Quantifying Absolute Amounts of Electrolyte Components in Lithium-Ion Cells Using HPLC

To quantify absolute amounts of electrolyte components in lithium-ion cells, we developed a method for electrolyte extraction from pouch cells using a diluent and subsequent analysis by high-performance liquid chromatography (HPLC) coupled to an electrospray ionization mass spectrometer and an ultraviolet/visible light detector. From LiNi1/3Co1/3Mn1/3O2 (NCM111)/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), electrolytes were extracted using diethyl carbonate as diluent and HPLC analyses were subsequently performed to investigate electrolyte decomposition upon electrochemical aging. 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 were applied. During formation, only VC decomposition and associated deposition of polymerized VC on the anode was identified. However, after long-term cycling, EC decomposition was dominant. Furthermore, LiPF6 decomposition and deposition of LiF on the anode were detected. Electrochemically untreated cells showed no change in electrolyte composition during storage. Unlike analyzing relative values (concentrations/quantity ratios), where only VC decomposition is detected, our approach determining absolute amounts reveals more details and allows to identify decomposition mechanisms in electrolytes upon electrochemical aging.