Non-reactive sintering enhances density and ionic conductivity of NASICON solid electrolytes

NASICON materials are promising solid electrolytes for room-temperature sodium solid-state batteries and are typically synthesized via solid-state reaction. While sintering has been extensively studied, the effect of calcination on electrolyte properties remains poorly understood. In this work, the temperatures at which the NASICON phase forms in Na3Zr2Si2PO12 and Na3.4Zr2Si2.4P0.6O12 are identified. Calcination temperature is then varied between 900 °C and 1200 °C to obtain powders with different degrees of reaction prior to sintering. Under identical sintering conditions, higher NASICON phase content in the calcined powder is shown to yield denser electrolytes. Non-reactive sintering also improves grain boundary conductivity, increasing it by 130% for Na3.4Zr2Si2.4P0.6O12 and raising total conductivity from 1.60 to 2.95 mS·cm⁻¹ for powders calcined at 900 °C and 1200 °C, respectively. Finally, it is shown that phosphorus loss during processing compromises cycling stability against Na metal electrodes, and that adding off-stoichiometric phosphorus resolves this issue, while reaching a critical current density of 5.0 mA·cm⁻² and a room-temperature conductivity of 3.82 mS·cm⁻¹ with over 400 hours of stable cycling. Overall, these findings directly relate synthesis and processing conditions to the final material properties and electrochemical performance of NASICON solid electrolytes.