Stabilizing Na_0.7MnO₂ Cathodes in PEO-Based Sodium Metal Batteries via Composite Interlayer Design

Poly-(ethylene oxide) (PEO)-based solid polymer electrolytes are attractive for sodium metal batteries but suffer from interfacial instability when coupled with layered oxide cathodes. Here, sodium β″-alumina (SBA) was tested as an ion-conducting filler in a PEO-based composite solid polymer electrolyte (CSPE) and as a major component of a composite cathode interlayer deposited via electrophoretic deposition using a polymeric ionic liquid (PIL). Although the solid polymer electrolyte without ceramic filler and the CSPE exhibited comparable ionic conductivity at 60 °C, the CSPE displayed lower activation energy and improved cell durability, underscoring the importance of ceramic reinforcement beyond bulk transport considerations. Within this stabilized CSPE platform, the SBA-PIL interlayer effectively regulated the cathode-electrolyte interface of layered Na_0.7MnO₂. As a result, polarization growth was suppressed, Coulombic efficiency increased (99.1% vs 97.9%), and capacity retention was significantly improved (76% vs 46% after 50 cycles) compared to pristine cathodes. Impedance analysis further revealed moderated charge-transfer resistance evolution and more stable interfacial kinetics. These findings demonstrate that combining ceramic-reinforced polymer electrolytes with targeted cathode interfacial engineering is an effective strategy to mitigate degradation in PEO-based sodium metal solid polymer batteries.