Edge and lithium concentration effects on intercalation kinetics for graphite anodes

Abstract

Graphite interfaces are an important part of the anode in lithium-ion batteries (LIBs), significantly influencing Li intercalation kinetics. Graphite anodes adopt different stacking sequences depending on the concentration of the intercalated Li ions. In this work, we performed first-principles calculations to comprehensively address the energetics and dynamics of Li intercalation and Li vacancy diffusion near the non-basal edges of graphite, namely the armchair and zigzag-edges, at high Li concentration. We find that surface effects persist in stage-II that bind Li strongly at the edge sites. However, the pronounced effect previously identified at the zigzag edge of pristine graphite is reduced in LiC12, penetrating only to the subsurface site, and eventually disappearing in LiC6. Consequently, the distinctive surface state at the zigzag edge significantly impacts and restrains the charging rate at the initial lithiation of graphite anodes, whilst diminishes with an increasing degree of lithiation. Longer diffusion time for Li hopping to the bulk site from either the zigzag edge or the armchair edge in LiC6 was observed during high state of charge due to charge repulsion. Effectively controlling Li occupation and diffusion kinetics at this stage is also crucial for enhancing the charge rate.