Polycarbonate‐Based Solid‐Polymer Electrolytes for Solid‐State Sodium Batteries

Abstract Solid‐polymer electrolytes comprised of polypropylene carbonate (PPC) and varied sodium bis(fluorosulfonyl)imide (NaFSI) salt concentrations are investigated for implementation as a conductive solid polymer electrolyte into solid‐state cathode composites utilizing a sodium‐layered oxide active material. The ionic conductivity generally increases with NaFSI salt content, reaching ≈1 mS cm ⁻¹ at 80 °C at the highest salt concentration (PPC:NaFSI = 0.5:1). Through an all‐in‐one slurry casting method, Na _2/3 Ni _1/3 Mn _2/3 O ₂ cathode composites are fabricated in which the dispersed PPC electrolyte acts as the primary binder. Enabled by a bilayer polymer electrolyte system, cycling performance with the PPC cathode electrolyte is optimized with respect to salt concentration and anode material. The best cyclability is achieved with a moderate salt concentration electrolyte (PPC:NaFSI = 5:1), showcasing an initial capacity of 83 mA h g ⁻¹ with a remarkable 80% capacity retention after 150 cycles at C/5 rate and 60 °C. The superior performance of the lower salt concentration electrolyte is attributed to better electrochemical stability, as confirmed by linear sweep voltammetry and online electrochemical mass spectrometry measurements. These results underscore the potential of carbonate‐based polymer electrolytes and the importance of balancing electrolyte conductivity and stability in cell design.