While lithium bis(fluorosulfonyl) imide (LiFSI) is widely used in current Li-ion batteries (LIBs), the role of LiFSI in the LIB performance remains elusive. We herein elucidate the effects of LiFSI on the electrochemical performance of graphite anodes in comparison with those of lithium hexafluorophosphate (LiPF6). An in-depth electrochemical analysis using graphite/Li half cells and graphite/graphite symmetric cells confirms that LiFSI provides little improvement to the cyclability of the graphite anode at 25 °C, but enables far better performance in cycle and storage tests at 60 °C. The superior thermal stability of the graphite anode in LiFSI electrolyte is attributed to the formation of a thin, inorganic-rich solid electrolyte interphase (SEI) layer as indicated by differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) measurements.
