[en] The cycling performance, impedance variation, and cathode surface evolution of the Li/LiCoO_2 cell using LiFSI–KFSI molten salt electrolyte are reported. It is found that this battery shows poor cycling performance, with capacity retention of only about 67% after 20 cycles. It is essential to understand the origin of the instability. It is noticed that the polarization voltage and the impedance of the cell both increase slowly upon cycling. The structure and the properties of the pristine and the cycled LiCoO_2 cathodes are investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). It is found that the LiCoO_2 particles are corroded by this molten salt electrolyte, and the decomposition by-product covers the surface of the LiCoO_2 cathode after 20 cycles. Therefore, the surface side reaction explains the instability of the molten salt electrolyte with LiCoO_2. (paper)