For lithium-ion batteries of C/͑spinel Li-Mn-O͒, severe capacity loss occurs after storage of the battery at Ͼ50°C. According to our previous studies, this occurrence is predominantly attributable to degradation of the carbon anode, which was induced by electroreduction of Mn͑II͒ dissolved from the spinel; this step is followed by the irreversible electrochemical reaction at the graphite/͑Mn deposits͒/electrolyte interface. 2-Vinylpyridine ͑VP͒ 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 poly͑2-vinylpyridine͒ 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 Mn͑II͒ on the graphite electrode.

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