Surface Layer Formation on Thin-Film LiMn[sub 2]O[sub 4] Electrodes at Elevated Temperatures

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Abstract
Thin-film LiMn2O4LiMn2O4 electrodes, which were exposed to pure dimethyl carbonate (DMC) or to a mixture of ethylene carbonate (EC) and DMC (1:1 by volume) containing 1 M LiPF6LiPF6 at elevated temperatures, were studied by using X-ray diffraction, current-sensing atomic force microscopy (CSAFM), cyclic voltammetry, ordinary Raman spectroscopy, and surface-enhanced Raman spectroscopy (SERS). Thin, electronically insulating surface layers were detected by CSAFM and SERS on all electrodes. The surface layer formed by exposure to DMC at 70°C was uniform and preserved the electrode structure, however, it led to complete electrode deactivation, probably due to loss of surface electronic conductivity and slower lithium-ion transport rates through the surface layer. A similar surface layer was formed when the electrodes were exposed to EC­DMC­LiPF6,EC­DMC­LiPF6, however, the layer formed at 70°C did not prevent LiMn2O4LiMn2O4 decomposition and consequent electrode capacity loss. In this case, manganese dissolution was observed, accompanied by the formation of λ­MnO2.λ­MnO2. SER spectra of the surface layers suggest that they were formed as a result of DMC decomposition at the LiMn2O4LiMn2O4 surface. The SER spectra displayed bands characteristic of Li-O-R, carbonate, and carboxyl groups. Derivatives of carbon-oxygen triple bonds or silver-carbon-oxygen groups, which are possibly a result of interactions between the surface layer and silver microparticles, were also detected. © 2001 The Electrochemical Society. All rights reserved.