Multi-functional filler ensuring high-performance composite quasi-solid polymer electrolyte for large-scale sodium metal pouch cells

Commercial-level sodium metal batteries require electrolytes with high ionic mobility and excellent thermo-mechanical and electrochemical stability. Conventional flammable liquid electrolytes, prone to dendrite growth and unstable interfacial reactions, rarely perform beyond coin-cell demonstrations. To address these shortcomings, a multifunctional composite quasi-solid polymer electrolyte (QSPE) that incorporates boron nitride (BN) as an engineered filler in a highly conductive polymer blend system has been developed. The optimized formation (15BN QSPE) delivers a room-temperature ionic conductivity of 2.15 mS cm−1 and a sodium-ion transference number of 0.80. Molecular dynamics simulations elucidate the coordination environment and show improved transport in the presence of BN. BN is chemically active and bifunctional: boron acts as an electron acceptor, interacting with solvents and macromolecules, while nitrogen coordinates with sodium ions, tailoring the solvation environment and transport pathways to promote efficient ion migration. The 15BN QSPE is self-extinguishing, resists oxidative thermal degradation, and enables stable cycling in symmetric sodium cells for >1400 h at 0.5 mA cm−2. A Prussian blue full cell achieves >1500 stable cycles at 1C with ∼99 % Coulombic efficiency in coin-cell configuration. A two-layer pouch cell with dual 15BN QSPE layers delivers 600 stable cycles at 0.125C and withstands rigorous mechanical abuse. These results position 15BN QSPE as a scalable, high-performance electrolyte offering enhanced safety and efficiency for next-generation sodium metal batteries.