The study of surface phenomena related to electrochemical lithium intercalation into LixMOy 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 mix ture of the commonly used ethylene and dimethyl carbonates. The: surface ch emistry of the electrodes in these solutions was studied by surface-sensiti ve Fourier transform infrared spectroscopy, X-ray photoelectron spectroscop y, 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 stron gly 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 migra te in order to reach the active mass. The impedance spectroscopy of these e lectrodes clearly reflects their surface chemistry. It demonstrates the ser ial nature of the Li insertion-deinsertion processes, which includes, in ad dition 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 surfa ce films and hence, the solution chosen. LiNiO2 is considerably more reacti ve with these solutions than LiMn2O4, probably due to its stronger nucleoph ilic nature. In addition, in LIPF6 solutions, the electrodes' impedance is higher due to precipitation of Films comprising LiF, which is highly resist ive to Li ion transport (probably produced by reactions of the LixMOy activ e mass with trace HF). (C) 2000 The Electrochemical Society. S0013-4651(99) 09-015-1. All rights reserved.