Aging mechanisms of NMC811/Si-Graphite Li-ion batteries

Electrode degradation processes at various Li-ion batteries' state-of-health (SoH 100 %, 80 %, 50 %, and 30 %) and cycling temperatures (5 degrees C, 23 degrees C, and 45 degrees C) were investigated. For this purpose, the standard format of Li-ion cylindrical 18,650 batteries with Si-Graphite negative and LiNi08Co01Mn01O2 (NMC811) positive electrodes were cycled with registering battery parameters and the electrochemical impedance spectrum were recorded after every 200 cycles. Once reaching their end-of-life, electrodes from cycled batteries were subjected to post-mortem analysis. NMC811 positive electrode was observed to crack during the charge and discharge processes, suffered by irreversible phase transition, transition metal dissolution, cathode electrolyte interphase growth, and cation mixing. The Si-Graphite negative electrode material was also affected by crack formation, layer exfoliation, solid electrolyte interphase (SEI) recompositing, Li dendrite growth, transition metal contamination, and Si dissolution. Degradation of components leads to an increase of the contact resistance, Li+ diffusion limitations, reduction of active materials participating in Li-ion storage and, as a result, capacity fade that finally rendered the battery utilization unfeasible. Degradation processes can be detected by capacity fade and impedance growth of the full battery. High temperature accelerates electrode degradation processes when low temperature leads to SEI and Li dendrite growth.