Instability of lithium bis(fluorosulfonyl)imide (LiFSI)–potassium bis(fluorosulfonyl)imide (KFSI) system with LiCoO₂ at high voltage

The cycling performance, impedance variation, and cathode surface evolution of the Li/LiCoO₂ 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 LiCoO₂ 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 LiCoO₂ particles are corroded by this molten salt electrolyte, and the decomposition by-product covers the surface of the LiCoO₂ cathode after 20 cycles. Therefore, the surface side reaction explains the instability of the molten salt electrolyte with LiCoO₂. (paper)