Li₂CO₃ decomposition in Li-ion batteries induced by the electrochemical oxidation of the electrolyte and of electrolyte impurities

Layered lithium transition metal oxides are state-of-the-art cathode materials for Li-ion batteries. Nickel-rich layered oxides suffer from high surface reactivity toward ambient air. Besides hydroxides, carbonates are known to be the major surface impurities formed. While the decomposition of Li₂CO₃ in a battery cell has been studied extensively, the mechanistic aspects of its decomposition during cell formation/cycling are still highly controversial. The decomposition reaction of Li₂CO₃ in a standard Li-ion battery electrolyte is studied by on-line electrochemical mass spectrometry, employing an electrode only consisting of Li₂CO₃ and conductive carbon. By modifying the electrode configurations in the cell, we are able to show that the decomposition of Li₂CO₃ occurs as a chemical process without any direct electrochemical oxidation of the Li₂CO₃ particles. Their decomposition proceeds by a chemical process via protons that are formed upon anodic oxidation of the electrolyte solvent and of trace impurities in alkyl carbonate based electrolytes. By adding common impurities in Li-ion battery electrolytes as ethanol and ethylene glycol, whose electrochemical oxidation at rather low anodic potentials (≈ 3.5 V vs Li⁺/Li) results in the formation of protons, the onset of CO₂ evolution from Li₂CO₃ is accordingly shifted to such low potentials. Tracing the proton-induced LiPF₆ decomposition products PF₅/POF₃, the formation of protons can be followed quantitatively and a direct correlation with the CO₂ produced by the proton-induced Li₂CO₃ decomposition is shown. Implications of these findings for transition metal oxide based cathode materials in Li-ion batteries are discussed based on the here found decomposition mechanism.