The critical role of in-situ lithium fluoride in gel polymer electrolyte for high-performance rechargeable batteries

Lithium-ion (Li-ion) batteries are continuously being developed to enhance safety and energy density. One promising approach involves combining high nickel cathode material (NCM811) with Gel Polymer Electrolyte (GPE). However, achieving compatibility between these components remains a challenge. In this study, we investigate the use of Ethyl difluoroacetate (DFEA) to enhance the mechanical stability, ionic conductivity, and heat-resisting of polyethylene glycol dimethacrylate (PEGDMA) based GPE, thereby improving battery performance. The modified GPE/DFEA exhibits a smoother surface, increased Young's modulus (2.2 GPa), and high ionic conductivity (7.10 mS cm−1). Additionally, it enables the in-situ formation of a LiF-rich interface film on the electrode/electrolyte surfaces. Moreover, the Li||Li symmetrical cell with GPE/DFEA demonstrates stable cycling for 500 h without dendrite growth. Furthermore, Li|Cu asymmetrical cells using GPE/DFEA achieve an average Coulombic efficiency (CE) of 99.0% over 200 cycles at 1 mA cm−2, compared to only 62.2% for liquid electrolyte (LE). Importantly, the GPE/DFEA allows for operation beyond the conventional voltage limit of 4.25 V, reaching 4.38 V. It is successfully implemented in an Ah-level NCM811/graphite pouch cell, resulting in an energy density of 250 Wh/kg and improved cycle stability at 45 °C compared to standard electrolytes. Moreover, the GPE/DFEA-based batteries exhibit enhanced safety performance under conditions such as needle puncture and overcharge. The developed GPE can meet the practical requirements for battery applications.