Li-ion batteries are commonly used in electric vehicles. However, the finite lifetime of these batteries is a common problem and its resolution is a significant challenge for electric vehicle companies. Frequently, the lifetime of Li-ion cells is studied using full-charge/full-discharge cycling protocols. Since many users of electric vehicles travel mostly locally, full discharges should occur infrequently if the users are charging nightly. Consequently, many laboratory experiments don't describe reality very well. The present research looked at the cycling performance of NMC622/graphite 2% VC, 1% MMDS and 1% TTSPi (VC211) cells for eight different depth of discharge ranges and three different C-rates at 20°C and 40°C. Testing has been in progress for more than 2 years for most cells. The capacity fade trend versus the change in depth of discharge (ΔDOD) and C-rate was studied. Additionally, operando pressure measurements versus voltage were acquired, to quantify the non-linear expansion of the graphite electrode versus cell state of charge, and compared to the capacity fade trend versus ΔDOD. According to past research¹, the effects of different DOD ranges on capacity fade can be explained by the cracking of the SEI assuming linear graphite expansion with state of charge, however there was little data available to compare to the model predictions. The mathematical model developed by Deshpande et al.¹ has been compared to our extensive experimental data and conclusions were made.

1. R. D. Deshpande and D. M. Bernardi, J. Electrochem. Soc., 164, A461–A474 (2017).