Solid-state sodium-ion batteries with composite polymer electrolytes and ALD-modified Na0.7MnO2 cathodes

• A composite polymer electrolyte (CPE) incorporating TiO 2 and LLTO was developed for solid-state sodium-ion batteries. • Ultrathin TiO 2 films were deposited on Na 0.7 MnO 2 cathodes via atomic layer deposition (ALD) to enhance interface stability. • The combination of ALD TiO 2 coatings with TiO 2 /LLTO-based CPE effectively improved overall electrochemical performance. Achieving high ionic conductivity in solid state electrolytes and reducing the interfacial resistance between solid state electrolytes and electrode materials are critical challenges in the development of solid-state batteries. This study explores the integration of the high ionic conductivity of inorganic ceramics and the flexibility of organic polymers to create a composite polymer electrolyte (CPE) for sodium-ion batteries. We developed a CPE comprising a poly(ethylene oxide) (PEO) polymer matrix, and lithium lanthanum titanium oxide (LLTO) and titanium dioxide (TiO 2 ) as ceramic components. The resulting CPE exhibited a sodium-ion conductivity of 0.20 mS cm −1 at 55 °C, maintaining the thermal stability and inherent flexibility of polymer electrolytes up to 330 °C. The combination of 5 wt% LLTO and 10 wt% TiO 2 in the CPE reduced interfacial resistance and enhanced ion transport, resulting in an initial reversible capacity of 138.1 mA h g −1 and stable charge/discharge cycling performance with negligible capacity loss over 70 cycles in Na 0.7 MnO 2 (NMO)/CPE/Na coin cells at 55 °C. To further increase the interface stability and enhance electrochemical performance, TiO 2 ultrathin films were coated on NMO particles using atomic layer deposition (ALD). The NMO particles with ten cycles of TiO 2 ALD exhibited an ionic conductivity of 0.37 mS cm −1 and demonstrated the highest discharge capacity of 160 mAh g −1 , maintaining this performance over 100 cycles of charge/discharge with significantly reduced interfacial resistance, suppressed undesirable side reactions, and minimized Jahn-Teller distortion in the NMO structure. This study highlights the potential of combining ALD TiO 2 coatings with advanced CPE formulations to develop high-performance solid-state sodium-ion batteries suitable for large-scale energy storage applications.