Submicron Na₃Zr₂Si₂PO₁₂ solid electrolyte for low-temperature solid-state sodium batteries

Na₃Zr₂Si₂PO₁₂ (NZSP) represents a promising solid electrolyte for solid-state sodium metal batteries owing to its wide electrochemical stability window and excellent thermal stability. However, its practical implementation is constrained by insufficient ionic conductivity and poor interfacial compatibility with sodium metal. Herein, we investigate a microstructure regulation strategy to simultaneously achieve high conductivity and superior interfacial performance through comparative analysis of submicron-grained NZSP (0.36 μm) prepared by dry ball-milling and micron-grained NZSP (5.72 μm) prepared by wet ball-milling with ethanol. The submicron NZSP delivers an ionic conductivity of 1.77 × 10⁻³ S cm⁻¹ at 30 °C, representing a 3.2-fold enhancement over its micron-grained counterpart. It simultaneously demonstrates substantially improved sodium metal compatibility with a reduced interfacial resistance from 272.25 to 38.13 Ω cm² and achieves a high critical current density of 1.2 mA cm⁻² at 30 °C. Stable sodium plating/stripping is sustained for 9000 h at 0.20 mA cm⁻². When assembled in Na₃V₂(PO₄)₃-based quasi-solid-state full cells (comprising a porous NVP cathode wetted only with a small amount of liquid electrolyte, an NZSP ceramic electrolyte, and a Na metal anode), the submicron NZSP enables a high-rate capacity of 100.8 mAh g⁻¹ with 95.8% retention after 3000 cycles at 5C and 30 °C. Remarkably, even at −10 °C, the cells maintain stable operation for 650 cycles at 1C with an initial capacity of 80.1 mAh g⁻¹ and 95.0% retention. These findings highlight the importance of a submicron-grained and dense microstructure in improving the ionic transport, Na-metal compatibility, and low-temperature electrochemical performance of NZSP electrolytes.