Abstract
Preventing the decomposition reactions of electrolyte solutions is essential for extending the lifetime of lithium-ion batteries. However, the exact mechanism(s) for electrolyte decomposition at the positive electrode, and particularly the soluble decomposition products that form and initiate further reactions at the negative electrode, are still largely unknown. In this work, a combination of operando gas measurements and solution NMR was used to study decomposition reactions of the electrolyte solution at NMC (LiNi x Mn y Co1-x-y O2) and LCO (LiCoO2) electrodes. A partially delithiated LFP (Li x FePO4) counter electrode was used to selectively identify the products formed through processes at the positive electrodes. Based on the detected soluble and gaseous products, two distinct routes with different onset potentials are proposed for the decomposition of the electrolyte solution at NMC electrodes. At low potentials (<80% state-of-charge, SOC), ethylene carbonate (EC) is dehydrogenated to form vinylene carbonate (VC) at the NMC surface, whereas at high potentials (>80% SOC), 1O2 released from the transition metal oxide chemically oxidises the electrolyte solvent (EC) to form CO2, CO and H2O. The formation of water via this mechanism was confirmed by reacting 17O-labelled 1O2 with EC and characterising the reaction products via 1H and 17O NMR spectroscopy. The water that is produced initiates secondary reactions, leading to the formation of the various products identified by NMR spectroscopy. Noticeably fewer decomposition products were detected in NMC/graphite cells compared to NMC/Li x FePO4 cells, which is ascribed to the consumption of water (from the reaction of 1O2 and EC) at the graphite electrode, preventing secondary decomposition reactions. The insights on electrolyte decomposition mechanisms at the positive electrode, and the consumption of decomposition products at the negative electrode contribute to understanding the origin of capacity loss in NMC/graphite cells, and are hoped to support the development of strategies to mitigate the degradation of NMC-based cells.