Graphite anodes are the mainstream of the lithium battery anode market, but because the specific capacity of graphite materials is close to the theoretical specific capacity limit of 372mAh/g, there is limited room for further improvement, which limits the development of high-performance lithium-ion batteries. Silicon-based anode materials are regarded as the most potential next-generation anode materials due to their high theoretical capacity, low lithium-deintercalation potential, environmental friendliness, and abundant reserves.
Problems with silicon-based anodes
In order to achieve large-scale application of silicon-based anode materials, three challenges need to be faced:
One is the volume expansion problem. The lithium intercalation of the silicon anode is an alloying reaction mechanism, which not only brings a high theoretical specific capacity (4200 mAh/g), but also causes a huge volume change (about 300%).
The second is the problem of stress accumulation and crack generation. The huge volume change of the silicon anode induces the accumulation of internal stress of the electrode, resulting in cracks, resulting in electrode pulverization, separation of the active material from the current collector, and performance degradation.
The third is unstable SEI. The huge volume effect of the silicon anode can also lead to the instability of the interface, resulting in the continuous growth of SEI, the loss of active lithium source, and the reduction of Coulombic efficiency.
Research progress of silicon carbon anode
In order to make full use of the high specific capacity of silicon and improve its defects, researchers have done a lot of research. Among them, one of the most effective strategies is to prepare silicon/carbon composites, which can comprehensively utilize the high specific capacity of silicon and the good mechanical properties and electrical conductivity of carbon materials.
Silicon/graphite composites
Silicon/graphite composites can be prepared by chemical vapor deposition or mechanical ball milling. Although the samples prepared by chemical vapor deposition are more uniform and regular, the cost is relatively high. Physical methods such as mechanical ball milling are more advantageous in terms of scale and production costs. Sun et al. used boron-doped microstructured silicon and graphite composites, which showed high capacity and retention. At present, in the production of silicon carbon materials, the proportion of silicon added is about 1%-5%. Due to the low price of graphite, silicon/graphite composite materials with graphite as the main composite component have become the first choice in production.
Silicon/Carbon Nanotube Composites
Among several well-known carbon materials, carbon nanotubes have excellent electrical conductivity and good physical stability, and are attractive as additives to improve the electrochemical properties of silicon-based materials. Studies have shown that the uniform distribution of nano-silicon particles along the carbon nanotubes can optimize the electrochemical performance of silicon. Depositing 10 nm of silicon on carbon nanotubes with a diameter of 5 nm resulted in a composite with a capacity of up to 3000 mAh/g (charge-discharge rate of 1.3 C). It was found that carbon nanotubes can alleviate the volume expansion of silicon and provide a continuous path for charge transfer along the axial direction, improving the electronic conductivity and electrochemical performance of the composite.