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 ) O 2 ) 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 (O 2 2– ) and oxygen gas (O 2 ) 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 Li 2 CO 3 oxidizes electrochemically on charge to evolve CO 2, but not O 2, implying that reactive oxygen species form instead that then react with cell components to form nonvolatile products. To study the effect of Li 2 CO 3 on degradation processes at the cathode surface, a Li-excess cathode material Li 1.17 (Ni 0.2 Mn 0.6 Co 0.2 ) 0.83 O 2 (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 13 CO 2 or C 18 O 2 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 Li 2 CO 3 itself. Our results therefore suggest that reactive oxygen released from the NMC lattice is triggered by the oxidation of surface lithium carbonate.