Li[(FSO2)(n-C4F9SO2)N] versus LiPF6 for graphite/LiCoO2 lithium-ion cells at both room and elevated temperatures: A comprehensive understanding with chemical, electrochemical and XPS analysis

•LiFNFSI-based electrolyte does not decompose after storage at 85°C for two weeks.•Decompositions of LiPF6-carbonate electrolyte would be initialized by the presence of trace amounts of HF and protic impurities.•SEI films formed on graphite anode in the LiFNFSI-based electrolyte are majorly dominated by the reductive products of FNFSI− anions.•Graphite/LiCoO2 cells with LiFNFSI show better higher-temperature storage, current-rate and cycling performances than those with LiPF6.