Electrografting solid polymer electrolytes for separator-less structural sodium batteries

Abstract

Sodium ion batteries (SIBs) are emerging as an attractive energy storage technology due to the accessibility, global abundance and low cost of sodium. However, improving their energy density to reduce system weight, particularly for mobile applications, remains a challenge. Structural batteries address this issue by integrating energy storage and mechanical load-bearing functionality into a single material. Here, we present electrografting as a single-step method to uniformly coat individual carbon fibres with a 1.1 μm thick, chemisorbed PEG-acrylate/NaTFSI-based solid polymer electrolyte (SPE). This enables the fabrication of separator-less structural sodium batteries with high energy and power density. The SPE rapidly forms a passivating layer on the carbon fibre surface, exhibiting excellent electrochemical stability, thermal resilience, and low overall resistance. A post-synthesis leaching step is critical to remove unreacted monomer, thereby minimising irreversible first-cycle capacity loss and the SPE resistance. The resulting SPE-coated electrodes deliver high specific capacities (150 mAh g⁻¹) and coulombic efficiencies >99% over 100 cycles. This approach opens new pathways for lightweight, high-performance structural and conventional sodium battery systems.