Comprehensive Study of Lithium Diffusion in Si/C-Layer and Si/C3N4 Composites in a Faceted Crystalline Silicon Anode for Fast-Charging Lithium-Ion Batteries

By using silicon (Si) as an anode of lithium-ion batteries, the capacity can be significantly increased, but relatively large volume expansion limits the application as an efficient anode material. Huge volume expansion of the silicon anode during lithiation, however, leads to cracking and losing its connection with the current collector. This shortcoming can be improved by the deposition of a nanometric carbon- or nitrogen-doped carbon coating on the silicon surface, resulting in Si/C-layer and Si/C₃N₄ interfaces. In this work, Li⁺ diffusion in Si/C-layer and Si/C₃N₄ composite materials along three Si surfaces and various ion pathways were carefully analyzed by using density functional theory and ab initio molecular dynamic (AIMD) simulations. Both Si/C and Si/C₃N₄ interfaces and three Si surfaces of (100), (110), and (111) were investigated. The formation of nitrogen holes and monatomic carbon binders in the composite increases ion diffusivity and limits volume expansion. Furthermore, the Bader analysis shows that the type and orientation of the surfaces have important effects on ion distribution. The results indicated that the C₃N₄ composite increases Li⁺ diffusion in Si (100) from 7.82 × 10^-5 to 3.17 × 10^-4 cm²/s. The presented results provide a guide for the appropriate design of stable and safe high-energy-density batteries.