Surface Lithium Carbonate Influences Electrolyte Degradation via Reactive Oxygen Attack in Lithium-Excess Cathode Materials

Lithium-excess layered oxide cathode materials (Li_(1+x)TM_(1–x)O₂) for lithium-ion batteries achieve high specific capacities (≥250 mA h/g) via redox participation of both transition metals and oxygen anions. While oxygen is initially present as O^2– in the cathode, oxidized oxygen species such as peroxo-like oxygen (O₂^2–) and oxygen gas (O₂) are known to form on charge. In this work, differential electrochemical mass spectrometry (DEMS) is used to study the mechanisms by which lithium carbonate, a common impurity, influences how these oxygen species form and react within the battery. We first show, in agreement with prior studies, that Li₂CO₃ oxidizes electrochemically on charge to evolve CO₂, but not O₂, implying that reactive oxygen species form instead that then react with cell components to form nonvolatile products. To study the effect of Li₂CO₃ on degradation processes at the cathode surface, a Li-excess cathode material Li1.17(Ni_0.2Mn_0.6Co_0.2)_0.83O² (NMC) is synthesized using a method that prevents formation of carbonate impurities in the synthesized material. Isotopically tagged lithium carbonate is then deposited on the NMC surface through controlled exposure to ¹³CO₂ or C¹⁸O₂ gas. DEMS results show that when lithium carbonate is present on the cathode surface, organic fragments containing diatomic oxygen are formed on the cathode surface during charge above 4.2 V versus Li/Li⁺. Isotopic analysis indicates that the diatomic oxygen within these fragments primarily originates from the NMC lattice, with only a minor fraction originating from the Li₂CO₃ itself. Our results therefore suggest that reactive oxygen released from the NMC lattice is triggered by the oxidation of surface lithium carbonate.