To leverage the strengths and compensate for the weaknesses of silicon, what processes can be used to modify and optimize it? Combining silicon with other substances can achieve good results, and silicon-carbon composite materials are one such material that has been extensively studied.
Carbon materials are currently the most widely used anode materials. Carbon materials can be divided into three types: soft carbon (graphitizable carbon), graphite, and hard carbon (amorphous carbon). Their charge-discharge chemical equations can be expressed as follows:
Carbon anode materials have good cycle stability and excellent conductivity, and lithium ions have no significant effect on their interlayer spacing. To a certain extent, they can buffer and adapt to the volume expansion of silicon, so they are often used to combine with silicon.
Composite materials can generally be divided into two categories based on the type of carbon material used: traditional silicon-carbon composite materials and novel silicon-carbon composite materials. Traditional composite materials refer to composites of silicon with graphite, MCMB, carbon black, etc., while novel silicon-carbon composite materials refer to composites of silicon with novel carbon nanomaterials such as carbon nanotubes and graphene.
Silicon-carbon anode materials are mainly classified into coated, intercalated, and molecular contact types based on the distribution of silicon, and into particulate and thin-film types based on their morphology. They are also classified into silicon-carbon binary composites and silicon-carbon multi-component composites based on the amount of silicon and carbon. The following figure shows silicon-carbon anode materials with different distribution methods:
The preparation processes for silicon-carbon composite materials include ball milling, high-temperature pyrolysis, chemical vapor deposition, sputtering deposition, and vapor deposition. Silicon-carbon anodes prepared using ball milling can achieve a reversible capacity of 500-1000 mAh/g. Ball milling promotes uniform mixing of raw material particles and yields smaller particle sizes; simultaneously, the gaps between particles also contribute to improved battery cycle performance.
High-temperature pyrolysis is a method to obtain Si/C composite materials by pyrolyzing nano-silicon particles and organic precursors or by directly pyrolyzing organosilicon precursors. The specific capacity of the silicon-carbon composite material prepared by this method is lower than that of Si/C composite materials prepared by high-energy ball milling, but higher than that of graphite, approximately 300–700 mAh/g. This is because the electrode materials prepared by pyrolysis contain a large amount of electrochemically inactive substances, which reduces the capacity of the electrode material.
Nano-silicon particles are among the earliest researched anode materials, but their large volume expansion effect limits their application. Composite materials made by combining silicon and carbon allow for the expansion of silicon, while also mitigating the drawbacks of poor conductivity and SEI film instability in silicon to some extent. This has garnered widespread attention and application from battery cell manufacturers. The Model 3, launched by Tesla in 2016, uses silicon-carbon anode materials, achieving a 0-60 mph (approximately 96.6 km/h) acceleration time of just 6 seconds and a range of 215 miles (approximately 346 km). Those interested can take a closer look.
