Optimizing the Na/NASCION electrolyte interface for stable solid-state Na metal batteries

Na super ionic conductor (NASICON)-type Na ₃Zr ₂Si ₂PO ₁₂ (NZSP) electrolytes, with their excellent safety and high energy density, are a promising candidate material for all-solid-state sodium-ion batteries. However, poor sintering performance and interfacial issues have limited its practical application. Herein, we propose the addition of sintering aids to densify grain boundaries and fine-tune the grain to achieve the densest microstructure, improve interfacial contact with sodium, and realize stable Na plating/stripping cycles at room temperature. (CuO) ₂-(B ₂O ₃) ₃ (CBO) is selected to reinforce grain boundary densification, inhibit dendrite growth, and ensure interfacial stability upon sintering at the lower temperature of 1000 °C. The optimized NZSP-0.2CBO demonstrates an ion conductivity reaching 6.83×10 ⁻⁴ S cm ⁻¹ at room temperature, exhibiting a lower activation energy of 0.27 eV compared to bare NZSP. The interface impedance of Na/NZSP-0.2CBO (34.48 Ω cm²) is significantly lower than that of bare NZSP (373.43 Ω cm²). The corresponding Na/NZSP-0.2CBO/Na exhibits stable deposition/stripping cycles over 850 h at 0.1 mA cm ⁻² and room temperature, with a low polarization voltage of 65 mV. These superior performances can be attributed to the densification effect that enhances conductivity, enabling a uniform and enhanced Na ⁺ flux across the electrode/electrolyte interface without dendrite formation. A Na/NZSP-0.2CBO/Na ₃V ₂(PO ₄) ₃ battery is assembled accordingly, exhibiting a capacity of 87.4 mAh g ⁻¹for 100 cycles with a capacity retention rate of 84.9 % and a high average coulombic efficiency of 97.6 % at room temperature.