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.

No abstract

Electrochemical reduction and oxidation of a graphite electrode were carried out in nonaqueous electrolytes that consisted of ethylene carbonate/diethyl carbonate ͑1:1͒ solvents where LiClO 4 and NaClO 4 were codissolved. Reversible lithium intercalation into graphite occurred accompanied with no remarkable side reactions in the potential range between 1.5 and 0.02 V vs Li/͑1 mol dm −3 Li + ͒, that is, no sodium deposition and no sodium intercalation were observed. Comparing to results in a sodiumfree electrolyte, i.e., a conventional electrolyte in Li-ion batteries, the initial irreversibility was noticeably suppressed by codissolving sodium ions, and kinetics of the lithium intercalation were enhanced with better capacity retention during cycling. Additionally, the redox behavior of graphite for lithium-ion batteries was improved by sodium ion coexistence, with no inferior influences on the negative and LiMn 2 O 4 positive electrodes and electrolyte. When the sodium ion concentration increased to 0.22 mol dm −3 , the superior reversibility of lithium intercalation was observed in comparison with that in a sodium-free system; however, the higher concentration of sodium ions deteriorated the lithium intercalation properties. We found that a surface layer formed on graphite electrode got less resistive as a lithium-ion conductor by entrapping sodium during the initial electroreduction with optimal Na + concentration.

No abstract

For lithium-ion batteries of C/(spinel Li–Mn–O), the sever capacity loss occurs after storage of the battery at >50 °C. This is mainly due to degradation of the carbon 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. However, 2-vinylpyridine as an additive into the electrolyte was capable of suppressing this degradation of graphite anode, therefore, improved the battery performances. During the first charge, electropolymerization of 2-vinylpyridine from about 0.9 V vs Li/Li+ resulted in film formation of poly(2-vinylpyridine) on the anode surface. The polymer protected the graphite from dissolved Mn(II).