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

Abstract

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.