Antimony-doped NASICON-type solid electrolyte with homogeneous sodium-ion flux for high-temperature solid-state sodium batteries

High operation temperatures increase the sodium-ion conductivity of solid-state sodium batteries but may cause early short-circuiting due to sodium-ion flux inhomogeneity and rapid sodium dendrite penetration caused by poor contacts between solid electrolytes particles. This study characterizes Sb-doped Na₃Zr₂Si₂PO₁₂ (Na_3.1Zr_1.9Sb_0.1Si₂PO₁₂, NZSbSP) as a prospective solid-state electrolyte and determines its compatibility with sodium-metal electrodes by examining the cycling performance of symmetric Na/NZSbSP/Na cells at 60 °C. Compared with Na₃Zr₂Si₂PO₁₂, NZSbSP exhibits a higher sodium-ion conductivity and sodium-ion transference number while featuring a lower electronic conductivity and activation energy for sodium-ion conduction. The Na/NZSbSP/Na symmetric cell sustains 3055 h of cycling at 0.1 mA cm⁻², which reflects the superior compatibility of NZSbSP with sodium metal. The postmortem analyses of NZSbSP after high-temperature operation reveal suppressed dendrite formation and the homogeneity of the sodium-ion flux at the NZSbSP–sodium metal interface. A Na_0.67Fe_0.5Mn_0.5O₂/NZSbSP/Na coin cell exhibits a discharge capacity retention of 58.84 % after 50 cycles as well as a high coulombic efficiency and sodium-ion diffusion coefficient. The oxidation of Sb during cycling is shown to prevent electrolyte degradation during high-temperature operation and stabilize the electrode interface. These results demonstrate the feasibility of using NZSbSP in solid-state sodium batteries operated at high temperatures.