Carbon-based anode materials are known for their minimal volume change during charge and discharge cycles, offering excellent cycle stability and being inherently a mixed conductor of both ions and electrons. Additionally, silicon and carbon share similar chemical properties, allowing them to bond tightly together. As a result, carbon is often chosen as the ideal substrate when combined with silicon.
In Si/C composite systems, silicon acts as the active material that provides lithium storage capacity, while carbon serves multiple roles—buffering the volume changes of silicon during cycling, improving the overall conductivity of the material, and preventing silicon particles from aggregating. This synergy between silicon and carbon leads to high specific capacities and long cycle life, making Si/C composites a promising candidate to replace traditional graphite in next-generation lithium-ion batteries.
When it comes to the structure of silicon-carbon composites, they can generally be categorized into two main types: coating structures and embedded structures. Coating structures involve covering silicon particles with a carbon layer to mitigate the volume expansion of silicon and enhance its conductivity. These coatings can further be classified into core-shell, egg yolk-shell, and porous structures based on the morphology and design of the carbon layer.
The egg yolk-shell structure is a unique nano-composite that builds upon the core-shell concept by introducing a void space between the inner core and outer shell. This configuration allows the silicon core to expand freely within the cavity, reducing structural stress and maintaining the integrity of the carbon shell. This design helps in forming a stable solid electrolyte interphase (SEI) film, which is crucial for long-term battery performance.
Researchers like Zhou et al. have successfully fabricated Si@void@C composites using a sol-gel method followed by pyrolysis of sucrose. After etching the SiO₂ layer with HF, they achieved a composite with 28.54% silicon content. Compared to raw silicon nanoparticles or hollow carbon, the Si@void@C showed better cycle performance, with a first specific capacity of 813.9 mA·h/g and a capacity retention of 500 mA·h/g after 40 cycles.
Similarly, Tao et al. developed a stable Si@void@C composite and found that at a carbon loading of 63%, the specific capacity reached 780 mA·h/g after 100 cycles, higher than the 690 mA·h/g observed at 72% carbon loading. This suggests that optimizing the egg yolk-shell structure is essential for maximizing performance.
Liu et al. used polydopamine as a carbon source to create a Si@void@C composite, where sufficient space was left between the silicon core and the thin carbon shell. This prevented damage to the carbon layer during lithiation and helped form a stable SEI film on the surface.
The Si@void@C composite demonstrated remarkable performance, achieving a reversible capacity of up to 2800 mA·h/g at 0.1 C, retaining 74% of its capacity after 1000 cycles, and maintaining a Coulombic efficiency of 99.84%.
More recently, researchers have introduced a multi-shell concept to enhance the mechanical strength of the carbon layer and improve resistance to silicon volume expansion stress. Sun et al. prepared a double-shell composite (Si@DC) by using a vesicle template method, followed by pyrolysis and HF etching. The resulting structure had two carbon shells, significantly boosting electrical conductivity. At 50 mA/g, the Si@DC retained a capacity of 943.8 mA·h/g after 80 cycles, outperforming single-shell and pure silicon samples.
Yang et al. also developed a double-shell composite (Si@void@SiO₂@void@C) using the Stöber method and pyrolysis. This structure exhibited excellent cycle stability, maintaining a capacity of 956 mA·h/g after 430 cycles at 460 mA/g, with an 83% capacity retention rate. In contrast, the core-shell Si@C sample showed significant capacity decay.
The dual-layer protection provided by the SiOâ‚‚ and carbon shells not only enhances structural stability but also prevents direct contact between silicon nanoparticles and the electrolyte, reducing unwanted side reactions. This dual-layer mechanism ensures a more reliable and durable anode material for advanced lithium-ion batteries.
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