Densification of a Solid-State NASICON Sodium-Ion Electrolyte below 400 °c by Cold Sintering with a Fused Hydroxide Solvent

A modified cold sintering process is described, which permits the densification of the prototypical sodium-ion electrolyte, Na3Zr2Si2PO12, to greater than 90% relative density at a process temperature below 400 °C. The room-temperature grain boundary ionic conductivity is greater than 2 × 10-4 S/cm. Sintering of Na3Zr2Si2PO12 to such densities and conductivities typically requires sintering near 1200 °C for many hours. We modify the cold sintering process by replacing the aqueous transient solvent with a fused hydroxide (NaOH, Tm = 312 °C) to increase the reactivity of the solvent-particle interaction while also retaining the increased driving forces for densification characteristic of cold sintering, namely, the transient nature of the solvent and uniaxial pressure applied to an open system. We demonstrate the changes in phase purity, conductivity, and density by varying the process temperature, weight fraction hydroxide, and dwell time. The best results are obtained near 375 °C, 10 w/w NaOH, and 3 h of sintering.