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 O2– in the cathode, oxidized oxygen species such as peroxo-like oxygen (O22–) 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(Ni0.2Mn0.6Co0.2)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.