Zwitterionic liquid electrolyte with efficient Li+ transfer modulated by ion pair for low-temperature lithium battery

Abstract

Zwitterions, covalently tethered by cation and anion, show great potential in the development of safe and high-performance electrolyte owing to the inherent characteristics of non-volatility, non-flammability, and electrification but non migration. The interaction between cations and anions in zwitterionic-lithium salt electrolyte is supposed to influence the lithium ions (Li⁺) migration efficiency, but being rarely studied. Herein, zwitterionic liquid (ZIL) electrolytes with three different anionic groups (−PO₄⁻, −SO₃⁻, −COO⁻) were synthesized. We demonstrate that the −PO₄⁻ group with the most negative electrostatic potential exerts the strongest competitive coordination effect on Li⁺, weakening Li⁺-TFSI⁻ binding and promoting lithium salt dissociation. This coordination also facilitates a lower migration energy barrier for Li⁺ and lessens intermolecular forces between zwitterions, reducing the freezing point. The ZIL-P electrolyte with the −PO₄⁻ group shows high Li⁺ transference number (0.54 at −40 °C) and ionic conductivity (1.57 × 10⁻⁴ S cm⁻¹ at −20 °C), outperforming most liquid electrolytes. The enhanced Li⁺ transfer results in excellent cycling stability, with a 98.1 % capacity retention after 500 cycles at 0.5C. Furthermore, the gel electrolyte constructed with ZIL-P and silicon-based framework improved battery performance: LFP|SiGE-ZIL-P|Li after 600 cycles with capacity retention rate of 98.5 % at 0.5C and −20 °C.