Mechanisms of Li deposition on graphite anodes: surface coverage and cluster growth

Abstract

Li plating on the anode is a side reaction in Li-ion batteries which competes with Li intercalation and leads to loss of capacity. Growth of Li clusters into dendrites is a potential safety hazard for batteries which can lead to internal short-circuit and fires. We consider two possibilities of Li deposition on the surface of graphite anode: deposition of Li⁺ ions uniformly on the surface and deposition of clusters of metallic Li. Using ab initio simulations, we predict the operating voltage for the occurrence of the above processes and safety measures to prevent dendrite growth in batteries. We find that Li deposition occurs in the following stages: at positive voltages vs. Li, surface deposition of Li⁺ ions is the dominant process. Below a critical cross-over voltage, the process of reduction of aggregated Li⁺ ions and the formation of metallic Li clusters takes over. This cross-over voltage is found to be −12 mV on the basal plane of unlithiated graphite and −29 mV on lithiated graphite. To prevent formation of Li clusters and for safe operation of Li-ion batteries, the voltage on the graphite anode should be kept above the cross-over value.