Highlighting the Advantages of Operating NMC811 Cells to Voltages below 4.20 V Compared to NMC Grades with Lower Ni Content

Abstract

Ni-rich Li-ion cells suffer from severe capacity fade at high states of charge (SOC) due to oxygen release and/or lattice volume collapse. We share long-term cycling results (12000 h) at 40 °C for NMC811, NMC532, and NMC640 cells cycled with different upper cut-off voltages (UCV), C-rates, and depths of discharge (DOD). We show that cycling to a greater UCV results in more capacity loss and resistance growth as expected. However, NMC811 suffered from 5–10 times greater resistance growth at 4.20 V compared to 4.06 V, while the resistance for NMC532 and NMC640 cycled to 4.30 V was roughly twice that of the 4.10 V UCV cells. This highlights a challenging problem with Ni-rich materials that can be mitigated if the UCV is limited to avoid NMC811 volume change. Finally, we used competitive graphite materials reported in previous work to examine the impact of UCV on the rate of SEI growth in NMC811 cells, which was found to increase with temperature and UCV, particularly at ≥ 4.08 V UCV. We hope this brief report will highlight the importance of lifetime energy output, rather than initial energy density, and encourage battery users to limit the UCV of Ni-rich cells when possible.