Engineering ionic conduction and interface stability in Ta-doped NZSP for all-solid-state Na-ion batteries

The growing demand for safe, affordable, and sustainable energy storage necessitates the development of advanced solid electrolytes for sodium-ion batteries. In this study, we report the first demonstration of Ta^{5 +} doping in monoclinic NZSP (Na_2.9 Zr_1.9 Si₂ Ta_0.1 PO₁₂) with synthesis and comprehensive characterization. Dielectric and modulus studies over a frequency range of 4 Hz to 8 MHz and temperatures spanning 30–150 °C show thermally activated relaxations. In Na/Au/solid electrolyte/Au/Na cells, Ta-NZSP exhibits a stable cycling for over 90 h at 100 µA·cm⁻². To demonstrate practical applicability, we then incorporated Ta-NZSP into full all-solid-state Na-ion cells using NNMO as the cathode and Na metal as the anode, with thin Au interlayer on Na-electrolyte interfaces. The full cell cycled for 100 cycles at 1 C, retaining 57 % of their initial capacity. Our results highlight Ta doping as an effective strategy to engineer lattice distortions, create Na⁺ vacancies, and optimize both bulk and interfacial conduction pathways for next-generation all-solid-state sodium batteries. © 2025 Elsevier B.V.