Composition manipulation of bis(fluorosulfonyl)imide-based ionic liquid electrolyte for high-voltage graphite//LiNi0.5Mn1.5O4 lithium-ion batteries

Ionic liquids (ILs), with wide electrochemical stability window, high thermal stability, and nonvolatility, are promising electrolytes for lithium-ion batteries. Among ILs with various types of anion, bis(fluorosulfonyl)imide (FSI-)-based ILs are particularly appealing owing to their high ionic conductivity, low viscosity, and great anode compatibility. However, strong corrosivity of FSI- toward the Al current collector at high potential restricts their practical utilization. In this study, three strategies are implemented to overcome this limitation. Li+ fraction modulation, FSI-/bis(trifluoromethyl)sulfonylimide (TFSI-) molar ratio optimization, and their synergistic combination are used to achieve optimal charge?discharge of a high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode with an Al substrate. The effects of the IL composition on the electrochemical properties of a graphite anode are also investigated. The proposed IL electrolyte, which lacks any organic solvents, has an optimal Li+/FSI-/TFSImolar ratio and thus can effectively suppress the Al corrosion, allowing a 5-V graphite//LNMO full cell to be realized. A reversible capacity of - 135 mAh g- 1 (based on LNMO) and a capacity retention of - 85% after 200 cycles are found for the full cell. This study opens a new route for FSI--based IL electrolytes in the field of highvoltage and high-safety lithium-ion battery applications.

Automated summary

This study explores strategies to overcome the limitation of FSI- corrosion towards the Al current collector, achieving optimal charge-discharge of a high-voltage LNMO cathode with an Al substrate. The effects of IL composition on the electrochemical properties of a graphite anode are also investigated. The proposed IL electrolyte, without organic solvents, effectively suppresses Al corrosion and enables a 5-V graphite//LNMO full cell.