Exploring lithium-ion diffusion and electronic properties in defective graphite via molecular dynamics and density functional theory

The thermodynamic, kinetic, and mechanical properties of graphite anodes significantly influence the performance of lithium-ion batteries. Molecular dynamics simulations and density functional theory calculations were employed to examine the effects of defects in graphite on these properties. In particular, the influence of three types of defects-Stone-Wales (SW), single vacancy (SV), and double vacancy (DV)-at defect densities below 0.4% was analyzed, including their impact on graphite density, charge transfer, voltage, lithium-ion diffusion, and mechanical stability. The results show that defects in graphite form bridge, ylide, and spiro configurations, with structural stability decreasing in the order of SV > SW > DV. As defect density increases, the lithium-ion diffusion coefficient decreases significantly from 4.71 × 10-8 to 3.75 × 10-11 Å2/ps as lithium concentration increases from Li0.02C6 to LiC6. In contrast, for Li0.02C6, the diffusion coefficient rises with increasing defect density, from 2.94 × 10-9 to 1.29 × 10-9 Å2/ps. Mechanical analysis reveals that increasing defect density reduces Young's modulus from 936.49 to 743.54 GPa and ultimate tensile strength from 94.59 to 58.50 GPa, highlighting the detrimental effect of defects on graphite's mechanical stability. Defects introduce localized electronic states within the bandgap, promoting lithium-ion diffusion at higher concentrations and disrupting the graphite structure to create new diffusion paths. These findings underscore the critical role of defect engineering in optimizing graphite anode performance and provide insights for the design of high-performance anode materials.