Mechanism of Electrolyte Decomposition in Practical Lithium-Ion Cell after Long Charge and Discharge Cycles

To elucidate the degradation mechanism of the electrolyte in practical lithium-ion batteries (the C／l M-LiPF6十PC十EMC十DEC十DMC／LiCoO2system)after IQng charge and discharge cycling，the electrolyte decomposition was investigated by the analysis of generated gases in the cells．The test cells were charged and discharged at different cutoff vOltages and currents over1500cycles．Small amount of methane gas，which was regarded as the product frQm DMC， was observed for the cells operated with＆cutoff voltages range between4．2and2，5V．The change in composition of the electrolyte with the ester exchange reactions such as EMC→DMC 十DEC occurred independently of the charge／discharge current arld the cutoff voltage，On the other hand，a Iarge amount Qf carbon dioxide，methane and Other hydrocarbons were observed in the over-charged an(i over-discharge(i ce1Ls． These results have suggeste(i that a decomposition of electrolyte solutions(PC，EMC，DEC and DMC)took place with the release of CO2，CH4，C2H6and C3H6gases． 1 緒 言 分散型電池電力貯蔵用電池としてリチウムニ次電池が 開発されている1)．電力貯蔵用電池は軽量・コンパクト であることが重要であるとともに，大容量で高出力・高