Oxide-Based Composite Electrolytes Using Na₃Zr₂Si₂PO₁₂/Na₃PS₄ Interfacial Ion Transfer

All-solid-state sodium batteries using Na3Zr2Si2PO12 (NASICON) solid electrolytes are promising candidates for safe and low-cost advanced rechargeable battery systems. Although NASICON electrolytes have intrinsically high sodium-ion conductivities, their high sintering temperatures interfere with the immediate development of high-performance batteries. In this work, sintering-free NASICON-based composites with Na3PS4 (NPS) glass ceramics were prepared to combine the high grain-bulk conductivity of NASICON and the interfacial formation ability of NPS. Before the composite preparation, the NASICON/NPS interfacial resistance was investigated by modeling the interface between the NASICON sintered ceramic and the NPS glass thin film. The interfacial ion-transfer resistance was very small above room temperature; the area-specific resistances at 25 and 100 °C were 15.8 and 0.40 Ω cm2, respectively. On the basis of this smooth ion transfer, NASICON-rich (70–90 wt %) NASICON–NPS composite powders were prepared by ball-milling fine powders of each component. The composite powders were well-densified by pressing at room temperature. Scanning electron microscopy observation showed highly dispersed sub-micrometer NASICON grains in a dense NPS matrix to form closed interfaces between the oxide and sulfide solid electrolytes. The composite green (unfired) compacts with 70 and 80 wt % NASICON exhibited high total conductivities at 100 °C of 1.1 × 10–3 and 6.8 × 10–4 S cm–1, respectively. An all-solid-state Na15Sn4/TiS2 cell was constructed using the 70 wt % NASICON composite electrolyte by the uniaxial pressing of the powder materials, and its discharge properties were evaluated at 100 °C. The cell showed the reversible capacities of about 120 mAh g–1 under the current density of 640 μA cm–2. The prepared oxide-based composite electrolytes were thus successfully applied in all-solid-state sodium rechargeable batteries without sintering.