Polyether/Na₃Zr₂Si₂PO₁₂ Composite Solid Electrolytes for All-Solid-State Sodium Batteries

All-solid-state sodium (Na) batteries are attracting attention as alternative systems to lithium-ion batteries because of the high abundance and safety of Na. Although inorganic solid electrolytes have mainly been investigated because of their high ionic conductivity, such electrolytes show low interfacial stability with electrodes in bulk-type all-solid-state batteries. Compared with inorganic solid electrolytes, solid polymer electrolytes show high formability at electrode/electrolyte interfaces and relatively low ionic conductivity. To improve the ionic conductivity and mechanical properties of polymer electrolytes, addition of inorganic powder has been broadly investigated. However, few composite electrolytes of a polymer and Na-conductive inorganic electrolyte with a high content of inorganic electrolyte have been reported. In this study, composite solid electrolytes (CSEs) of a polyether-based polymer and inorganic Na₃Zr₂Si₂PO₁₂ (NZSP) were prepared to utilize the advantages of both materials. The prepared CSEs exhibited increasing ionic conductivity with decreasing NZSP ratio (<50 wt %). Differential scanning calorimetry and Fourier transform infrared spectroscopy analyses indicated that this behavior was caused by the improved segmental mobility of polyether at low NZSP content, which promoted dissociation of the NaTFSA salt. Conversely, CSEs with a high NZSP ratio showed decreased ionic conductivity because the aggregation of NZSP particles increased grain boundary resistance. With a Na metal electrode, the CSE with a high NZSP content (200 wt %) showed lower interfacial resistance and apparent activation energy compared with those of the NZSP-free system. The Na⁺ transference number of the CSEs was highest for that with a high NZSP content of 200 wt %. Higher NZSP content is therefore expected to realize faster charge transfer during the charge–discharge process than a lower NZSP content. All-solid-state Na–NaCoO₂ and Na–sulfur batteries containing the CSE with 200 wt % NZSP operated at 333 K with capacities of 115 and 170 mAh g–¹, respectively.