We investigated effects of high lithium bis(fluorosulfonyl)imide (LiFSI) salt concentration or heat treatment before initial charge on charge-discharge performance of a graphite electrode in a 1-ethyl-3-methylimidazolium (EMImFSI)-based ionic liquid (IL) electrolyte. LiF was observed at the surface of graphite electrodes taken out from 2.00 mol kg −1 LiFSI/EMImFSI system and from 0.32 mol kg −1 LiFSI/EMImFSI system with pre-heat treatment before the initial charge. The surface LiF effectively suppresses the reductive decomposition of EMIm + . As a result, the irreversible capacity of initial cycle was significantly suppressed and the coulombic efficiency of subsequent cycles was greatly improved.