成果報告書詳細
管理番号20110000000505
タイトル*平成22年度中間年報 次世代自動車用高性能蓄電システム技術開発 要素技術開発 リチウム二次電池の安全性に資するイオン液体電解質の開発
公開日2012/6/27
報告書年度2010 - 2010
委託先名独立行政法人産業技術総合研究所
プロジェクト番号P07001
部署名スマートコミュニティ部
和文要約リチウム二次電池の安全性を高める液体電解質媒体として、難燃性・難揮発性が主な特徴である“イオン液体( 常温溶融塩) ”の適用を検討し、既存の有機電解液系リチウム二次電池の構成を生かしつつ、安全性に優れた車載用リチウム二次電池システムの実現に資するイオン液体電解質を開発することを目的とする。
今年度はこれまでの研究を通じて見いだされた新規イオン液体やセパレータを用い、小型ラミネートセルによる出力評価を検討した。また、高性能イオン液体の開発指針を得るべく取り組んでいる計算機科学的手法としてリチウム金属/ イオン液体界面の第一原理DFT 計算によるリチウム金属界面でのアニオン種の効果についての検討、さらにこれまでの検討によりイオン液体の輸送特性の予測に効果を持つことを明らかにしてきた分子動力学計算を用い、計算結果から得られる回転緩和の詳細な検討を行うことにより、分子設計指針として有効であるとの知見を得た。電池構成材料のイオン液体への最適化に関し、新に炭素負極の可能性について検討し、その作動がアニオン種の構造に大きく依存することを明らかとした。
英文要約"Development of Novel Ionic Liquids as Safety Electrolytes for Lithium Secondary Battery" (National Institute of Advanced Industrial Science and Technology (AIST))Less-flammability is one of the most valuable properties of ionic liquids (ILs) containing perfluoroanions. Recently, such a unique "liquid" has been expected to be applied to an electrolyte of a lithium secondary battery in order to improve thermal stability and life, which have been increasingly required for an application especially to hybrid vehicles. The aim of this study is to elucidate the molecular design for highly conductive and thermally stable ionic liquids (ca. 2x10-2 S cm-1 at moderate temperature) and to demonstrate that these ionic liquid sufficiently attains the peak power of 1800 W kg-1 (middle target at 2009) and 2500 W kg-1 (final target at the last year of 2011) in a practical lithium battery system. The results obtained in 2010 were descried as follows.1) We have found that molecular dynamics simulations well reproduce the trend of the ion dependence of the self-diffusion coefficients of ions in ionic liquids. The simulations show that the translational motion of ions in ionic liquids accompanies rotational motion of ions. Therefore, we studied the relationship between the diffusion of ions and their rotation. We found that the experimental and calculated self-diffusion coefficients of ions and calculated rotational correlation functions of the C2-H bond of imidazolium cation in the ionic liquids. There exists the strong correlation between the rotation and translational motion of ions. The self-diffusion coefficients of ions become larger as the rotation of the cation is faster. These results show that molecular dynamics simulation is an effective tool for designing ionic liquids.2) The atomic and electronic structures of room-temperature ionic liquid (RTIL)/Li-metal interfaces have been examined by using periodic density-functional theory (DFT) calculations. We have dealt with various RTILs, consisting of common cationic molecule EMI+ and a series of perfluoroamide anions. We have examined the EMI-anion pair adsorption on a Li(100) surface as a model of RTIL/Li interfaces. For the amide systems, we observed that an anion molecule forms a close contact with partially ionized Li atoms via S-O-Li or S-F-Li ionic bonding, while electrons around the surface Li atoms substantially transfer to a cation molecule, resulting in partial reduction of EMI+. The adsorption energy reveals the order of FSA >> FTA > TFSA, opposed to the results of anion-Li+ binding energy. By using these results, we can discuss the mechanism of the experimental results of interface transport properties. 3) The thermal stability of LiFTA was investigated by TGA analysis under N2 gas flow comparing with LiTFSA and LiFSA. The thermal decomposition temperature of these Li amides were depend on the kind of the counter anion. LiFTA exhibited much higher decomposition temperature than that for LiFSA. This result suggests that the thermal stability of ionic liquids containing FTA anion must be much higher than that containing FSA anion as we expected. And also we found the melting point of LiFTA (100-C) was the lowest among these Li amides.4) Small coin-type two electrode cell containing RTIL (Room Temperature Ionic Liquid) electrolyte was manufactured and power output test during 10 seconds was conducted at several temperature to cofirm the potential of RTIL as electrolyte of the cell with power of 1800W/kg. Power density estimation for scaled-up cell was carried out based on the obtained data and actual cell weight of short-term target cell. It was revealed that cells with EMI[FSA] and EMI[FTA] electrolyte provided estimated power density more than 1800 W kg-1 at 55 oC. Further optimization of the cell is underway in order to improve both of ebnergy density and power density.
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