Using High Precision Coulometry Measurements to Compare the Degradation Mechanisms of NMC/LMO and NMC-Only Automotive Scale Pouch Cells

Large-scale nickel-manganese-cobalt/lithium manganese oxide (NMC/LMO):graphite pouch cells from the General Motors Chevrolet Volt program were tested on Dalhousie's Ultra-High Precision Cycler after being stored for 2 years at 30% state of charge. The difference in the amount of charge that the cells required to reach the end of charge from cycle to cycle, termed charge end point capacity slippage, was initially very high, but became very low after a few charge-discharge cycles. After the stabilization period, the charge end point capacity slippage rates were found to be relatively insensitive to temperature changes. Yet the capacities of the cells continued to fade at an increasing rate with increasing cell temperature. This capacity fade modeled well with respect to the square root of time (t1/2), consistent with the solid electrolyte interface (SEI) thickness growth model on the negative electrode. An Arrhenius relationship was fitted for the SEI growth and used to predict the life dependency on temperature. The potential cell failure modes of these cells were compared with those found for large-format NMC442/graphite pouch cells from another automotive supplier.