Functionality of 1‐Butyl‐2,3‐Dimethylimidazolium Bromide (BMI‐Br) as a Solid Plasticizer in PEO‐Based Polymer Electrolyte for Highly Reliable Lithium Metal Batteries

Abstract To address the challenges associated with solid polymer electrolytes, flame‐retardant organic ionic plastic crystals (OIPCs) have been utilized as a solid plasticizer in composite polymer electrolytes (CPEs). In this study, 1‐butyl‐2,3‐dimethylimidazolium bromide (BMI‐Br) is used as an OIPC material. BMI‐Br and LiTFSI are initially mixed in an acetonitrile (ACN) organic solvent for a certain time. Anion exchange takes place in this mixing, replacing the Br − in BMI‐Br with TFSI − . As a result, BMI‐TFSI and Li‐Br are formed. Here, BMI‐TFSI acts as an ionic liquid, while Li‐Br serves as a salt. The 10% BMI‐Br content (BMI‐Br‐10 CPE) exhibits significant ionic conductivity (σ = 2.34 × 10 −3 S cm −1 at 30 °C), wide window (up to 4.57 V), and flame retardancy. Furthermore, the BMI‐Br‐10 CPE demonstrates galvanostatic lithium plating stripping cycling stability at 100 and 300 µA cm −2 for 800 and 500 h against Li‐metal, respectively, without a significant overpotential shooting. Furthermore, at 60 °C, the BMI‐Br‐10 CPE in LiFePO 4 /BMI‐Br‐10/Li batteries demonstrates an initial capacity of 146.9 mAh g −1 , capacity retention of 99.7% and high coulombic efficiency (99.5%) after 300 cycles at 1C.