An in situ STM investigation of EMITFSI ionic liquid on Au(111) in the presence of lithium salt

We report an in situ scanning tunneling microscopic study of surface morphology changes in Au(111) electrode in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) ionic liquid containing LiTFSI salt. The surface processes can be divided into three stages: In the first stage, a reduction wave of dissolved oxygen in the ionic liquid appears at approximately 2.0 V and a network structure covers the surface afterward; in the second stage at around 1.5 V, reduction of trace water is initiated and a surface film containing lithium hydroxide is formed; in the third stage, as potential is further decreased to 0.85 V, decomposition of the EMITFSI ionic liquid occurs, which is accompanied by lithium underpotential deposition and Au-Li alloying. In this stage, the surface experiences significant morphological changes with formation of many clusters on the surface, and eventually becomes electronically less conductive. This unique surface film is understood to be the initial stage formation of a solid electrolyte interphase on gold, which may be a common feature in ionic liquids in the presence of lithium salt.