Facile Gel Polymer Electrolyte Enabling Fast Charging of Quasi-Solid-State Lithium Metal Batteries at −40°C

Abstract

Gel polymer electrolytes (GPEs) are promising for lithium metal batteries (LMBs) due to their high ionic conductivity and excellent mechanical properties. However, conventional GPEs often underperform at low temperatures and high rates. Herein, we develop a low-temperature fast-charging GPE (LFGPE) by incorporating LiPO₂F₂ as an additive into a LiBF₄/DOL/FEC-based precursor. The LFGPE exhibits a high ionic conductivity of 0.49 mS cm⁻¹ (−40°C). Notably, LiPO₂F₂ participates in Li⁺ solvation structures at −40°C, competing with poly-DOL and BF₄⁻, which weakens their tight coordination with Li⁺. Moreover, LiPO₂F₂ exhibits a narrow gap of HOMO-LOMO energy level, preferentially decomposing and inducing formation of P-containing solid electrolyte interphase (SEI). Thus, the LFGPE system generates inorganic-rich SEI containing Li₃PO₄, which accelerates Li⁺ desolvation and Li⁺ migration across the SEI, improving Li⁺ transport kinetics and interfacial stability at −40°C. Consequently, Li//LiCoO₂ (LCO) cells with the LFGPE yield a high reversible capacity of 128 mAh g⁻¹ at 1 C (−40°C). Impressively, the LFGPE even enables Li//LCO cells to sustain over 200 cycles under 3 C charging/1 C discharging at −40°C. This work offers an effective approach of simultaneously enhancing ion transport kinetics and interfacial stability to develop quasi-solid-state LMBs with low-temperature fast charging capability.