Preventing dehydrofluorination of PVDF based solid polymer electrolytes via TFA/Acetone additive

Abstract

Solid polymer electrolytes (SPEs) have emerged as a promising electrolyte system for improving the safety of lithium-ion batteries compared with conventional liquid electrolytes. However, their intrinsically low ionic conductivity remains a critical obstacle to practical implementation. To address this limitation, composite polymer electrolytes (CPEs) embedding inorganic fillers have been developed to improve ion transport. Among various fillers, garnet-type Li₆.₄La₃Zr₁.₄Ta₀.₆O₁₂ (LLZTO) is particularly attractive due to its high intrinsic ionic conductivity, ability to provide additional ion- conduction pathways, and excellent chemical stability against lithium metal. Nevertheless, when incorporated into PVDF-based electrolytes, the alkaline surface environment of LLZTO induces excessive dehydrofluorination of the polymer matrix, leading to deteriorated mechanical integrity, interfacial stability, and overall cell performance. To mitigate these issues, surface modification of LLZTO via acid treatment has been widely investigated. Recent studies have demonstrated that trifluoroacetic acid (TFA) treatment followed by thermal processing can remove surface contaminants and form a LiF-rich layer to suppress impurity reformation. However, such approaches typically require multi-step procedures and high-temperature treatment, which can induce lithium depletion in the LLZTO lattice and reduce ionic conductivity. Herein, we present a simplified and effective one-step in- situ strategy by directly incorporating TFA into the polymer electrolyte precursor solution. This approach enables simultaneous surface purification of LLZTO and its immediate encapsulation within the PVDF matrix, effectively suppressing the reformation of Li₂CO₃ and LiOH even under ambient exposure, without the need for high-temperature processing. Furthermore, acetone is introduced as a co-solvent to moderate the acidity of TFA, preventing excessive lithium leaching from LLZTO and avoiding polymer degradation. As a result, the resulting PVDF-based CPE exhibits improved electrochemical stability, enhanced cycling performance, and prolonged cell lifetime. This work provides a simple yet robust route for tailoring LLZTO–polymer interfaces and offers a practical strategy for achieving high-performance, long-life composite polymer electrolytes for solid-state lithium metal batteries.