Protective behavior of phosphonate-functionalized imidazolium ionic liquid and its impact on the Li-ion battery performance

The commercial lithium-ion batteries (LIBs) rely on lithium hexafluorophosphate (LiPF6), which is extremely sensitive to moisture and liable to thermal decomposition. Lithium bis (trifluoro methane sulfonyl) imide (LiTFSI), as a promising electrolyte salt, possesses high thermal stability and excellent moisture tolerance. However, LiTFSI is closely related to severe corrosion of the aluminum (Al) current collector at high voltage. Herein, phosphonate-functionalized imidazolium ionic liquid (PFIL) is developed and utilized as an electrolyte co-solvent to inhibit the oxidative dissolution of the Al current collector. PFIL can suppress Al corrosion by participating in the interface reaction and forming a stable and reliable protective film on the surface of Al foils, as confirmed by X-ray photoelectron spectroscopy. Thanks to the corrosion suppression of the Al current collector, the Li||LiNi0.8Mn0.1Co0.1O2 (NCM811) cells with PFIL-containing electrolytes exhibit better cycling performance and improved capacity retention. This work proposes an effective strategy for the advancement of high-voltage LIBs and contributes to promoting the widespread use of the sulfone imide-based lithium salts.