No abstract

We report herein on the comparative study of LiNiO2 and LiMn2O4 electrodes in three salt solutions, namely, LiAsF6 , LiPF6 , and normalLiCfalse(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 LixMOy active mass with trace HF). © 2000 The Electrochemical Society. All rights reserved.

No abstract

No abstract

We studied the electroanalytical response of Li x Mn 2 O 4 electrodes around 3 V (vs. Li/Li ϩ ) using slow scan rate cyclic voltammetry (SSCV), potentiostatic intermittent titration technique, and electrochemical impedance spectroscopy. Broad anodic and cathodic peaks of typical 50 to 60 mV half-peak width, and a surprisingly large peak potential separation (ca. 150 mV and higher) appear on the SSCV curves of thin Li x Mn 2 O 4 electrodes around 3 V (vs. Li/Li ϩ ). The subsequent potentiostatic titrations performed at 10 mV steps revealed quite an unusual response; an increase in the current with time at 2.94 and 3.00 V (vs. Li/Li ϩ ) during discharge and charge, respectively. The above features of the cyclic voltammograms (at slow scan rate) and the current vs. time response can be understood in terms of a phase transition controlled by slow formation of droplets of a new phase in the bulk of the old one. A model describing such a process, based on the refined Frumkin-type sorption isotherm, which relates to both the electronic and the ionic species which participate in the intercalation reaction, is developed. The SSCV curves simulated according to this model agree well with the experimental ones. We also show a semiquantitative comparison of the electroanalytical characteristics of Li x Mn 2 O 4 electrodes around 3 and 4 V (vs. Li/Li ϩ ).