Study of surface phenomena related to electrochemical lithium intercalation into Li_xMO_y host materials (M = Ni, Mn)

We report herein on the comparative study 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. The surface chemistry of the electrodes in these solutions was studied by surface-sensitive Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis, and their electrochemical behavior was studied by variable-scan-rate voltammetry and impedance spectroscopy. It was found that the electrochemical behavior of these electrodes is strongly dependent 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 Li_xMO_y active mass with trace HF).