Solid-state sodium-ion batteries with composite polymer electrolytes and ALD-modified Na_0.7MnO₂ cathodes

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 (TiO₂) 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% TiO₂ 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_0.7MnO₂ (NMO)/CPE/Na coin cells at 55 °C. To further increase the interface stability and enhance electrochemical performance, TiO₂ ultrathin films were coated on NMO particles using atomic layer deposition (ALD). The NMO particles with ten cycles of TiO₂ 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 TiO₂ coatings with advanced CPE formulations to develop high-performance solid-state sodium-ion batteries suitable for large-scale energy storage applications.