A Medium-Temperature All-Solid-State Sodium Battery Utilizing Sodium-Beta Alumina and a Polymeric Composite Positive Electrode

Li-ion batteries often use transition metal oxide (TMO) positive electrodes. Sodium-based analogs are also an excellent option for all-solid-state sodium-based batteries. However, combining TMOs with solid electrolytes is challenging, both being rigid in nature. In this work, we use a polymeric secondary electrolyte to combine a sodium manganese oxide composite positive electrode with a sodium-beta alumina solid electrolyte (BASE) to an all-solid-state sodium battery. The composite electrode exhibits intimate interface contact with the solid electrolyte and well-established intra-electrode conduction pathways because mechanical flexible poly(ethylene oxide) acts as an ionically conducting auxiliary agent. The composite electrode shows an ionic conductivity of 5.5·10-6 S cm-1 at 80 °C. The careful co-treatment of the Na|BASE interface, resulting in a superior cycling stability of 1000 h at 0.35 mA cm-2 in a symmetric cell, enhances sodium's wettability to the BASE surface. The full cell achieves an initial discharge capacity of 80 mAh g-1 at an average voltage of 2.78 V vs Na+/Na, corresponding to a specific energy of 155 Wh kg-1 (positive electrode). Hence, we provide a proof-of-concept for all-solid-state cells, which paves the way for numerous combinations of well-conducting polymeric secondary electrolytes with TMO active materials and BASE.