Surface layer formation on thin-film LiMn2O4 electrodes at elevated temperatures

Thin-film LiMn2O4 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 LiPF6 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 degreesC 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 EC-DMC-LiPF6, however. the layer formed at 70 degreesC did not prevent LiMn2O4 decomposition and consequent electrode capacity loss. In this case, manganese dissolution was observed, accompanied by the formation of lambda -MnO2. SER spectra of the surface layers suggest that they were formed as a result of DMC decomposition at the LiMn2O4 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. (C) 2001 The Electrochemical Society.