Cycle parameter dependent degradation analysis in automotive lithium-ion cells

In this study, we report on the operational parameter dependent degradation mechanisms occurring in cycled large-format automotive lithium-ion cells. The comprehension of these mechanisms is a prerequisite for design and operation of long-life lithium-ion cells. The degradation mechanisms are evaluated in dependence of cycle temperature, cut-off voltage, depth of discharge and discharge current, performing an extensive post-mortem analysis on cells subjected to a one-year-long cycle test. The main degradation mechanisms in the cells cycled at 60 °C are the large formation of gas, gas-assisted lithium plating, and, additionally, temperature-accelerated growth of the solid electrolyte interphase (SEI), as revealed by XPS depth-profiling. The growth of the SEI is intensified by using higher cut-off voltages, while transition metal dissolution is observed via STEM. The manganese ions incorporate into the SEI, causing a strong blue coloration of the anodes’ surface. The major effect in the cells cycled at high depth of discharge is the loss of cathode active material, as revealed by ICP-OES, XRD, and FIB-SEM measurements. The variation of the discharge current has no effect on the type of degradation mechanism occurring in the cells cycled at 20% depth of discharge.