Comparative Study on Surface Films from Ionic Liquids Containing Saturated and Unsaturated Substituent for LiCoO2

A comparative study is made on the surface films that are deposited on a LiCoO2 electrode by oxidative decomposition of room-temperature ionic liquids (RTILs) that have either a saturated (propyl) or an unsaturated (allyl) substituent on the pyrrolidinium cation. The surface film deposited from the former RTIL does not so perfectly cover the LiCoO2 electrode surface that the film deposition continues with cycling, leading to a gradual increase in the electrode polarization and an eventual capacity fading. From the allyl-containing RTIL, however, a uniformly covered surface film is deposited even after a single charging to give better cyclability to the LiCoO2 electrode. The latter film contains a larger amount of organic carbon species relative to that found in the former. An enhanced film property is also observed by adding vinylene carbonate that has an unsaturated moiety, ensuring that the unsaturated functional groups are responsible for such favorable film properties.