Surface oxygen drives electrolyte degradation at Ni-rich battery cathodes

The rapid uptake of lithium-ion batteries for large scale electric vehicle and energy storage applications requires a deeper understanding of the degradation mechanisms that contribute to fading performance. Capacity fade arises because of complex and interlinking degradation mechanisms such as phase transitions, electrolyte decomposition and transition metal dissolution. These degradation reactions are still poorly understood; however, many of them evolve gases as a side product. Here we present detection of gaseous species evolving from operating lithium-ion batteries with chip-based electrochemistry mass spectrometry. We observe oxygen (O2) evolution from a Li(Ni0.8Mn0.1Co0.1)O2 (NMC811) cathode, and electrolyte solvent degradation to CO2 and CO. We study isotopically labelled NMC to monitor where lattice oxygen is incorporated into electrolyte oxidation products. A previously unobserved deleterious catalytic CO oxidation mechanism is identified as we provide the first direct evidence that lattice oxygen loss is coupled to electrolyte decomposition on the oxide surface, rather than by reactive oxygen species. We also report a difference in electrolyte decomposition onset potential and quantify the extent of lattice oxygen loss between EC containing and EC-free electrolytes. The ensuing understanding of the newly identified and deconvoluted degradation mechanisms will facilitate the development of longer lasting lithium-ion batteries, as well as facilitate the development of more accurate battery lifetime models.