Lithium-ion batteries are currently one of the most widely used secondary battery systems. Compared with other rechargeable batteries such as NiCd and NiMH, Li-ion batteries have higher energy density, higher operating voltage, limited self-discharge and lower maintenance costs. However, current commercial graphite anodes cannot meet the increasing energy density and operational reliability requirements of energy storage applications such as portable electronic devices and electric vehicles. Therefore, silicon, as a new-generation anode material, has attracted extensive attention from academia and business circles.

1. Overview of silicon anode materials

Compared with traditional graphite materials, silicon has a high theoretical specific capacity, and the voltage platform of silicon is slightly higher than that of graphite. It is not easy to cause lithium precipitation on the surface during charging, and has better safety performance. Silicon is the second most abundant element in the earth's crust, and its abundant reserves make it an abundant source of raw materials and low prices. However, as a semiconductor material, silicon has low electrical conductivity. The insertion and extraction of lithium ions will cause huge expansion and contraction of the silicon volume, pulverize the material, collapse the structure, and finally separate from the current collector, which greatly reduces the battery cycle performance.

At present, methods such as nanometerization, compounding and alloying of silicon anode materials are usually used to improve their structural stability and improve the cycle performance of silicon anodes.

2. Silicon Nanomaterials

In order to improve the cycle stability of silicon-based anode materials, silicon materials are usually nanosized. The main research directions are: silicon nanoparticles, silicon nanowires, silicon thin films, and 3D porous silicon.

(1) Nano-silicon particles and their composite materials. Putting silicon particles into the buffer layers of different substrates can effectively improve the cycle performance of silicon anodes by adapting to volume expansion and absorbing stress. Especially carbon materials, such as graphite, carbon nanotubes, and graphene, have been widely used in Si /C composite. Embedded silicon-carbon composites mean that silicon particles are embedded in a continuous carbon matrix, which is usually continuous and dense. Therefore, the diffusion of lithium ions in the composite material is hindered. Studies have shown that the performance of silicon-based anodes can be significantly improved by adjusting the structure and morphology of the carbon matrix. Different types of carbon substrates can provide different transport routes for ions and electrons and facilitate the wetting of electrolytes.

Xu et al. synthesized mesoporous Si/C composites through evaporation-induced self-assembly, and the silicon nanoparticles were uniformly distributed in the mesoporous carbon, providing spatial and mechanical support for adapting to volume changes and stress release. At a current density of 500 m·g-1, the first-week capacity was 1410 mAh·g-1, and the capacity of 1018 mAh·g-1 remained after 100 cycles, which was much higher than that of pure nano-silicon anode.

Carbon nanotubes can be used as a good flexible silicon composite matrix due to their excellent mechanical strength, good electrical conductivity, high aspect ratio and flexible structure. Several research groups have found that the addition of carbon nanotubes during electrospinning of Si/C composites can enhance high-rate performance.

Besides carbon nanotubes, graphene is also often used as a Si/C composite. Graphene has two-dimensional structural characteristics and is easy to form a sandwich structure, which can well buffer mechanical stress, enhance lithium ion transport and electrochemical reactions. Xia et al. generated SiO2 in situ on graphene sheets through magnesium thermal reduction. This composite material has an initial reversible capacity of 1750 mAh·g-1 and good cycling performance, which still retains a capacity of 1374 mAh·g-1 after 120 cycles of cycling. .

(2) Silicon nanowires and their composites. Studies have shown that silicon nanowires have unusual capacity and cycle life, with a high reversible capacity of 3100 mAh g-1. Yang et al. used Cu catalysis to synthesize silicon nanowires on stainless steel foil by CVD method. The coulombic efficiency reached 89% in the first week, and the specific capacity of more than 2000mAh·g-1 was maintained for dozens of cycles. The introduction of hollow structures or voids into silicon nanowires can provide additional space to accommodate volume expansion.

Jing et al. prepared coral-like surface silicon nanowires on copper foam by a one-step CVD method. The copper foam is both a catalyst and a current collector, and the reversible capacity reaches 2745mAh. g-1 and 884mAh·g-1.

Silicon nanowires minimize volume expansion during electrochemical cycling by stress relaxation and providing an efficient electron path, however, in practice it is still possible to cause silicon nanowires to break, resulting in rapid capacity decay. Therefore, silicon nanowire composites have been widely developed.

By sputtering Cu coating on silicon nanowires, Ko et al. achieved an efficiency of 90.3% in the first cycle and a discharge capacity of 2700mAh·g-1 at a current density of 210mA·g-1, which is better than adding carbon coating on silicon nanowires. with better performance.

(3) Porous silicon and its composite materials. Low-dimensional silicon can well suppress the volume expansion of silicon anode during cycling, but it inherits the disadvantage of low mass loading density, so 3D porous silicon has attracted much attention.

Cho prepared 3D nanoporous silicon by depositing Si onto nanoporous SiO2 templates. The material has a capacity of up to 2800 mAh g-1 at 400 mA g-1, and the capacity does not decay significantly after 100 cycles. In addition to the preparation of porous silicon by template, magnesium-aluminum reduction has also been used for the preparation of porous silicon. Cui also exhibits good electrochemical performance by extracting nanoporous silicon from rice husks as a sustainable source of nanostructures through magnesium-aluminum reduction.

3. Silicon alloy material

In addition to silicon nanomaterials, adding metal elements to silicon can also effectively improve the cycle performance of silicon-based anodes. Metal forms an alloy with silicon. On the one hand, the metal can slow down the volume expansion; on the other hand, the increase in electron enrichment makes the intercalation of lithium easier. But metals as inactive species limit the specific capacity of the material. At present, there are Fe-Si, Ni-Si, Cu-Si, Ti-Si and other silicon alloy materials.

Lee et al. prepared Ti-Si and Ti-Si-Al alloys from metal powders by high-energy ball milling. The materials have good cycle performance, and the effect of ball milling time on electrochemical performance was studied. Yin et al. obtained a Si-Cu alloy by ball milling, and then further added carbon ball milling to obtain a SiCuC composite, which showed better cycle stability than pure silicon. There are also related studies on the mechanism of silicon alloy anode. The solid-electrolyte interface protects the silicon-based anode during cycling and also buffers the volume expansion of silicon. Therefore, in the research and application of silicon-based anode materials, the selection of electrolyte, binder and solvent is worthy of in-depth study.

Although the problems of silicon-based anode material properties and cycle stability have been deeply studied by researchers and have been well solved, with high specific capacity and long life, large-scale commercial applications have not yet been achieved. Several key issues, such as Coulombic efficiency, mass loading density, and fabrication cost, need to be further optimized. Binder selection and modification may be the best path to cost reduction and commercialization. However, in order to make silicon-based anodes play a greater and better value in power storage and other aspects, further research and excavation by researchers are still needed.