Investigating the dominant decomposition mechanisms in lithium-ion battery cells responsible for capacity loss in different stages of electrochemical aging

Cell aging is a major issue in battery cells, as it affects the application capabilities. The mechanisms contributing to aging and capacity loss are not yet fully understood, so that performance enhancements are difficult to achieve. Here, the decomposition mechanisms responsible for capacity loss in LiNi0.6Co0.2Mn0.2O2 (NCM622)/graphite lithium-ion pouch cells containing 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 by weight with 3 wt% vinylene carbonate (VC) are analyzed. For this purpose, absolute amounts of the electrolyte components are determined in cells at six stages of electrochemical aging using High Performance Liquid Chromatography. The resulting, absolute consumptions of the electrolyte components reveal the dominant degradations. Furthermore, complementary analysis methods, namely X-ray photoelectron spectroscopy, gas chromatography, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma optical emission spectrometry are applied. Two phases of electrochemical aging are identified: During formation and short-term cycling, preferential decomposition of EC and VC is observed accompanied by solid electrolyte interphase (SEI) buildup at the graphite particle edges. During long-term cycling, non-preferential decomposition of each electrolyte component is found associated with SEI growth at edges and basal planes of the graphite particles induced by manganese contamination and/or crack formation in the graphite during de-/lithiation.