An Investigation of Li-Ion Cell Degradation Caused by Simulated Autoclave Cycles

Abstract

Li[Ni0.6Mn0.4Co0.0]O2/graphite (NMC640, balanced for 4.1 V cut-off) and Li[Ni0.83Mn0.06Co0.11]O2/graphite (Ni83, balanced for 4.06 V cut-off) pouch cells were tested using lab-simulated autoclave conditions. After every cycle, the cells at either 3.4, 3.7, or 3.9 V were placed in a 120°C oven for 40 min to undergo an “autoclave” run, then continued for another cycle. Electrolyte blends using lithium bis(fluorosulfonyl)imide (LiFSI) salt were used to improve the cycle-life of autoclaved cells. The lab autoclave protocol was also performed on LiFePO4/graphite (LFP) and NMC commercial cylindrical cells, which were advertised for use in or found in autoclaved medical devices. LFP cells performed poorly in the simulated autoclave tests, while commercial high-temperature-tolerant NMC cylindrical cells and the pouch cells performed similarly. In continuous testing at 85°C, the pouch cells had better capacity retention than both cylindrical cell types. However, the pouch cells suffered from electrolyte permeation through the polymer seals. The pouch cell chemistries incorporated in cylindrical cell format would probably give superior performance to the commercial cells in the autoclave tests. Cell lifetimes were improved when cells were placed into the 120°C oven at a lower voltage suggesting that hospitals should charge Li-ion cells after the autoclaving process instead of standard practice of before.