Regulating Amine Substitution in Fluorosulfonyl-Based Flame-Retardant Electrolytes for Energy-Dense Lithium Metal Batteries

Sulfone-based electrolytes offer unusually high anodic and thermal stability that in principle makes them promising candidates for fabricating energy-dense lithium metal batteries (LMBs). Their uses in practical batteries are currently limited by their inability to sustain long-term Li metal plating/stripping processes due to their high reactivity toward the Li metal. Here, we report on the design and synthesis of a unique family of fluorosulfonyl group-based (FSO₂^-) molecules, modified with ethyl (FSE)/<i>N</i>,<i>N</i>-dimethyl (FSNDM)/<i>N</i>,<i>N</i>-diethyl (FSNDE)/<i>N</i>-pyrrolidine (FSNP) end groups to create exceptionally stable single-salt single-solvent electrolytes. The flammability, solvation structure, ion transport, Li metal deposition kinetics, and high-voltage stability of the electrolytes are systematically studied. It is shown that the electrolytes are nonflammable, possess weak solvation characteristics, yet manifest high room-temperature ionic conductivities (1.6-6.1 mS cm^-1) and low solution viscosities. In comparison to FSE, the FSNDM-, FSNDE-, and FSNP-based electrolytes exhibit an exceptionally reversible Coulombic efficiency for Li metal plating/stripping (>99.71% over 800 cycles) and exhibit typical oxidative stability at voltages exceeding 4.6 V. Deployed as electrolytes in Li metal batteries (20 μm Li anode and 3 g A h^-1 electrolyte) with high-loading (18.5 mg cm^-2) LiNi_0.8Co_0.1Mn_0.1O₂ cathodes, 329 cycles have been achieved before 80% capacity retention. Six Ah Li metal pouch cells based on the designed electrolytes also exhibit high stability and high energy density (496 W h kg^-1) for over 150 cycles with at most 2.7% volume expansion. Our findings demonstrate that through an intentional molecular design, sulfone electrolytes provide a robust route toward nonflammable Li metal compatible electrolytes with practical high-voltage cathodes.