Evaluation of cyclic aging tests of prismatic automotive LiNiMnCoO2-Graphite cells considering influence of homogeneity and anode overhang
M. Lewerenz, D.U. Sauer;Journal of Energy Storage Volume 18, August 2018, Pages 421–434
Cyclic aging tests of 20 compressed prismatic automotive Li(NiMnCo)O2|Graphite cells are evaluated. The shallow cyclic aging tests are conducted around five average SOCs with respect to the anode. The cells are cycled at two DODs and two C-rates. The irreversible capacity loss is evaluated by the slope of the near-linear part at the end of aging test. The homogeneity of lithium distribution (HLD) is associated with peak height of differential voltage analysis (DVA) and to capacity difference analysis (CDA). The evaluations of DVA, CDA and capacity fade curve are depending mainly on the average SOC and hardly on DOD or C-rate. The trends correlate with the volume expansion originated from the graphite. The highest HLD and the lowest capacity fade are reached around 50% SOC where hardly any additional volume expansion occurs. In the SOC regions with high volume expansion of the graphite the HLD reduces dramatically and the capacity fade rises towards 0% and 100%, respectively. Due to smeared characteristics in DVA, capacity loss cannot be directly separated into shares related to anode overhang, HLD, loss of active material and residual irreversible losses. The combination of cell compression and high gradients of volume expansion during shallow cycling is found to be the root cause for the flattening of DVA curves.