Molten salts have attracted wide attention as electrolyte for lithium battery due to their nonvolatile and nonflammable features. In this work, the phase diagram of high pure lithium bis(fluorosulfonyl)imide (LiFSI) and KFSI binary system has been measured. This system shows a high transfer number of lithium ion at 85°C and its conductivity reaches to 10−3–10−2 Scm−1 from 40°C to 150°C. The electrochemical windows of this system have been measured using Pt, Al and Cu as working electrode, respectively. The reversibility of lithium deposition in eutectic mixture (LiFSI:KFSI=0.4:0.6 by mole) were investigated on LiCoO2, artificial graphite and LiFePO4, respectively, by cyclic voltammetry using three-electrodes systems. It is found that the half cell with this eutectic system as electrolyte can charge and discharge reversibly for LiFePO4 cathode but fails for MCMB and Li4Ti5O12 anodes. This problem would be solved partially by forming a passivating layer in advance or coating a protection layer on anode.