Highly concentrated dual-anion electrolyte for non-flammable high-voltage Li-metal batteries

Abstract

With a higher demand towards the energy density of power batteries for electric vehicles, the Li-metal batteries (LMBs) have staged a comeback in recent years. However, the low cycle efficiency and safety issues associated with Li dendrite growth and electrolyte combustion severely limit their practical applications. Here we prepare a highly concentrated TFSI-FSI dual-anion electrolyte (HCDE) based on lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), N-ethyl-N-methyl-pyrrolidinium bis(fluorosulfonyl)imide ([C2mpyr][FSI]), and dimethyl carbonate (DMC). The co-solvent of [C2mpyr][FSI] and DMC exhibits promoted ionic conductivity, low viscosity and non-flammability. In HCDE, the TFSI-FSI dual-anion is preferentially decomposed to obtain a robust inorganic solid electrolyte interphase (SEI), which induces a dense and uniform Li deposition on the Li-metal surface and effectively suppresses Li dendrite growth. Meanwhile, it also enables good suppression of Al current collector corrosion and stable cycling performance at high-voltage owing to the high salt-to-solvent ratio. The LFP (LiFePO4)/Li cells steadily deliver a long-term cycle life and obtain ~80% capacity retention with an extremely high CE (~99.9%) after 500 cycles, and the NCM523 (LiNi0.5Co0.2Mn0.3O2)/Li cells retain ~95% of their original capacity after 100 cycles at a high cut-off voltage of 4.5 ​V. According to X-ray photoelectron spectroscopy (XPS) and molecular dynamic (MD)-density functional theory (DFT) simulations, the forming mechanism of SEI layer is suggested.