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 degrees C or 60 degrees 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 CC-CV 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 degrees C than at 60 degrees 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 degrees C, while this does not occur in the cells with potential-limited CC charge protocol at 60 degrees 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 degrees C.