High-conductivity, low-temperature sintering-compatible NASICON solid electrolyte for enhanced compositing with hard carbon electrode in all-solid-state batteries†

Oxide-based all-solid-state sodium-ion batteries present a safer, more robust and more sustainable alternative to lithium-ion batteries, though fabrication challenges persist, particularly during co-sintering. In this study, we demonstrate that by adding sodium borate-based sintering aids to highly conductive Na₃Zr₂(SiO₄)₂(PO₄) (NZSP)-based materials, both a lower sintering temperature and high ionic conductivity can be achieved. Specifically, the mixture of Na₂CO₃ and B₂O₃ as a sintering aid is crucial, and samples sintered at 900 °C with a Na_3.4Zr_1.95Al_0.05(SiO₄)_2.35(PO₄)_0.65 composition exhibits a Na⁺ ion conductivity exceeding 4 × 10⁻³ S cm⁻¹ at room temperature. These materials are also compatible with co-sintering alongside hard carbon anode materials. The all-solid-state cell, featuring a composite electrode of spherical hard carbon particles and optimized NZSP-based electrolytes, demonstrated stable charge–discharge performance at room temperature, retaining a capacity of 140–220 mA h g⁻¹ across 80 cycles.