Switch of the Charge Storage Mechanism of Li_xNi_0.80Co_0.15Al_0.05O₂ at Overdischarge Conditions

The specific charge loss during the first cycle of LiNi0.80Co0.15Al0.05O2 (NCA) is due to low Li+-ion diffusion kinetics on discharge down to 3.0 V. Discharge to 2.14 V pushes further reinsertion of Li+ into the layered NCA lattice; thus, part of the specific charge loss can be regained. Additional discharge to even lower potentials switches the charge storage mechanism from a solid solution to a two-phase reaction process. Pushing the NCA to work at potentials lower than 2.14 V vs Li metal irreversibly alters the material’s crystal structure and microstructure. The gradual overall change in the energy storage reaction mechanism, promoted by a repeated deep discharge, causes a decrease in NCA’s performance and lifetime, thus challenging the validity of cycling over a wide potential window in practical cells.