Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes

Electrochemical intercalation of lithium into a natural graphite anode was investigated in electrolytes based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion. Graphite electrode was less prone to forming effective passivation film in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Reversible intercalation/de-intercalation of TMHA cations into/from the graphene interlayer was confirmed by using cyclic voltammetry, galvanostatic measurements, and ex situ X-ray diffraction technique. Addition of 20 vol% chloroethylenene carbonate (Cl-EC), ethylene carbonate (EC), vinyl carbonate (VC), or ethylene sulfite (ES) into the ionic electrolyte resulted in the formation of solid electrolyte interface (SEI) film prior to TMHA intercalation and allowed the formation of Li–C6 graphite interlayer compound. In the ionic electrolyte containing 20 vol% Cl-EC, the natural graphite anode exhibited excellent electrochemical behavior with 352.9 mAh/g discharge capacity and 87.1% coulombic efficiency at the first cycle. A stable reversible capacity of around 360 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. Mechanisms concerning the significant electrochemical improvement of the graphite anode were discussed. Ac impedance and SEM studies demonstrated the formation of a thin, homogenous, compact and more conductive SEI layer on the graphite electrode surface.