Capacity degradation in commercial Li-ion cells: The effects of charge protocol and temperature

To examine the impact of charge protocol and temperature on Li-ion cell degradation, commercial 18650 Li-ion cells with NCA/C chemistry are cycled deeply1000 times at either 25 °C or 60 °C using two different charge termination protocols: (a) potential-limited constant-current (CC) without a constant potential dwell, or (b) constant-current constant-voltage (CC-CV). Analyses indicate that the cells that experienced the CCCV charge protocol show a lower degradation rate than those cycled using the potential-limited/CC charge protocol at both temperatures. Additionally, the cells that used the potential-limited/CC charge protocol exhibit a much higher capacity degradation rate at 25 °C than at 60 °C. Electron microscopy and surface chemistry analyses show a thick secondary solid-electrolyte interphase (SEI) is formed on the anode surface for the cell using the potential-limited CC charge protocol at 25 °C, while this does not occur in the cells with potential-limited CC charge protocol at 60 °C. Based on the electrochemical data, this thick secondary SEI shows large resistance and makes higher rate cycling difficult. This secondary SEI is thought to be a key contributor to the fast capacity degradation rate of the cells using the CC charge protocol at 25 °C.