Recent progress of silicon composites as anode materials for secondary batteries

Abstract

Silicon is recognized as one of the most promising anode materials for lithium-ion batteries because of its extremely high theoretical capacity, low delithiation potential and abundant availability. However, Si experiences a huge volume expansion during lithiation and delithiation processes, which results in rapid capacity degradation and very poor cycling performance. Numerous studies have been conducted to circumvent these problems through the utilization of various silicon composite materials. Furthermore, suitable materials as well as proper design of nanostructures are a great challenge at this time. This review mainly focuses on the latest research achievements of Si composites and their nanostructures, including Si–carbon, Si–metal, Si–transition metal oxides and Si–polymer, which can improve the electrochemical performance of lithium-ion batteries effectively. In addition, the latest application of Si to other secondary batteries, such as sodium-ion batteries and magnesium-ion batteries is also included. Moreover, this review describes the remaining issues and future study directions.

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Jingjing Wang

Jingjing Wang is currently a Master student under the supervision of Prof. Tingli Ma in the Graduate School of Life Science and Systems Engineering Kyushu Institute of Technology (KIT) in Japan since Oct 2014. She received her Bachelor Degree in Mechanical Engineering College, Yangzhou University, China, and majored in Automotive Engineering from Sep 2010 to Jun 2014. Now, her research interests mainly focus on anode materials for secondary batteries including lithium-ion batteries and sodium-ion batteries.

Tingting Xu

Dr. Tingting Xu is currently an associate professor in the Applied Chemistry Department at the School of Science, Northwestern Polytechnical University (NPU), Xi'an, P. R. China. She completed her PhD in Photovoltaics in the Electrical Engineering department at South Dakota State University (SDSU) in August 2012. She worked as a postdoctoral research associate at Kyushu Institute of Technology (KIT) in 2015–2016. She was awarded the Chinese Government Award for Outstanding Self-financed Students Abroad in 2011. Her research interests mainly focus on renewable energy including solar cells and batteries.

Xiao Huang

Xiao Huang is currently a PhD student in the Graduate School of Life Science and Systems Engineering at Kyushu Institute of Technology (KIT) in Japan since Oct. 2015. She completed her Masters Degree in the Institute of New Energy Material Chemistry, Nankai University, China, and majored in Li-ion battery from Sep. 2012 to Jun. 2015. She was awarded by the China Scholarship Council as a high level research scholar. Now, her research project deals with not only Li-ion batteries but also Na-ion batteries.

Huan Li

Huan Li is carrying out PhD research at the Graduate School of Life Science and Systems Engineering Kyushu Institute of Technology under the supervision of Prof. Tingli Ma. Since 2012 she has been actively involved in the development of anode materials for Li-ion and Na-ion batteries. She currently works on the synthesis and characterization of Ti-based materials as the anode for Na-ion batteries.

Tingli Ma

Dr Ma is a professor of chemistry and PhD supervisor at the Graduate School of Life Science and Systems Engineering Kyushu Institute of Technology, in Japan. Since 2007-present. She also works at the State Key Laboratory of Fine Chemicals at Dalian University of Technology in Dalian, China. She leads research teams studying solar energy conversion with over 15 years of experience. She also has interests in research and development of advanced fuel cells, metal ion batteries, and other renewable energy and nano materials.

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1. Introduction

With the intensification of climate change and shortage of energy resources, increasing concerns have been raised for the development of green renewable energy to replace fossil fuels in recent years.¹ The secondary battery, which possesses efficient, clean, and sustainable sources of energy, is regarded as one of the most promising techniques to improve this situation.^2,3 Since its introduction by Sony in 1991, the lithium ion battery (LIB) has been widely used in various portable electronic devices because of its high energy density, long shelf life, no memory effect, and environmental friendliness. Currently, it acts as a promising green power instead of fuel oil, and it has been gradually implemented in the new generation of hybrid electric vehicles (HEV), plug in hybrid electric vehicles (PHEV) and pure electric vehicles (PEV). However, the performance of LIBs cannot meet the present requirement of several properties, such as high energy density, high charge–recharge rate and long cycle life. Therefore, further improvement of the electrode materials and the system of LIBs is necessary.^4–10

There have been several studies on the various sections of LIBs, such as cathode, anode, electrolyte and electrode–electrolyte interface.¹¹ Among these areas, the anode is considered to be a crucial component which can vigorously affect the performance of LIBs.¹² At present, graphite as a commercially available anode material, possesses excellent properties of flat and low working potential vs. Li, high electrical conductivity, low volume change during lithiation/delithiation process and it is also abundant. However, with its practical capacity approaching its theoretical value of 372 mA h g⁻¹, other new anode materials with higher power capacity, longer cycle life and better safety are essential to the development of LIBs.¹³ Currently, a tremendous amount of research has been carried out in the investigation of both carbon and non-carbon materials as anodes in LIBs, such as insertion/de-insertion materials (hard carbon, graphene, TiO₂ etc.), alloy/de-alloy materials (Si, Ge, Sn, Al, SnO₂ etc.), conversion materials (Fe₂O₃, CO₃O₄, CuO etc.) and pure lithium metal.^14,15

Among the various investigated anode candidates, Si has been recognized as one of the most promising materials for LIBs, not only due to its high gravimetric capacity of 4200 mA h g⁻¹, but also due to its moderate working potential, abundance and environmentally benign property.^16–18 The operation principle of the lithium-ion battery based on an Si anode is shown in Fig. 1, which is accompanied with a morphology transformation from crystalline to amorphous during the charge and discharge processes, as follows.^19–21

Fig. 1 The operation principle of the lithium-ion battery.

During the discharge process:

Si (crystalline) + xLi⁺ + xe⁻ → Li_xSi (amorphous) + (3.75 − x)Li⁺ + (3.75 − x)e⁻ → Li₁₅Si₄ (crystalline)

During charge process:

Li₁₅Si₄ (crystalline) → Si (amorphous) + yLi⁺ + ye⁻ + Li₁₅Si₄ (residual)

However, there are still several obstacles that restrict the employment of pristine Si as an anode material in LIBs. Firstly, a large number of lithium ion insertion/extraction results in a huge volume variation (>400%) for the Si material, which leads to structural pulverization, electrical disconnection between the active material and current collector and finally reversible capacity fading quickly.²² As shown in Fig. 2(a), large mechanical stress caused by huge volume expansion, produces cracks, which then spread and lead to the loss of electrical contact and eventual degradation of electrochemical performance.²³

Fig. 2 (a) Failure mechanisms of the MIVA elements (Si, Ge, and Sn) as anode materials for LIBs²³ and (b) schematic of SEI formation on silicon surfaces.²⁵

The instability of the solid-electrolyte interphase (SEI) is another important factor that results in rapid capacity degradation during lithiation and delithiation processes.²⁴ This SEI layer is formed by electrolyte decomposition on the surface of the anode at a low potential during battery discharging, as shown in Fig. 2(b).²⁵ During charging and discharging processes, the large volume variation of Si particles cracks the SEI films, which are broken down into separate pieces, and a sequentially fresh Si surface is exposed to the electrolyte, and thus a new SEI layer continues to grow on the newly exposed Si surface. After several cycles, the SEI films eventually grow to a certain thickness, which decreases the electrochemical reactivity of the electrode, thus leading to irreversible capacity loss. Apart from the large volume change and unstable formation of SEI films, pristine Si possesses quite low electrical conductivity of 6.7 × 10⁻⁴ S cm⁻¹ and lithium ion diffusivity.

To overcome these issues, numerous studies have been conducted for the utilization of various nanostructured Si materials,²⁶ including nanowires,²⁷ nanotubes,²⁸ nanospheres,²⁹ and thin films,³⁰ which may provide high surface areas, free volume for accommodation of the Si expansion, and short lithium ion diffusion path length, and thus an improved capacity retention and rate capability can be obtained. However, the pristine nanostructured Si is likely to generate aggregation and irreversible electrochemical sintering during repetitive lithiation and delithiation.³¹ Moreover, the intrinsic low electrical conductivity and lithium ion diffusivity of pristine Si itself are also other limitation factors. Therefore, further progress has been made with the use of the Si nanostructure combined with composites as anode materials which could not only suppress volume expansion, but also enhance conductivity, and as a consequence realize high energy capacity, long cycle life and rapid charge–discharge capability for LIBs.³² Mostly, the materials studied for Si composites can be divided into four categories, Si–carbon (carbon nanotube,⁴⁰ graphene,⁴³ etc.), Si–metal (Ni,⁵² Cu,⁴⁸ Ag,⁵³ etc.), Si–transition metal oxides (TiO₂,⁵⁹ MoO₃,⁶¹ etc.) and Si–polymer (polydopamine,⁷¹ etc.). This review mainly emphasizes the recent research progress concerning Si composites as anodes for LIBs.

2. Si composites as anode materials in current lithium-ion batteries

2.1 Si/carbon composites

Carbon is an alternative material to composite with Si owing to its good electronic conductivity, small volume change during cycling and relative ease of availability.³³ In recent research, various forms of carbon materials, such as porous carbon,³⁴ carbon nanotubes,³⁵ graphite,³⁶ and graphene,³⁷ have been used to composite Si as a buffer to cushion volume changes during Li⁺ insertion and provide a conductive channel for electron transportation. The combination of the advantages of Si and carbon materials can effectively improve the specific capacity, cycle stability and rate capability for LIBs. Kim et al.³⁸ prepared silicon/carbon core–shell particles (Si@C) by thermal decomposition and subsequently formed three-dimensional (3D) electrode structures by the laser ablation process. This 3D Si@C electrode with a conformal carbon coating and empty voids is effective in mitigating large volume expansion and achieving a stable cycling performance. The enhanced performance exhibited a high specific capacity (>1200 mA h g⁻¹ over 300 cycles) and good rate capability (1170 mA h g⁻¹ at 8 A g⁻¹).

Si/carbon composites with a nano-fibrous structure have better architectural stability compared with nanoparticles, which is attributed to the high intensity, high tenacity and ample space between fibers. Wang et al.³⁹ designed and fabricated a type of novel silicon nanoparticle with porous carbon nanofiber (Si/PCNF) hybrids through a simple electrospinning method. A high specific surface area, fast transport of lithium ions and accommodation for Si volume expansion/shrinkage during cycling were effectively exhibited. As a consequence, the capacity of this Si/PCNF reaches 870 mA h g⁻¹ after 100 cycles at 0.1 A g⁻¹ without the use of binders and conducting additives. This superior electrochemical performance of the Si/PCNF electrode is mainly due to the benefits of uniform distribution of Si in the carbon nanofiber, stable SEI layer and three-dimensional unique porous structure. Wei Wang et al.⁴⁰ synthesized another binder-free connection between silicon decorated carbon nanotube cones and graphene for the preparation of LIB anodes. The innovative three-dimensional cone-shape CNT clusters deposited by amorphous silicon architecture demonstrate high a reversible capacity of 1954 mA h g⁻¹ and excellent cycling stability (>1200 mA h g⁻¹ capacity with ≈100% coulombic efficiency after 230 cycles), which are mainly attributed to the seamless connection that facilitates charge transfer in the system.

Hard carbon is also a type of promising compositing material with Si. Kim et al. successfully dispersed nanocrystalline Si (c-Si) in amorphous Si (a-Si) encapsulating hard carbon (HC) as an anode material for fast chargeable lithium-ion batteries.⁸² The HC@c-Si@a-Si anode showed excellent cycle retention of 97.8% even after 200 cycles at a 1 C discharge/charge rate. Furthermore, the LiCoO₂/HC@c-Si@a-Si full-cell showed excellent rate capability and very stable long-term cycle. Even at a rate of 10 C discharge/charge, the capacity retention of the full-cell was 50.8% of its capacity at a rate of 1 C discharge/charge and showed superior cycle retention of 80% after 160 cycles at a rate of 1 C discharge/charge, which indicates the possibility of its practical application.

Among the various carbon materials, graphene is an admirable candidate to improve the electrochemical performance of LIBs owing to its superior electrical conductivity, large surface area (theoretical value of 2630 m² g⁻¹) and relevant mechanical flexibility.⁴¹ Recent researches show that its sandwiched structure plays a vital role in Si/graphene composites.⁴² In this configuration, the alternately stacked elastic layers of graphene sheets could prevent direct exposure to the electrolyte and the aggregation of Si layers. Meanwhile, the inside void space can well mitigate the volume expansion of the Si material and buffer the strain during lithium insertion/extraction. Mori et al.⁴³ fabricated graphene/silicon multilayer sandwich structures using electron beam deposition without air exposure. Different numbers of layers and thickness of each layer for this sandwich structure have a direct effect on the properties of first discharge capacity values, coulombic efficiencies, and reversible efficiencies of batteries. In their further experiment, they determined that the structure possessing optimal seven layers which are 100 nm thick, achieved the highest discharge capacitance (>1600 mA h g⁻¹) at a current density of 100 mA g⁻¹ after 30 cycles. In addition, after assembling a soft package battery using the fabricated graphene and Si electrode as the anode, LiCoO₂ as the cathode, a separator and liquid electrolyte, it was found that this device could be used for commercial light-emitting diode (LED) lighting even under bending status, which indicates the potential application for thin film flexible electronic devices.

Similar to graphene, reduced graphene oxide (rGO) is also an ideal protective material for electrode materials and has shown great potential in improving battery performance.⁴⁴ Liu et al.⁴⁵ designed a sandwich nano-architecture of rolled-up Si/reduced graphene oxide bilayer nanomembranes via a strain released strategy, as shown in Fig. 3. Within this nanostructure, it is believed that the inner void space inside the configuration together with the mechanical feature of the nanomembranes can help buffer the strain during lithiation/delithiation. Furthermore, the alternately aligned rGO layers can not only enhance electrical conductivity, effectively release the volume change and aggregation of Si nanoparticles, but also can protect the nanomembranes from the excessive formation of thick SEI layers during the charge/discharge process. As anodes for lithium-ion batteries, this sandwiched Si/rGO nano-architecture demonstrates a long cycling life of 2000 cycles at 3 A g⁻¹ with a capacity degradation of only 3.3% per 100 cycles.

Fig. 3 (1) Schematic fabrication process of the rolled-up Si/rGO bilayer nanomembranes and (2) cycling performance at (a) 100 mA g⁻¹ (in comparison with the Si nanomembrane electrode), and (b) 1 A g⁻¹, and (c) 3 A g⁻¹.⁴⁵

To achieve a better electrochemical performance for LIBs, further research has been carried out on multiple composites. Wu et al.⁴⁶ synthesized a novel binder-free Si/carbon composite film consisting of alternately stacked Si–porous carbon layers and graphene layers by electrostatic spray deposition and subsequent heat treatment. This hierarchical architecture demonstrates a maximum reversible capacity of 1020 mA h g⁻¹ with 75% capacity retention after 100 cycles and good rate capability. These excellent electrochemical performances are attributed to the fact that the layer-by-layer porous carbon framework can efficiently suppress the volume expansion of Si particles, whereas the flexible graphene layer is able to facilitate electron conductivity and maintain the structural integrity of the electrode. This simple and controllable approach is demonstrated to be an effective way to alleviate the effects of volume changes of Si-based anode materials.

2.2 Si/metal composites

Metal material is another promising substance to composite with Si due to its good malleability, high electronic conductivity and high mechanical strength. For Si/metal composites, the morphology of metal materials can be original, or alloyed to metal silicides, such as an FeCuSi ternary composite.⁴⁷ In this composite system, the uniform dispersion of Si in the metal matrix is capable of suppressing the volume change during lithium ion intercalation and deintercalation and increasing electronic conductivity. Polat et al.⁴⁸ formed well-aligned nanorods of a multi-layered Cu/Si film using an ion assisted oblique angle (80°) electron beam deposition method. For comparison, a flat multilayered Cu/Si film was coated at a 0° angle with the same process. The results indicate that the flat thin film delivers 1100 mA h g⁻¹ as the first discharge capacity, then decreases to 150 mA h g⁻¹ in 10 cycles and retains that level for 100 cycles. On the other hand, the well aligned nanorods exhibit a better performance of 1700 mA h g⁻¹ as an initial discharge capacity, then decrease to 500 mA h g⁻¹ after the 1st cycle and become stable up to 100 cycles. The main reasons for this improved cycling capability are amorphous and nano-sized particles, the electron conductive pathway induced by Cu atoms along the coating, and homogenously distributed nanosized pores in the thin film.

Numerous successful 1D conductive nanostructure materials have been achieved, such as nanotubes,⁴⁹ nanocables,⁵⁰ and nanorods.⁵¹ However, the weak structure connection between these nanostructures and the current collectors results in capacity degradation after long term cycles. Given this, Wang et al.⁵² designed a novel 2D Ni/Si nanosheet network on Ni foam by combining Si film with an interconnected Ni nanosheet network, as shown in Fig. 4. This unique composite anode possesses large pores between the nanosheets, high conductivity and robust adhesion, which effectively facilitate the large volume vibration of Si during cycling, and transportation of both electrons and lithium ions. With these features, excellent performances were demonstrated such as a high specific capacity of 2038 mA h g⁻¹ at 840 mA g⁻¹ after 100 cycles, good rate capability of 803 mA h g⁻¹ at 16 800 mA g⁻¹ and remarkable cycling stability of 655 mA h g⁻¹ at 8400 mA g⁻¹ over 1000 cycles. It is believed that this developed electrode architecture promises great application potential for next generation of LIBs.

Fig. 4 Fabrication process for the Ni/Si nanosheet network.⁵²

Compared with the traditional preparation methods for porous Si and Si-based composites, Hao and his co-workers employed a one-step de-alloying method to fabricate a bimodal porous Si/Ag composite, which is easily operated and environmentally benign.⁵³ Furthermore, it is convenient to realize the controllable components and uniform distribution of Si and Ag in the product. Owing to the rich porosity of the unique bimodal porous structure and the incorporation of highly conductive Ag, the as-prepared Si/Ag composite possesses improved conductivity and alleviates volume changes of the Si network during repeated charging and discharging. As expected, this bimodal porous Si/Ag anode delivered a stable reversible capacity of above 1000 mA h g⁻¹ at a current rate of 1 A g⁻¹, and exhibited a capacity retention up to 89.2% of the highest capacity after 200 cycles.

A further strategy to improve the electrochemical performance of silicon is the formation of multiphase composite materials, such as Si/metal/carbon, which can conflate the merits of both metal and carbon. Zhang et al.⁵⁴ prepared a Si₂₀Co₁₀C₇₀ composite material by a simple high energy mechanical milling process. The uniformly dispersed cobalt nanoparticles act as an inactive matrix and can relieve the destruction of the graphite. Furthermore, the synergistic effect of carbon and cobalt plays an important role in alleviating the agglomeration and suppressing the volume expansion of Si efficiently. The electrochemical results show that the Si/Co/C anode obtained the best performance with a reversible capacity above 610 mA h g⁻¹ after 50 cycles.

A novel, porous NiSi₂/Si/carbon composite with a core–shell morphology was successfully prepared using a facile ball milling method by Jia et al.⁵⁵ After surface carbon coating by the CVD method, electrochemical characterization indicates that this carbon layer can enhance the surface electronic conductivity and stabilize the whole composite structure. Furthermore, the porous and core–shell structure can effectively accommodate the severe volume variation of silicon during the lithiation/delithiation process. As expected, this as-prepared NiSi₂/Si/carbon composite anode material displays an outstanding electrochemical performance, which gives a stable specific capacity of 1272 mA h g⁻¹ for 200 cycles at a charge–discharge rate of 1 C and a good rate capability with a reversible capacity of 740 mA h g⁻¹ at a rate of 5 C.

The sandwich structure design can also make contributions to the improvement of the electrochemical performance in terms of rate capability and cycle stability. Gao et al.⁵⁶ designed and fabricated a sandwich Ni/Si-nanoparticles/graphite clothing (Ni/Si-NPs/GC) nano-architecture electrode, where a mechanically robust, flexible graphite clothing (GC) was grown directly on an Ni foam coating with Si nanoparticles (Si-NPs). This unique architecture, in which Si-NPs are sandwiched between the Ni matrix and graphite clothing, offers efficient ion diffusion, high conductivity, and structure durability during cycling. As desired, this electrode shows an outstanding electrochemical performance, and reversible capacity of 1800 mA h g⁻¹ at 2 A g⁻¹ after 500 cycles. In addition, this simplified process without any polymeric binder and conductive additives holds great potential for application in next generation Li-ion batteries.

2.3 Si/transition metal oxides composites

Transition metal oxides are found to be potential substitutes for graphite as anode materials in LIBs owing to their wide ranging electronic, chemical and mechanical properties.⁵⁷ Nowadays much interest has been aroused in nano-structured TiO₂ for obtaining fast lithium insertion/removal due to its excellent safety, stability and cost effectiveness.⁵⁸ McCormac et al.⁵⁹ encapsulated Si nanoparticles within a highly porous TiO₂ nanofiber matrix using a unique sulphur-templating method combined with electrospinning. After electrochemical characterization, it was determined that the Si/TiO₂ composite nanofibers possess specific surface area as large as 387 m² g⁻¹ and the outside porous TiO₂ nanofiber matrix is able to compensate for ∼300% volume expansion during the Si lithiation/delithiation process. Therefore, the obtained Si/TiO₂ porous nanofibers display an outstanding electrochemical performance, which gives a stable specific capacity of above 1586 mA h g⁻¹ even after 180 cycles at 0.135 A g⁻¹.

Molybdenum oxide (MoO₃), which has a layered structure, is also a potential durable material to composite Si because there is little chemical and mechanical degradation during repeated lithium ion insertion and de-insertion.⁶⁰ Martinez-Garcia et al.⁶¹ designed a silicon decorated MoO₃ nanoplatelet composite material using a hot-wire chemical vapor deposition and ultrasonic spray method, which is desirable to realize a high rate and durable, binder free anode in lithium-ion batteries. The specific capacity of the as-synthesized silicon decorated MoO₃ started at 1475 mA h g⁻¹ and dropped to about 1000 mA h g⁻¹ at a cycling rate of 100 mA g⁻¹ in the second cycle, and then it retained this capacity for over 50 cycles. More importantly, even at a high current rate of 10 A g⁻¹, the charge capacity in the silicon sprayed MoO₃ composite remained above 1037 mA h g⁻¹ after 50 cycles, which means there is almost no steady state capacity fading during cycling. This experimental result is very useful in guiding the preparation of fast-charging Si-based anode materials with a good storage performance using a simple synthetic process.

Further research was carried out on the synthesis of multiphase composite materials. Li et al.⁶² fabricated hollow carbon nanospheres/silicon/alumina (CNS/Si/Al₂O₃) core–shell films by depositing Si and Al₂O₃ on hollow CNS interconnected films, which act as anode materials of lithium-ion batteries, as shown in Fig. 5. The electrochemical results exhibit a high specific capacity and remarkable capacity retention of 1560 mA h g⁻¹ after 100 cycles at a current density of 1 A g⁻¹. Moreover, even at a rate of 8 A g⁻¹, this as-fabricated electrode can still achieve a discharge capacity of 854 mA h g⁻¹. These excellent performances are mainly attributed to the structure of this material, where the hollow CNS film acts as a three dimensional conductive substrate and thus provides void space for silicon volume expansion. The Al₂O₃ thin layer is beneficial to the reduction of solid-electrolyte interphase (SEI) formation and the robust surface-to-surface contact between Si and CNSs is able to facilitate fast electron transport during electrochemical cycling.

Fig. 5 (1) Schematic of hollow CNS/Si/Al₂O₃ core–shell film fabrication processes. (2) Electrochemical characterization of different electrodes. Voltage profiles of (a) CNS/Si and (b) CNS/Si/Al₂O₃ with the potential window of 0.01–1 V. The current densities are 0.2 A g⁻¹ for the 1st cycle, 0.5 A g⁻¹ for the 2nd and 3rd cycles and 1 A g⁻¹ for the other cycles. (c) Charge/discharge capacity of four electrodes over 100 cycles: Si film (black squares), CNS/Si with Si mass percent of 22.1% (red circles), 62.9% (blue triangles) and CNS/Si/Al₂O₃ with Si mass percent of 62.5% (green stars). (d) Charge/discharge capacity and coulombic efficiency of CNS/Si/Al₂O₃ at high current densities ranging from 0.5 A g⁻¹ to 8 A g⁻¹.⁵⁷

2.4 Si/polymer composites

Conductive polymers, such as polythiophene,⁶³ polypyrrole⁶⁴ and polyaniline,⁶⁵ have also been used as effective coatings for nanoparticles due to their good conductivity, flexible mechanical property and easy structure modification. The silicon nanowire anodes with conducting polymer coatings fabricated by the Cui group demonstrated that such a polymer coating on Si not only functions as a buffer to cushion volume changes during lithium ion insertion, but also provides a conductive matrix to ensure a good cycling performance.⁶⁶ However, it still remains a challenge to realize the facile and cost effective fabrication of nanostructured composites of silicon and polymers as anode materials.⁶⁷

On the basis of this research, the addition of carbon materials in Si/polymer composites would lead to enhanced electro-chemical performances. Wang et al.⁶⁸ prepared Si@PVP–GCB [GCB = graphitized carbon black and PVP = poly N-vinyl-2-pyrrolidone] by a one step-assembly technique, in which a well-connected three dimensional nanoporous GCB network was uniformly embedded with core–shell nanoparticles with a silicon nanoparticle (SiNPs) core and PVP shell. In this work, it is believed that the GCB 3D network and PVP coating shell not only contributed to high electrical conductivity, but also alleviated the volume variation of the SiNPs effectively. As a result, the obtained Si@PVP–GCB delivered a reversible capacity of 545 mA h g⁻¹ at a current density of 50 mA g⁻¹ after 100 cycles, which is much higher than that of commercial graphite microspheres (GMs, 372 mA h g⁻¹).

Lee et al. demonstrated the surface engineering of sponge-like silicon particles by introducing a double coating layer consisting of electrically conductive carbon and ionically conductive polyimide.⁷⁰ The Si electrodes with a double shell exhibited high electrochemical performances, including a high specific capacity of 1780 mA h g⁻¹ after 80 cycles and excellent rate capability (a capacity retention of 89.1% at 10 C, compared to a 0.2 C rate). Moreover, the Si electrode with a double coating layer showed high thermal stability, compared to the carbon coated Si electrode. It is believed that this double layer coating process may open up an effective route for the fabrication of high performance lithium-ion battery anodes.

Fang et al.⁷¹ synthesized a graphene oxide/polydopamine-coated Si nanocomposite (GO/PDA-Si) by a novel facile solution-based chemical method. The combination of the PDA coating layer and GO can serve as a cushion to accommodate volume variation and inhibit direct contact with a liquid electrolyte of Si nanoparticles during repeated cycling, and thus play an important part in improving the electrochemical lithium storage performance. As desired, this GO/PDA-Si anode exhibits a high reversible specific capacity and excellent cycling stability of 1074 mA h g⁻¹ after 300 cycles at 2100 mA g⁻¹. Furthermore, this type of Si-based anode material paves a way to realize long-cycle-life and good rate performance using a simple surface-modification process for commercial lithium-ion batteries.

Polymers could also act as both binder and conductive additive for negative electrodes in lithium-ion batteries. Higgins et al. investigated the properties of negative electrodes consisting of silicon nanoparticles partnered with PEDOT:PSS, which here acts as a conductive binder, as shown in Fig. 6.⁶⁹ Even at a relatively high areal loading of 1 mg cm⁻², this system demonstrated a first cycle lithiation capacity of 3685 mA h g⁻¹ (based on the SiNP mass) and a first cycle efficiency of ∼78%. After 100 repeated cycles at 1 A g⁻¹, this electrode was still able to store an impressive 1950 mA h g⁻¹ normalized to Si mass. This excellent property could be attributed to the fact that this combination eliminates the occurrence of capacity loss due to physical separation of the silicon and traditional inorganic conductive additives during repeated lithiation/delithiation processes. An overview of the most recent studies of various Si-based anode materials for lithium-ion batteries is given in Table 1.^72–89

Fig. 6 (1) Scheme for the experimental PEDOT:PSS/SiNP electrode and high-magnification SEM image. (2) (A) Electrical conductivity for a 20 wt% PEDOT:PSS/SiNP sample with different amounts of FA secondary dopant added during slurry preparation (expressed as a percentage of the PEDOT:PSS volume). Electrochemical characterization of CB/Li-PAA/SiNP and PEDOT:PSS/SiNP composites with various FA treatment conditions. (B) Initial lithiation/delithiation profiles at a current density of 0.5 A g⁻¹ within 0.005 and 1.2 V vs. Li/Li⁺. Inset shows the enlarged lithiation profiles for all electrodes. (C) Cyclability of lithiation of these electrodes at 1 A g⁻¹ after initial cycling at 0.5 A g⁻¹. (D) Rate capability of these electrodes at various current densities from 0.5 to 5 A g⁻¹. In all cases, areal loadings of 1 mg cm⁻² were used.⁶⁹

Table 1 Electrochemical performance of Si composite anodes for lithium-ion batteries

	Anode	Specific capacity (mA h g^−1)	Cycling stability (mA h g^−1)	Rate capability (mA h g^−1)	Structure	Method	Ref.
Carbon	Si/C	1961	1045 at 100 mA g^−1 after 20 cycles	—	Porous composite nanofiber	Electrospinning and foaming processes	72
	Si/C	1909	1179 at 0.1 C after 120 cycles	493 at 4 C	Peanut shell	Ball milling and carbonization	73
	Si/C	1557	1391 at 0.54 A g^−1 after 100 cycles	1133 at 1.44 A g^−1	Interconnected core–shell composite films	Mixing, coating, and subsequent pyrolization	89
	Si@C	2300	>1200 at 0.1 A g^−1 after 300 cycles	1170 at 8 A g^−1 after 25 cycles	Core–shell 3D channel architecture	Thermal decomposition and laser ablation process	38
	Si/CNT	1954	≈1260 at 0.25 C after 230 cycles	≈400 at 8 C	Cone-shape clusters	Chemical vapor deposition and magnetron sputtering	82
	MWCNT@Si	1547	520 at 400 mA g^−1 after 70 cycles	536 at 1600 mA g^−1	Core–shell nanocable	Acid vapor steaming and magnesiothermic reduction	74
	Si/PCNF	2071	870 at 0.1 A g^−1 after 100 cycles	405 at 5 A g^−1	Nanoparticle/porous nanofiber hybrids	Electrospinning	39
	Hard carbon @c-Si@a-Si	964.2	—	Cycle retention of 97.8% at 1 C after 200 cycles	Core–shell	Chemical vapour deposition	40
	Si/graphite	1702.9	975.7 at 100 mA g^−1 after 100 cycles	672.2 at 2000 mA g^−1	Sheet-like morphology	Magnesium thermal reduction	75
	Si/graphene	1665	>1600 at 100 mA g^−1 after 30 cycles	—	Multilayer sandwich structure	Electron beam deposition	43
	Si NBs@G	1558	765 at 200 mA g^−1 after 600 cycles	—	Nanotube bundles	Hydrothermal method and modified Hummers method	76
	Si/rGO	2871	1433 at 100 mA g^−1 after 100 cycles	571 at 3 A g^−1 after 2000 cycles	Sandwich nanoarchitecture	Strain released strategy	45
	Si/rGO	3168.2	1004 at 50 mA g^−1 after 100 cycles	849 at 200 mA g^−1 after 100 cycles	Nano-porous network structure	Steam etching	77
	Si/C-CNFs	1940	1215.2 at 600 mA g^−1 after 50 cycles	350 at 5 A g^−1	Nanofiber	Electrospinning and carbonization	78
	Si@C/GF	4976	—	650 at 1 A g^−1 after 200 cycles	Framework	Modified Hummers method and hydrogel and hydrothermal method	83
	Si/C/graphene	1650	760 at 0.2 A g^−1 after 100 cycles	508 at 2 A g^−1	Multilayer structure	Electrostatic spray deposition and heat treatment	46
	Si–CNT–graphene	2155	1337 at 1.0 A g^−1 after 100 cycles	>1000 at 5.0 A g^−1	Sponge-like robust architectures	Freeze-drying process and sintering	84
	C–Si–SiO–SiO2	1670	1280 after 200 cycles	—	Core–shell	High-temperature annealing process	85
Metal	Si/Ni	4043	2038 at 840 mA g^−1 after 100 cycles	655 at 8400 mA g^−1 over 1000 cycles	2D nanosheet network	Solvothermal method and chemical vapor deposition	52
	Si/Cu	2183.19	500 after 100 cycles	—	Well-aligned nanorods of a multi-layer	Electron beam deposition method	48
	Si/Ag	2343.6	1656 at 200 mA g^−1 after 150 cycles	Over 1000 at 1000 mA g^−1 after 200 cycles	Micro-nano bimodal porous structure	One-step dealloying method	53
	Si/Ag	1825	1751 after 60 cycles	—	Film	Magnetron sputtering	79
	Si20Co10C70	1283.3	610 at 50 mA g^−1 after 50 cycles	—	Sheet structure	Mechanical milling process	54
	NiSi2/Si/C	1765	1272 at 1 C for 200 cycles	740 at 5 C	Porous and core–shell structure	Ball-milling method and chemical vapor deposition method	55
	Ni/Si/graphite	2900	1800 at 2 A g^−1 after 500 cycles	1500 of 4 A g^−1	Sandwich structure	Dip-coating and carbon deposition	56
Metal oxide	Si/TiO2	4316	1586 at 0.135 A g^−1 after 180 cycles	—	Porous and nanofibres structure	Sulphur-templating and electrospinning	59
	Si/TiO2	3874	1250 at 100 mA g^−1 after 50 cycles	—	Core–shell nanostructure	Sol–gel method	80
	Si/MoO3	1475	1000 at 100 mA g^−1 after 50 cycles	1037 at 10 A g^−1 after 50 cycles	Nanoplatelets	Hot-wire chemical vapor deposition and an ultrasonic spray	61
	Si/SnO2	2020	1000 at 200 mA g^−1 after 50 cycles	500 at 1.5 A g^−1	Core–shell structure	Sol–gel method	81
	CNS/Si/Al2O3	2055	1560 at 1 A g^−1 after 100 cycles	854 at 8 A g^−1	Core–shell film	Template-directed carbon segregation and electrophoretic deposition technique	62
	Si@Li2SiO3–Li2Si2O5–Li4Ti5O12	1032	∼65% capacity retention at 1 C after 1000 cycles	∼800 at 10 C	Core–shell	A simple sol–gel process	86
Polymer	Si@PVP–GCB	637	545 at 50 mA g^−1 after 100 cycles	—	Core–shell and 3D nanoporous network	One step-assembly technique	68
	Si/PEDOT:PSS	3685	1950 at 1 A g^−1 after 100 cycles	—	Nanoparticles	In situ secondary doping treatment	69
	Si@C@polyimide	2590	1780 at 0.2 C after 80 cycles	—	Sponge-like particles	One-step silver catalytic etching process and thermal decomposition	70
	GO/PDA-Si	2643	1475 at 840 mA g^−1 after 130 cycles	1074 at 2100 mA g^−1 after 300 cycles	Sheet structure	Novel facile solution-based chemical method	71
	C/Si/polymer	872	722 at 100 mA g^−1 after 100 cycles	—	Foam structure	Chemical vapor deposition	87
	SiNW–cPAN	—	>1700 over 700 cycles	—	Porous network of clusters	Porous anodized alumina templates	88

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3. Si composites as anode materials in other secondary batteries

In addition to lithium-ion batteries, silicon-based anodes can also be employed in lithium sulfide secondary battery system, where the high capacity of silicon anode and the high efficiency and cycling rate of lithium sulfide cathode could allow optimal full cell balance.⁹⁰ However, with the increasing commercial application of lithium secondary batteries, the availability of Li resources and subsequent rising price of Li severely restrict its development. Given this, new energy storage devices with a high electrochemical performance and cost effectiveness are desired to be explored.^91,92 Very recently, Na and Mg have drawn significant attention due to their similar chemical property to Li, abundant natural resource and corresponding low cost, which promise to be the new communities for secondary batteries instead of the lithium-ion battery, as shown in Table 2.⁹³

Table 2 Comparative qualities of lithium, sodium, and magnesium for alkaline (earth)-ion battery applications⁹³

Parameter	Lithium	Sodium	Magnesium
Cationic radius (Å)	0.76	1.02	0.72
Atomic weight (g mol^−1)	6.9	23.0	24.3
E (V vs. SHE)	−3.04	−2.71	−2.37
Carbonate cost ($ per ton)	∼6000	∼150	∼1000
Metallic capacity (mA h g^−1)	3862 (Li^+)	1166 (Na^+)	2205 (Mg^2+)
Metallic capacity (mA h cm^−3)	2046	1129	3833
Coordination preference	Octahedral and tetrahedral	Octahedral and prismatic	Octahedral

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Sodium-ion batteries (SIBs) were explored in parallel with LIBs since the 1970s.^94–96 Since sodium and lithium are alkali metals, they both possess similar chemical properties including ionicity, electronegativity and electrochemical reactivity. Thus the operation principle of these batteries is also similar, where sodium ions are shuttled between the cathode and anode through the electrolyte during the discharge and charge processes of SIBs. However, the higher electrochemical properties of LIBs drew all the interest and the development of SIBs was stagnant. With the restriction of lithium resources, increasing research on SIBs has been conducted to address this hurdle. Recent reports have shown the potential commercial application of SIBs, which operate at room temperature, have a very suitable redox potential, and are considerably low cost.^97–101 Magnesium ion batteries (MIBs) have also been proposed as a potential alternative to lithium-ion secondary batteries.¹⁰² Compared to LIBs and SIBs, MIBs function under the same principles, but they are attractive because of their extremely high volumetric capacity (3832 mA h cm⁻³), much lower reactivity which can ease constraints on safety and manufacturing, and twice as much charge delivery per atom. In addition, their cheap and abundant raw materials make them ideal prospective secondary batteries for energy storage devices.^103–105 It is believed that SIBs and MIBs, as renewable sources, can act as a complement to LIBs for stationary storage and significantly reduce cost.

In order to realize the practical utility of commercial sodium-ion and magnesium-ion batteries, there are still several challenges for these emerging fields that need to be researched. For instance, in the sodium-ion battery, the larger ionic radius of Na⁺ often causes the electrode to be pulverized under repeated intercalation/deintercalation, and thus result in rapid capacity degradation.¹⁰⁶ In the magnesium-ion battery, the slow diffusion kinetics of Mg²⁺ and quite low energy density would decrease the total electrochemical properties of batteries.¹⁰⁷ To improve these limitations, much work has to be done in the field of advanced electrode materials. Among these, one of the most troublesome components is the anode material, where the typical graphitic carbon employed in LIBs makes it difficult to incorporate Na⁺ and Mg²⁺.^108–110 Therefore, the discovery of suitable anode materials with large insertion space and high electron/ion conductivity is a major challenge.

Due to its excellent performance in lithium-ion battery, Si has been considered as a potential material for the applications in other secondary batteries. However, previous research has indicated that crystalline Si does not intercalate sodium and magnesium at room temperature.^111–113 With the latest theoretical exploration, Legrain et al.¹¹⁴ presented a comparative computational study of the energetics of Li, Na, and Mg atoms in crystalline Si (c-Si) and amorphous Si (a-Si) structures using first-principles calculations to assess the potential of Si as an anode material for different metal-ion batteries. They showed that Si preamorphization increases the average anode voltage and slightly reduces the volume expansion of the anode during the insertion of the metal atoms. In addition, the analysis of computed formation energies suggests that a-Si is advantageous over c-Si for Li, Na, and Mg insertion energetics and volume expansion, as shown in Table 3. Therefore it is believed that appropriate control of the charge/discharge process may realize the insertion of Na and Mg atoms into a-Si. Furthermore, Legrain and his co-workers¹¹⁵ also demonstrated that Al doping of Si significantly improves the energetics for Na and Mg insertion, specifically, making it thermodynamically favored versus vacuum reference states. These findings therefore show a possible way of making Si suitable not only for Li but also for Na and Mg storage.

Table 3 Average voltages V (vs. respective metal reference state) and relative volume expansions (%) of c-Si and a-Si anodes for Li, Na, and Mg-ion batteries and lowest defect formation energies (in eV) of Li, Na, and Mg defects vs. bulk and vacuum reference states in c-Si and a-Si¹¹⁴

	Li (Si4Li15)		Na (SiNa)		Mg (SiMg2)
	c-Si	a-Si	c-Si	a-Si	c-Si	a-Si
Voltage	0.21	0.24	0.02	0.14	0.10	0.13
Expansion	292	282	133	127	216	207
Vs. bulk	0.3	−0.13	2.19	0.67	1.51	0.09
Vs. vacuum	−1.37	−1.80	0.64	−0.88	0.37	−1.05

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An atomic level study on the applicability of the Si anode in sodium-ion batteries using ab initio molecular dynamics simulations was proposed by Jung et al.¹¹⁶ While crystalline Si is not suitable for alloying with Na atoms, amorphous Si can accommodate 0.76 Na atoms per Si atom, which corresponds to a specific capacity of 725 mA h g⁻¹. The amorphous Na_0.76Si phase undergoes a volume expansion of 114% and shows the Na diffusivity of 7 × 10⁻¹⁰ cm² s⁻¹ at room temperature. Two representative local structures of Na@Na₃Si₅ and Na@Na₇Si₁ for Na_0.75Si and Na_1.5Si are shown in Fig. 7. Overall, the amorphous Si phase turns out to be quite attractive in performance compared to other alloy-type anode materials. This work suggests that amorphous Si might be a competitive candidate for sodium-ion battery anodes.

Fig. 7 Two representative local structures of Na@Na₃Si₅ and Na@Na₇Si₁ for Na_0.75Si and Na_1.5Si, respectively.¹¹⁶

The most recent research shows a unique amorphous Ge anode material coated on 3D hexagonal Si nanorod (NR) arrays using a TiN/Ti interlayer as the current collector for rechargeable sodium-ion micro/nanobatteries, which was designed and fabricated by Yue and coworkers.¹¹⁷ This optimized Si/TiN/Ti/Ge composite NR array anode displays superior areal/specific capacities and cycling stability, which benefits from the favourable 3D nanostructures and effective conductive layers of TiN/Ti thin films, as shown in Fig. 8. Therefore, the successful configuration of the Si/TiN/Ti/Ge composite NR array anode can provide insight into the exploration and design of new Si-based electrode materials, which provides a promising way to fabricate and optimize Na-ion micro/nanobatteries in the future.

Fig. 8 Electrochemical performance of the Si/TiN/Ti/Ge composite NR anode. (a) CV measurement of Si/TiN/Ti/Ge composite NR anode at a scan rate of 0.5 mV s⁻¹ between 0.001 and 1.5 V versus Na/Na⁺ from the 1st to 10th cycle; (b) cycling performances of the 3D Si/TiN/Ti/Ge composite NR anode compared with that in 3D Si/Ge NRs, 3D Si NRs, and planar Si/TiN/Ti/Ge composite as anodes in Na-ion half-cell under a current density of 10 μA cm⁻² within a voltage window of 0.001–1.5 V versus Na/Na⁺; (c) sodiation/desodiation voltage profiles of 3D Si/TiN/Ti/Ge composite NR anodes; and (d) the rate performances of the 3D Si/TiN/Ti/Ge composite NR anode under different current densities of 10, 20, 50, 100, 150, 200, and 10 μA cm⁻² between 0.001 and 1.5 V versus Na/Na⁺.¹¹⁷

4. Summary and perspective

Silicon-based composites are one of the most potential materials applied to the new generation of lithium-ion batteries, which are beneficial to their extremely high theoretical capacity, proper discharging potential and low cost. However, severe volume expansion of Si during the charge and discharge processes results in the crushing and pulverization of materials and then leads to a rapid fall in cycling performance. Moreover, the instability of the solid-electrolyte interphase (SEI), the low electronic conductivity and ion diffusivity of pristine Si material ultimately lead to a poor electrochemical performance. Nowadays, methods to overcome these hindrances include decreasing the scale of the Si structure to the nanoscale and synthesizing composite materials. This paper briefly summarizes the recent progress in Si-based composites that act as anode materials in lithium-ion batteries, including Si–carbon, Si–metal, Si–transition metal oxides and Si–polymer, which can not only effectively suppress the volume change during lithium ion intercalation and deintercalation, but also increase electronic and ionic conductivity, and ultimately realize high energy density, high charge–recharge rate and long cycle life of lithium-ion batteries.

However, it is difficult for a single material compositing Si to guarantee good electrochemical performance completely. Although carbon nanotubes or graphene have high electronic conductivity and good flexibility, it is difficult to realize entire silicon capsulation. Similarly, several types of inert metals, such as Fe, Ti and Ni, act as a dispersion matrix for Si, but possess very poor electrochemical activity, which can be improved by adding a certain amount of carbon materials to improve Li⁺ conductivity. Moreover, compositing conductive polymers also can play a part in effectively releasing the volume change and improving electrical contact. Therefore, the combination of different materials to realize multiple composites is an effective approach to achieve an excellent electrochemical performance completely. Furthermore, much effort has been made on Si-based composites in which Si can be homogenously dispersed in an active/inactive matrix to accommodate strain and maintain structural integrity. Therefore, not only the choice of materials, but also a proper design of the nanostructured hybrid materials is quite necessary to achieve the desired properties of batteries.

Furthermore, although considerable advances have been achieved in the last decade in the design and synthesis of Si-based anode materials, and good electrochemical performance can be obtained in the present research, comparatively complicated fabrication methods, poor electrochemical properties of full cells, and limited lithium resources restrict the practical application and large scale industrialization of lithium-ion batteries. Therefore, we believe that the future directions for research efforts are as follows: (1) to carry out deep mechanisms studies by using in situ measurement methods, such as SEM, XRD, Raman, and XPS; (2) to develop predictive theoretical tools for fundamental studies on the relationships between nano-structures and electrochemical characteristics; (3) to develop more stable materials to improve electrochemical properties for lithium-ion full cells; and (4) to reduce the cost of materials and LIBs.

Using simple synthetic processes and fabrication techniques, the further commercial application of lithium-ion batteries can be accelerated. In addition, it is believed that the new communities for the secondary batteries, such as sodium-ion batteries and magnesium-ion batteries, could make a great contribution complementary to lithium-ion batteries for stationary storage and significantly reduce costs. Meanwhile, the exploration of Si-based composites for secondary batteries will start from a new perspective.

Acknowledgements

This work was supported by Grant-in-Aid for Scientific Research of (KAKENHI) program, Japan (C, Grant Number 15K05597).

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