Ultrathin, room temperature cycling polyphosphazene polymer electrolyte for lithium metal solid-state batteries

Traditional liquid electrolytes in lithium metal batteries pose serious safety risks due to their flammability and volatility, motivating the development of solid polymer electrolytes (SPEs) as safer alternatives. However, achieving both ideal interfacial compatibility and high ionic conductivity remains challenging. Herein, we report the construction of a novel SPE, mPEG350PDCP, by grafting polyethylene oxide (PEO) onto phosphazene via ring-opening polymerization using a straightforward macromolecular nucleophilic substitution strategy. This design allows fine-tuning of side-chain properties to enhance solid electrolyte/electrode interface contact, enabling thinner membranes and improved ionic transport. Comprehensive characterization, including gel permeation chromatography (GPC), electrochemical impedance spectroscopy (EIS), and nuclear magnetic resonance (NMR), confirmed the polymer’s structural and electrochemical features. The mPEG350PDCP electrolyte exhibited a thermal decomposition onset at 209 °C with a 26.5 wt% char yield, demonstrating superior thermal stability. Its electrochemical stability window extended to 4.42 V, surpassing traditional PEO (3.94 V). Symmetric Li/Li cells maintained stable cycling over 2000 h, effectively suppressing dendrite formation. LiFePO₄/Li cells showed stable cycling for 80 cycles at 25 °C/0.2 C with >120 mAh·g⁻¹ capacity and 350 cycles at 60 °C/1.0 C, underscoring excellent performance at elevated temperatures. These results highlight the promise of mPEG350PDCP polymer electrolytes for safe, high-performance lithium metal batteries.