The study of surface phenomena related to electrochemical lithium intercalation into LixMOy host materials (M = Ni, Mn)

LiNiO2 and LiMn2O4 are important as cathode materials in rechargeable Li ion batteries. The behaviour of LiNiO2 and LiMn2O4 electrodes in three salt solutions, namely, LiAsF6, LiPF6, and LiC(SO2CF3)3 in a mixture of the commonly used ethylene and dimethyl carbonates was studied. The surface chemistry of the electrodes in these solutions was studied by surface-sensitive FTIR spectroscopy, XPS and EDX analysis, and their electrochemical behaviour was studied by variable-scan-rate voltammetry and impedance spectroscopy. It was found that the electrochemical behaviour of these electrodes strongly depends on their surface chemistry. Complicated reactions between the active mass and solution components, which include the solvents, the salt anions, and unavoidable contaminants such as HF and perhaps, HSO3CF3, lead to the precipitation of surface films through which the Li ion has to migrate in order to reach the active mass. The impedance spectroscopy of these electrodes clearly reflects their surface chemistry. It demonstrates the serial nature of the Li insertion-deinsertion processes, which includes, in addition to solid-state diffusion and accumulation, Li-ion migration through surface films and their charge transfer across the surface film/active mass interface, which strongly depends on the chemical composition of the surface films and hence, the solution chosen. LiNiO2 is considerably more reactive with these solutions than LiMn2O4, probably due to its stronger nucleophilic nature. In addition, in LiPF6 solutions, the electrodes' impedance is higher due to precipitation of films comprising LiF, which is highly resistive to Li ion transport (probably produced by reactions of the LixMOy active mass with trace HF). 36 refs.