High-Performance Multifunctional Electrolytes and Additives for Next-Generation Lithium-Ion Batteries

As the demand for rechargeable batteries surges, concerns about the safety of lithium-ion batteries (LIBs) have intensified. Issues with thermal stability, such as fires and explosions resulting from thermal runaway, are primarily caused by the use of volatile and flammable liquid electrolytes that contain linear or cyclic organocarbonate compounds. Thermal runaway can be triggered by overcharging, external shocks, cell defects and excessive heat, which destabilizes the organic liquid electrolytes. This destabilization leads to overheating, heat accumulation, combustion, and ultimately battery explosions. Despite the development of various flame-retardant additives to prevent combustion and explosions, these often come with a trade-off between reducing flammability and maintaining electrochemical performance. Achieving thermal stability has typically resulted in the reduced electrochemical performance, including lower coulombic efficiency and unstable cycle retention properties. Common flame-retardant additives include organophosphorus-based compounds like trimethyl phosphate, triphenyl phosphate, and tris(2,2,2-trifluoroethyl) phosphite, which work by generating phosphorus-containing radicals to trap free radicals. Fluorine or sulfur-based flame retardants, known for their moderate-to-good electronegativity, have relatively low HOMO energy levels, providing higher oxidation stability. Compounds such as fluoropropanesultone, trifluoromethyl sulfones, and bis(4-fluorophenyl) sulfone have been shown to improve the interfacial stability of nickel (Ni)-rich cathodes and 5 V-class LIBs. It is believed that achieving simultaneous improvements in thermal and interfacial stabilities could result in better flame retardancy without compromising electrochemical performance. This study introduces a flame retardant with dual fluorosulfate moieties for a Ni-rich LiNi_0.9Co_0.05Mn_0.05O₂/Lithium cell. The study investigates the thermal and interfacial effects of this synthetic functional additive through surface analysis and mechanistic study, demonstrating its unique flame-retardant properties without sacrificing high capacity and cell cyclability. Lastly, a synergistic effect was confirmed by introducing a small amount of a salt-type additive into the developed trade-off-free fluorosulfate-based flame retardant electrolyte.

Acknowledgement
This research was supported by the Materials Innovation Initiative Project of the National Research Foundation (NRF) funded by the Korean Ministry of Science and ICT (2020M3H4A3081880).