For lithium-ion batteries of C/(spinel Li- Mn- O), severe capacity loss occurs after storage of the battery at > 50 degrees C. According to our previous studies, this occurrence is predominantly attributable to degradation of the carbon anode, which was induced by electroreduction of Mn(II) dissolved from the spinel; this step is followed by the irreversible electrochemical reaction at the graphite/(Mn deposits)/electrolyte interface. 2-Vinylpyridine (VP) used as an additive in the electrolyte suppressed this degradation; therefore, improving the battery performances. During the first "charge," the electrochemical reductive polymerization of VP monomers at about 0.9 V vs. Li/Li+ resulted in the film formation of poly(2-vinylpyridine) on the graphite surface. The quantity of charge passed for the polymeric film formation depends on the amount of VP addition. Surface analyses using X-ray photoelectron spectroscopy and electron microscopy confirmed that the electrodeposited film blocked the electroreduction of dissolved Mn(II) on the graphite electrode. (c) 2005 The Electrochemical Society.