Synergistic dipole and pseudo-hydrogen bonding in phosphate electrolytes for stable elevated temperature lithium batteries

Abstract

Tributyl phosphate (TBP) is regarded as a potential solvent for elevated temperature lithium metal batteries due to its high boiling point, low vapor pressure and excellent thermal stability. However, the strong interaction between the polar P=O groups of phosphate and Li⁺ slows down Li⁺ kinetics, and the vigorous reductive decomposition of phosphate ester compounds on the anode leads to an unstable and fragile solid electrolyte interphase (SEI), resulting in poor cycling stability at elevated temperatures. Herein, an electrolyte system that employs functionalized ethoxylated (pentafluoro)cyclotriphosphazene (PFPN) and TBP establishes a dipole/pseudo-hydrogen bond synergistic solvation relationship to resolve the strong coordination between TBP and Li⁺ and the resulting SEI instability. Specifically, the synergistic dipole-dipole interactions between the N-P ring (δ⁺P) of PFPN and the P=O group (δ^-O) of TBP, combined with pseudo-hydrogen bonds between δ^-F of PFPN and δ⁺H of TBP alkyl chains, generate a drag effect that weakens Li⁺-TBP coordination, thereby enhancing Li⁺ transport kinetics. Moreover, the low lowest unoccupied molecular orbital level of PFPN enables its preferential decomposition on the anode, which cooperates with the FSI^--rich inner solvation sheath to construct a robust =P-N/LiF-rich SEI and cathode electrolyte interphase (CEI), effectively suppressing side reactions between TBP and the Li metal anode. Consequently, the NCM811||Li cells using the electrolytes deliver outstanding rate capability and stable long-term cycling at 60 °C. In addition, the Ah-level Li||NCM811 pouch cell shows excellent safety, characterized by no ignition upon nail penetration at full charge and a high thermal runaway onset of 224 °C from accelerating rate calorimetry testing.

Keywords

Lithium metal batteries / tributyl phosphate / dipole/pseudo-hydrogen bond / high-temperature / safety

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