Degradation identification of LiNi_0.8Co_0.1Mn_0.1O₂/graphite lithium-ion batteries under fast charging conditions

Cycle aging of commercial 2.5 Ah 18650 cylindrical lithium-ion batteries with LiNi0.8Co0.1Mn0.1O2(NCM)/graphite chemistry is investigated at different charging rates. The cells charged at 1C-3C follow a similar aging path, and the degradation mechanisms under 2C and 3C charging are characterized by non-destructive electrochemical techniques and post-mortem analyses. Electrochemical impedance spectroscopy measurements indicate that the impedance rise of cells is primarily attributed to the increase of charge transfer resistance. Post mortem analyses reveal that the side reactions on NCM cathode are secondary particle cracking and transition metal dissolution, and anode degradation is caused by the growth of solid-electrolyte interface layer and the lithium plating. Voltage fitting analyses of 3C charging demonstrate that the main degradation mode is the insufficient active lithium that is available for intercalation/deintercalation in highly-lithiated anodes. Quantitative analyses of the individual electrodes based on differential voltage curves identify that loss of lithium inventory (LLI) contributes the dominant aging mode to full cell, followed by loss of active material of delithiated negative electrode (LAM_deNE) and LAM of lithiated negative electrode (LAM_liNE), while LAM of positive electrode (LAM_PE) exerts a minor effect. Accurate identification of the battery degradation mechanisms cycling under fast charging conditions helps to provide guidance for charging optimization.