Nanostructured Si-FeSi₂-graphite-C composite: An optimized and practical solution for Si-based anodes for superior Li-ion batteries

To obtain anodes with a high reversible capacity, high initial coulombic efficiency (ICE), long cycle durability, and fast rate-capability for Li-ion batteries (LIBs), a nanostructured Si-based composite comprising Si, a Li-inactive FeSi₂ matrix, and carbonaceous matrices was developed by combination of simple solid-state synthetic methods. Firstly, various Fe_xSi_y alloys with different atomic compositions were synthesized by simple high-energy ball milling (HEBM) and their potential as LIB anodes was investigated. Among these Fe_xSi_y alloys, the Fe₁₀Si₉₀ alloy (comprising Si-FeSi₂) exhibited the highest first reversible capacity of 2609 mAh g⁻1 and ICE of ∼91%. To further enhance the electrochemical performance of the Si-FeSi₂ alloy, a graphite-modified (Fe₁₀Si₉₀-graphite (Si-FeSi2-G)) composite was synthesized by using the physical force generated during HEBM, and the Si-FeSi₂-G composite was further hierarchically carbon-coated (Fe₁₀Si₉₀-graphite-C (Si-FeSi2-G-C)) with polyvinyl chloride (PVC) by pyrolysis. The final product (Si-FeSi₂-G-C composite) was comprised of well-dispersed nanocrystalline Si (active for Li) and FeSi₂ (inactive for Li) within the graphite and amorphous carbon matrices. The Si-FeSi₂-G-C composite exhibited excellent electrochemical performance with a high first reversible capacity of 1045 mA h g⁻¹, a high ICE of 87%, a long cycle durability of 925 mAh g⁻¹ over 80 cycles, and fast rate-capability of 700 and 550 mAh g⁻¹ at 1.2C and 2C rates, which meets the requirements for commercial use as a high-capacity Si-based anode for LIBs.