Binary-borate-based electrolytes enabling 100 °C stable lithium metal batteries without additives

Electrolytes are critical for high-temperature lithium metal batteries. Here we present an additive-free binary borate electrolyte of 0.5 M lithium difluoro(oxalato)borate (LiDFOB) and 0.5 M lithium bis(oxalato)borate (LiBOB) in a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) (1:1:1:3, v / v ) (denoted as Dual-4S), compared with commercial baseline (denoted as Base, 1 M lithium hexafluorophosphate in EC/EMC/DEC = 1:1:1, v / v ) in Li||lithium iron phosphate cells. Dual-4S delivers 90.12% capacity retention after 216 cycles at 100 °C and 1C, while Base fails within 10 cycles. Impedance shows a unique charge transfer resistance (R ct ) decrease in Dual-4S, directly evidencing the synergistic interplay between the two salts. Combined with distribution of relaxation times analysis, these results further confirm the preferential interphase formation of LiDFOB at the electrode surface. Storage tests of Dual-4S electrolyte and cell (8 days, 100 °C) confirm that Dual-4S has excellent thermal stability and cathode protection. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) confirm that Dual-4S forms a thin, dense solid electrolyte interphase rich in lithium fluoride, lithium carbonate, and boron oxide, suppressing parasitic reactions and dendrites. Collectively, this work demonstrates that cooperative salt design can achieve high-temperature stability without complex additives, offering a simpler and more sustainable electrolyte strategy.