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

Lithium (fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide (LiFNFSI) is investigated as conducting salt to replace conventional used LiPF6 for lithium-ion batteries. The stabilities of electrolytes of 1.0 M LiFNFSI- and LiPF6-ethylene carbonate (EC)/ethyl methyl carbonate (EMC) are comparatively studied at both room (25 °C) and elevated temperatures (60 and/or 85 °C) by using NMR. It is found that the electrolyte of LiFNFSI does not decompose after storage at 85 °C for 14 days; however, both LiPF6 and carbonate solvents occur degradations even after storage at 25 °C, and degrade prominently with increasing the temperature. A new mechanism for continuous decompositions of both LiPF6 and carbonate solvents, being initialized by trace amounts of HF and protic impurities, has been suggested. The electrochemical performances of LiFNFSI for graphite/LiCoO2 Li-ion cells have been comparatively investigated with those of LiPF6 at both 25 and 60 °C, with particular attention to characterizing the electrode/electrolyte interphases formed on both electrodes by using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). It is demonstrated that LiFNFSI is advantageous over LiPF6 in the high-temperature storage, current-rate and cycling tests, particularly at elevated temperature; however, the impedances are all larger for the LiFNFSI-based cells than for the LiPF6-based ones. Analyses of XPS reveal that the chemical compositions of electrode/electrolyte interphases formed on both electrodes are highly dependent on the types of lithium salts. Particularly, on graphite anode, the solid-electrolyte-interphase (SEI) films formed in the LiFNFSI-based electrolyte are majorly dominated by the reductive products of FNFSI− anions, and are relatively stable at elevated temperature, while those formed in the LiPF6-based electrolyte are largely governed by the reductive products of carbonate solvents, and occur significant dissolutions and regrowth at elevated temperature. All above results suggest that the improved capacity retention for the cells with LiFNFSI is mainly attributable to the robust nature of the SEI films formed on graphite anode, and the superior stability and absence of HF contamination for the LiFNFSI-based electrolyte, while the rapid capacity fading of the cells with LiPF6 is essentially due to the decompositions and regrowth of SEI films on graphite anode, and the detrimental impact of HF and protic residues in the electrolyte of LiPF6.