Wide-temperature solid-state sodium metal batteries using Na⁺ superionic conductor-type solid electrolytes

Solid-state sodium metal batteries (SSMBs) are considered as one of the critical technologies for safe and high-energy-density batteries. However, most SSMBs encounter poor cycling performance due to the sluggish charge transfer processes across the solid-solid interfaces. Based on a cation doping strategy, Al³⁺ and Zn²⁺ doped Na₃Zr₂Si₂PO₁₂ solid electrolytes (SEs) are comprehensively examined to decouple their ionic conductivities and interfacial resistances with sodium metal in a wide temperature range of -20–80 °C. The Zn²⁺ doping signifies more favorable effect than the Al³⁺ doping on improving the conductivity and reducing the interfacial resistance. The Na_3.20Zr_1.90Zn_0.10Si₂PO₁₂ SE shows an optimal conductivity of 1.58 mS cm^-1 at 30 °C, which is over 4 times higher than that of Na₃Zr₂Si₂PO₁₂, and possesses intrinsically small interfacial resistances of 229.80, 24.72, and 2.96 ohm cm⁻² at -20, 30, and 60 °C, respectively. Stable sodium plating/stripping cycles over long terms are achieved, specifically demonstrating accumulated capacities of 90, 300, and 582 mAh cm^-2 at 0, 30, and 60 °C, respectively. Moreover, full cells using a Na₃V₂(PO₄)₃ cathode exhibit notable cycling stability at 0 °C with a high retention of 90.4 % over 1800 cycles, providing insights into the practical SSMBs operating in diverse temperature conditions.