Composition manipulation of bis(fluorosulfonyl)imide-based ionic liquid electrolyte for high-voltage graphite//LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄ (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⁻/TFSI⁻ molar 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 high-voltage and high-safety lithium-ion battery applications.