Dual-cationic pyrrolidinium ionic liquid-reinforced poly (ethylene oxide) electrolyte for lithium metal batteries

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are considered commercially promising for lithium metal batteries (LMBs) due to their high lithium-salt solubility, flexibility, electrochemical stability, and interfacial compatibility. Nevertheless, their practical application is hindered by the high crystallinity of PEO, which leads to low ionic conductivity of PEO at room temperature. In this work, a dicationic ionic liquid, Butyl-3-dimethylpyrrolidine bis[(trifluoromethanesulfonyl)imide] (C 4 PyTFSI), is incorporated as a functional filler into PEO-LiTFSI matrix to form a composite SPE. The C 4 Py 2+ boosts ion conductivity by both disrupting PEO crystallinity via spatial hindrance and promoting Li + ion dissociation through charge shielding. The optimized electrolyte with 5wt%C 4 PyTFSI exhibit a high lithium-ion transference number ( t Li + ) of 0.61, a wide electrochemical stability window of up to 4.5  V , and superior lithium plating/stripping stability exceeding 550 h at 0.1 mA cm −2 . Consequently, the LFP|SPE-5|Li cell delivered excellent rate performance at 30 °C, specific discharge capacities of 162, 154.6, 152.1, 147.6, and 130.2 mAh g −1 were achieved at 0.1C, 0.2C, 0.3C, 0.5C, and 1.0C, respectively. At 40 °C, the cell maintained robust performance with capacities of 159.1, 158.4, 157.1, 154.6, and 147 mAh g −1 at the same rates. Moreover, the cell exhibited remarkable cycling stability, retaining 95% of its capacity after 100 cycles at 0.5C and 30 °C, and maintaining 80% even after 120 cycles under the more demanding conditions of 1C and 40 °C. This work provides new insights into the application of PEO-based SPEs in LMBs.