [83] used in situ polymerized electron-conducting PAni hydrogel to connect SiNPs and graphene sheets to prepare the three-dimensional sandwich structure of Si/Polyaniline/Graphene (Figure 7A). This sandwich structure has high conductivity and elasticity, which can accommodate the large volume expansion of silicon. After 50 cycles, the structure of the Si/Polyaniline/Graphene did not rupture. The Si/Polyaniline/Graphene showed a discharge-specific capacity of 2708 mAh g⁻¹ in the first cycle at 160 mA g⁻¹. After several cycles, the specific capacity stabilized at around 1400 mAh g⁻¹. There is no significant decrease in capacity after even 250 cycles. The special feature of this sandwich structure is that the connection between SiNP and graphene is formed by conducting APni hydrogel as an “adhesive”.

Wei et al. [84] prepared a sandwiched silicon anode that protects silicon under a soft multiwall carbon nanotube (MWCNT) membrane. Liu et al. [85] designed a Si/Reduced Graphite Oxide (Si/rGO) bilayer nanomembrane sandwich structure (Figure 7B). The characteristic of this structure is that the sandwich structure curls into a multi-layer structure. The gaps within the structure can accommodate volume expansion, and the close combination of rGO and Si increases the conductivity of electrons while preventing excessive SEI caused by direct contact between Si and electrolyte. At a current density of 3 A g⁻¹, the discharge-specific capacity of the first cycle is 1642 mAh g⁻¹. After 2000 cycles, it is found that the specific capacity loss rate for every 100 cycles is only 3.3%. Unlike other sandwich structures, this electrode consists of only Si and rGO layers, which are crimped to form a sandwich structure. The alternating sandwich structure improves the migration of electrons and also buffers the volume expansion of silicon. Meanwhile, such a structure could prevent the accumulation of silicon particles.

Huang et al. [86] prepared a symmetrical sandwich structure SiN/Si/SiN composite anode with good cycling performance. Jia et al. [87] used self-assembly technology to prepare biomimetic sandwich structure carbon/silicon/TiOx nanofiber composites. Based on self-assembled biomimetic-inspired materials, the excellent electrochemical performance proves that biomimetic materials have great potential in the development of high-performance lithium-ion batteries. Zhang et al. [88] prepared a carbon/silicon/hematite (C–Si–Fe₂O₃) multilayer electrode by simple mixing and heat treatment (Figure 7C). Dissolving FeCl₃·6H₂O in oleic acid and undergoing simple treatment, carbon sheets with iron oxide embeddings can be obtained. Finally, silicon is mixed and annealed to obtain electrodes with silicon sandwiched between the carbon sheets. Iron oxide expands in addition to silicon in this multi-layer electrode, but the space between the carbon layers can accommodate huge volume expansion. At the same time, the carbon layer also improves the flexibility and conductivity of the electrode. At a current density of 750 mA g⁻¹, a high specific capacity of 1980 mAh g⁻¹ could be obtained after 250 cycles.

Table 5 summarizes and compares the above electrochemical performance of silicon anode material modified by sandwich structure.

Figure 7. (A) Fabrication process for 3D Sandwich-like Si/Polyaniline/Graphene nanoarchitecture. Reproduced with permission [83]. Copyright 2018, Ceramics International. (B) Schematic fabrication process of the rolled-up Si/rGO bilayer nanomembranes. Reproduced with permission [85]. Copyright 2015, ACS Nano. (C) Schematic of the prepared C-Si-Fe₂O₃ composite. Reproduced with permission [88]. Copyright 2016, Journal of Materials Chemistry.

Table 5. Electrochemical performance of silicon anode material modified by sandwich structure.

Sample	Synthesis Method	Cycling Stability
Sample	Synthesis Method	Discharge Capacity [mA h g⁻¹]	After nth Cycle	Current Density	Ref.
sandwich electrode	many times coated	1090	30	100 mA g⁻¹	[75]
sandwich structure electrode	drop and press and calender	1000	400	0.5 A g⁻¹	[76]
Si/Ti₃C₂Tx	etching and electrostatic self-assembly	643.8	100	300 mA g⁻¹	[77]
Si/MXene	etching and vaccum filtration	1672	200	1000 mA g⁻¹	[78]
MS-G@C	ball milling and thermally treated	830	100	0.5 C	[79]
SG-Si	Hummer method and thermally treated	1033	2275	2 A g⁻¹	[80]
G/CNT/Si	ball milling and thermally treat	420	60	808 mA g⁻¹	[81]
Si/HG@LG	high-temperature annealing and grinding	1050 mA h cm⁻³	300	2000 mA g⁻¹	[82]
Si/Polyaniline/Graphene	Ultrasonication and mix	1400	250	160 mA g⁻¹	[83]
Si/rGO bilayer nanomembrane	Hummer method and sequentially deposit	571	2000	3 A g^–1	[85]
SiN/Si/SiN	PE-CVD and thermal annealing	1702	100	0.6 C	[86]
carbon/silicon/TiO_x	deposition and carbonization and Mg reduction	792.6	160	100 mA g⁻¹	[87]
C–Si–Fe₂O₃	mixing and thermally treat	1980.5	220	750 mA g⁻¹	[88]

5. Artificial SEI

The severe capacity degradation of silicon anode lithium-ion batteries is not only due to the expansion and pulverization of silicon particles, but also partly due to the rupture and regeneration of SEI caused by volume expansion, which continuously consumes lithium ions. The continuous thickening of SEI is also not conducive to ion transport. SEI can transport lithium ions while isolating electrons. An ideal SEI should have high ion transport capability and low electron transport capability. Artificial SEI, also known as an artificial solid electrolyte, prevents direct contact between silicon particles and the electrolyte. Some artificial SEI can provide a portion of lithium ions during the first cycle, improving the initial Coulombic efficiency of batteries. It can also reduce the capture of lithium in long-term cycles, improving the capacity retention rate and Coulombic efficiency of lithium-ion batteries. Moreover, artificial SEI can also provide stress to confront and buffer the expansion of silicon anodes.

Chen et al. [89] prepared S-containing artificial SEI by nucleophilic reaction of sulfide and ester-based electrolyte (Si@S-ARSEI). The results show that the S-ARSEI layer can effectively inhibit the decomposition of the electrolyte. Li et al. [90] used solid electrolyte material lithium nitride phosphorus oxide (Lipon) to prepare artificial SEI to solve the problem of electrochemical performance degradation. Lipon can transport ions and isolate electrons. It was found that the ideal thickness of the Lipon SEI is 40–50 nm. When the thickness does not exceed 50 nm, the capacity does not significantly decrease after 100 cycles. When the thickness is greater than 50 nm, the reversible capacity decreases due to the additional ion resistance caused by the excessively thick SEI. However, when the thickness is 20 nm, the prevention of charge loss is limited, and the Coulombic efficiency is about 98%. On the contrary, when the thickness exceeds 50 nm, the effect of preventing electrolyte decomposition is significant, and the Coulombic efficiency increases to over 99%. The Lipon SEI could isolate electrons, prevent the electrolyte from continuously decomposing and thus relieve capacity loss and improve Coulombic efficiency. Lipon is an amorphous ceramic material. Although preventing the decomposition of the electrolyte, Lipon cannot adapt to the expansion of silicon electrodes. Sufficient mechanical properties should be considered for commercial artificial SEI.

In the study of Wang et al. [91], N-containing functional groups form a uniform and thin layer on the surface after strong interaction with Si particles, and decompose to form an N-rich inorganic solid electrolyte interface (SEI) layer. Using PVA adhesives, PVAm wraps Si particles and preferentially reacts with Li ions to form N-rich SEI. Bolloju et al. [92] reported a novel artificial SEI by adding polymer artificial SEI (A-SEI) to the porous structure of Si/GR composites (Figure 8A). The A-SEI through a simple and scalable early wetting impregnation (IWI) method, formed by in situ polymerization of SCS and GA. This preparation method induces more porosity, prevents the electrolyte from entering the porous structure, and prevents direct contact between Si and the electrolyte. Adding A-SEI improves cycle stability, and batteries containing 3 wt% polymers perform best. After 300 cycles, the capacity retention rate was 74.1%.

To inhibit the reaction of solid electrolytes, Ronneburg [93] designed alumina as artificial SEI. Except for alumina, covalent organic frameworks (COFs) are another kind of widely used artificial SEI due to their high porosity and controllable shape. Ai et al. [94] synthesized COF artificial SEI on the surface of Si particles using a two-step method (Figure 8B). The SEI coating of COF effectively reduces the decomposition of the electrolyte, resulting in higher Coulombic efficiency and cycling stability of Si electrodes, as well as improving rate performance. Meanwhile, the SEI coating of COF improves ion transfer efficiency. At a high current density of 2 A g⁻¹, the COF-coated electrode (Si@COF NPs) showed a reversible specific capacity of 1864 mAh g⁻¹ and a cycling retention rate of 60% after 1000 cycles. Si@COF NPs has a higher vibrational density than Si NPs and is highly conductive, improving ion transport.

Cao et al. [95] designed a multifunctional artificial solid electrolyte, which was processed and finally self-polymerized to obtain poly-4-trifluoromethylphenylboronic acid (PTFPBA) with repeated B–O chains. TFPBA molecules have a rich chain structure, in which C–F and benzene ring provide toughness to SEI. Zhou et al. [96] directly constructed a multifunctional polypyrrole protective layer on the surface of silicon nanoparticles as artificial SEI. Chen et al. [97] constructed a multi-component puzzle-shaped artificial SEI by combining fluoro silane and polyether silane (Figure 8C). The application of fluorinated groups produces a uniform and dense structure for the electrode, and ethylene glycol as the main chain promotes ion transport efficiency. The multi-component spliced network improves the mechanical properties of the SEI structure. This SEI structure allows the electrode to maintain stable lithium removal and insertion for up to 500 h in a corrosive carbonate-based electrolyte.

Jin et al. [98] designed a self-repairing artificial SEI (aSEI) that includes Si in TiO₂ shells and thin carbon shells (Figure 8D), with pores between the two, which can accommodate the expansion of silicon particles and inject electrolyte. Initially, TiO₂ may not be completely sealed, leading to the electrolyte influx. However, during the first cycle, the expansion of silicon will squeeze out the electrolyte, and the contact between the electrolyte and silicon will generate natural SEI (nSEI), which will automatically repair aSEI. At 0.5 C, the specific capacity exceeds 990 mAh g⁻¹, and the Coulombic efficiency is above 99.9% after 1500 cycles. When the SEI breaks, the penetrative electrolyte will be squeezed out by silicon expansion. This dynamic design is very advanced.

Harpak et al. [99] deposited the parylene layer as an artificial elastic SEI layer for three-dimensional nano silicon anodes. Parylene F has excellent elasticity and can act as a protective layer for Si. Mu et al. [100] prepared artificial SEI through molecular layer deposition (MLD), depositing a conformal polyurea layer with hydrogen bonds and polar functional groups on a silicon electrode (Figure 8E). This organic coating improves the ion diffusion rate, which is conducive to forming stable SEI. At the same time, the mechanical properties of the interface buffer the expansion of silicon volume, ensuring the integrity of the electrode. The electrode with added coating has good cycling stability and rate performance, with a specific capacity of 1010 mAh g⁻¹ at a current density of 800 mA g⁻¹ after 1000 cycles.

Table 6 summarizes and compares the above electrochemical performance of silicon anode material modified by artificial SEI.

Figure 8. (A) Schematic diagram of in situ crosslinking polymer coating on Si/Gr composite particles using the IWI method. Reproduced with permission [92]. Copyright 2023, American Chemical Society. (B) Schematic illustration of preparation of Si@COF NPs. Reproduced with permission [94]. Copyright 2020, Nano Energy. (C) Schematic illustration of designing multi-component jigsaw-like artificial SEI, where the fluorine-donating group (i.e., –CF₃) could yield a dense structure to prevent the formation of Li dendrites and the decomposition of electrolytes, the ethylene glycol backbone is favorable for rapid Li⁺ transport, the robust cross-linking network could endow the SEI layer with excellent mechanical strength. Reproduced with permission [97]. Copyright 2023, Commun Mater. (D) Schematic illustrations of self-healing in a yolk-shell SiMA. Reproduced with permission [98]. Copyright 2017, Energy & Environmental Science. (E) The preparation process diagram of the MLD-polyurea coated silicon electrode. Reproduced with permission [100]. Copyright 2022, Nano Energy.

6. Summary and Outlook

In response to the problem caused by excessive volume expansion of silicon anode, this article summarizes the research on the structural design and optimization of silicon anodes from the aspects of reducing the dimension of silicon particles, optimizing the surface structure of silicon anodes, and artificial SEI. It is worth mentioning that except for the typical structure optimization approaches summarized in this work, there are still many promising structural designing strategies for silicon anodes, such as prelithiation, composite with metal-oxides, hierarchical structures etc. The combination of multiple strategies may be more effective to solve the problems of silicon anodes, which is an important future research direction of the structural design and optimization of such anodes.

Silicon particles with smaller sizes have a larger specific surface area and smaller expansion stress, resulting in a smaller volume expansion effect. Many studies have prepared silicon into various nanoscale structures, and the results have shown that nanoscale silicon particles have smaller expansion and lower degrees of pulverization. Nanosized particles shorten the ion migration distance and improve reaction efficiency, thus creating higher cycling stability and specific capacity.

There are two primary purposes for optimizing silicon particles from the surface. On the one hand, it provides a buffering effect for the expansion of silicon particles. On the other hand, it prevents direct contact between silicon particles and the electrolyte. Most silicon anode surface optimizations are achieved through various forms of carbon, and carbon can also improve the poor conductivity of silicon particles.

Unlike graphite anodes, silicon anodes cannot form a passive SEI, and volume expansion leads to the rupture of SEI. Si contacting with the electrolyte will continuously regenerate SEI and consume some lithium ions, leading to severe capacity fade and a decrease in Coulombic efficiency. The addition of artificial SEI could solve these problems by preventing Si from contact with the electrolyte, which reduces electrolyte consumption, and improves specific capacity and cycling performance.

Except for addressing the issues caused by volume expansion and poor conductivity of silicon anode, achieving commercialization requires a simple and low-cost preparation technology. Preparing nano silicon particles or nanocomposite structures is insufficient to achieve this goal. In future research, while improving electrochemical performance, simple and scalable methods should be explored to achieve the commercialization goal of silicon anode.

It should be noted that realizing the commercial application of silicon anodes in lithium-ion batteries cannot rely on only structural design and optimization of silicon anodes themselves. Lithium-ion battery is a complex energy storage system that involves the electrolyte, anodes, cathodes and the match among them. Especially the electrolyte, which directly contacts with silicon anodes, and has a remarkable influence on the performances of silicon anodes. Combining structure optimization of silicon anodes and electrolyte modification can improve the practical application performances of the silicon anode-based lithium-ion battery. It is also an important future research direction in the field of silicon anodes.

In the context of high oil prices and carbon neutrality, the penetration rate of new energy vehicles has increased, and the global electric vehicle market is expanding. Abundant, inexpensive, environmentally friendly, silicon-based materials with high specific capacity are moving towards the next generation of lithium-ion battery anode. At present, the content of silicon in the commercial silicon-based anode is low, and there is still a lot of room for development. Reducing the size of silicon particles and constructing porous structures will become the driving force for the commercial development of silicon anodes, and successfully open the market door to silicon-based anodes.

Author Contributions

Conceptualization, Z.Y.; methodology, Z.Y., L.C.; software, Z.Y. and L.C.; validation, Z.Y., L.C., B.Z. and G.Q.; formal analysis, B.Z.; investigation, Z.Y. and B.Z.; resources, G.Q.; data curation, B.Z.; writing—original draft preparation, Z.Y.; writing—review and editing, G.Q.; visualization, L.C.; supervision, G.Q.; project administration, Z.Y. and G.Q.; funding acquisition, G.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Thousand Talents Plan Project of Jiangxi Province (jxsq2020101053 to G.Q.) and supported by Jiangxi Provincial Natural Science Foundation (20232BAB213012).

Acknowledgments

The authors would like to acknowledge the support from the open fund of the key laboratory of advanced energy materials chemistry of Nankai University.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Table 6. Electrochemical performance of silicon anode material modified by artificial SEI.

Sample	Synthesis Method	Cycling Stability
Sample	Synthesis Method	Discharge Capacity [mA h g⁻¹]	After nth Cycle	Current Density	Ref.
Si@S-ARSEI	grinding and thermally treat	1387	500	0.5 C	[89]
Si coated with Lipon	radio-frequency magnetron sputtering	1600	100	0.5 C	[90]
Si-PVAm	mix	2000	200	0.1 C	[91]
Si/Gr	IWI	550	300	1 C	[92]
Si@COF NPs	solvothermal polymerization	1864	1000	2 A g⁻¹	[94]
Si@TTFPB	wet coating and heating	1778	500	0.2 C	[95]
Si-e-PPy	stirring and sonication	1153.2	500	1 A g⁻¹	[96]
FO–Si@Li (Full battery)	uniform mixing	130	500	1 C	[97]
Si@TiO₂	hydrothermal and calcination	990	1500	0.5 C	[98]
parylene F coated Si	CVD and parylene coating	500	468	2 C	[99]
Si@ 25-PU	MLD	1010	1000	800 mA g⁻¹	[100]

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Yu, Z.; Cui, L.; Zhong, B.; Qu, G. Research Progress on the Structural Design and Optimization of Silicon Anodes for Lithium-Ion Batteries: A Mini-Review. Coatings 2023, 13, 1502. https://doi.org/10.3390/coatings13091502

AMA Style

Yu Z, Cui L, Zhong B, Qu G. Research Progress on the Structural Design and Optimization of Silicon Anodes for Lithium-Ion Batteries: A Mini-Review. Coatings. 2023; 13(9):1502. https://doi.org/10.3390/coatings13091502

Chicago/Turabian Style

Yu, Zhi, Lijiang Cui, Bo Zhong, and Guoxing Qu. 2023. "Research Progress on the Structural Design and Optimization of Silicon Anodes for Lithium-Ion Batteries: A Mini-Review" Coatings 13, no. 9: 1502. https://doi.org/10.3390/coatings13091502

APA Style

Yu, Z., Cui, L., Zhong, B., & Qu, G. (2023). Research Progress on the Structural Design and Optimization of Silicon Anodes for Lithium-Ion Batteries: A Mini-Review. Coatings, 13(9), 1502. https://doi.org/10.3390/coatings13091502

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Research Progress on the Structural Design and Optimization of Silicon Anodes for Lithium-Ion Batteries: A Mini-Review

Abstract

1. Introduction

2. Problems with Silicon Anodes

3. Silicon Anode Structure Optimization

3.1. Size Reduction

3.2. Porous Structure

3.3. Binder/Current Collector Free Silicon Anode

4. Optimization of Surface Structure of Silicon Anode

4.1. Coating Structure

4.2. Sandwich Structure

5. Artificial SEI

6. Summary and Outlook

Author Contributions

Funding

Acknowledgments

Conflicts of Interest

References

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