Stabilizing Na0.7MnO2 Cathodes in PEO-Based Sodium Metal Batteries via Composite Interlayer Design

Abstract

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 Na0.7MnO2. 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.