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

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 [LiFePO₄/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.