Understanding and Mitigating Acidic Species in All-Fluorinated Electrolytes for a Stable 572 Wh/kg Lithium Metal Battery (LMB)

Abstract

Fluorine-rich electrolytes hold promise to significantly enhance the energy and the safety of lithium metal batteries (LMBs). However, they generate acidic species, especially when lithium hexafluorophosphate (LiPF₆) is used as the lithium salt. This critical issue impedes their wide-scale utilization but has to date received minimum analysis. Herein, we reveal the mechanisms behind the exacerbation of HF generation in LiPF₆-based all-fluorinated electrolytes and propose a universally applicable mitigation strategy. The screened additive Tris(trimethylsilyl)phosphate (TMSPa) reacts with HF and stabilizes PF₅, preventing its further hydrolysis and thereby effectively reducing the HF content in fluorine-rich electrolytes. TMSPa contributes to preferentially form a conductive and protective solid electrolyte interphase (SEI), suppressing interface parasitic reactions and ensuring the structural integrity of electrode materials throughout battery cycling. The all-fluorinated electrolytes developed in this work with the addition of TMSPa (AFE-TMSPa) demonstrates a wide electrochemical window (4.6 V), high-temperature stability (up to 55°C), and enhanced safety for LMBs (flame-retardant and dendrite-suppressing). A Li metal pouch cell (7.2 Ah) employing AFE-TMSPa (NCM811 double sided cathode with a mass loading of 80.72 mg/cm²), and lean electrolytes at 1.23 g Ah⁻¹, achieves an energy density of 572 Wh kg⁻¹ at a 0.1 C rate. In a Li||NCM811 coin cell with a 50 µm thick Li-metal anode and a high-loading NCM811 cathode (19.8 mg cm⁻², 3.96 mAh cm⁻²), the system supports 160 stable cycles with a capacity retention of 89% at a 0.2 C charge and 0.5 C discharge rate.