Glass-Free NaSICON with High Ionic Conductivity and Chemical Stability for Sodium Metal and Redox Flow Batteries

Sodium batteries are increasingly recognized as cost-effective alternatives to lithium batteries, particularly for applications such as grid-scale energy storage and light-duty vehicles. These systems often rely on energy storages like redox flow batteries (RFBs) and solid-state batteries (SSBs), which require solid electrolytes that are both mechanically robust and exhibit high ionic conductivity while exclusively transporting sodium ions. Among the potential sodium-ion conductors, NaSICON (Sodium Super Ionic Conductor) stands out as the most promising candidate due to its excellent ionic conductivity and structural stability.
Despite extensive research reporting ionic conductivities of 1–5 mS cm^-1 in NaSICON, the presence of a chemically unstable and ionically resistive glassy phase remains a persistent challenge. While this secondary phase might not significantly impact solid-state battery performance, it is prone to degradation in aqueous environments, thereby limiting its use in RFBs that require chemical stability against aqueous electrolytes.
In this study, Na_3.4Zr₂Si_2.4P_0.6O₁₂ was synthesized using a solution-assisted solid-state reaction followed by reactive rapid induction hot-pressing. The resulting NaSICON exhibits a glass-free microstructure with nanosized grains and demonstrates exceptional ionic conductivity of 7.3 mS cm^-1. Furthermore, it was found to be stable against sodium metal and showed negligible degradation over 14 days in harsh aqueous electrolytes.
This work presents a novel synthesis route for producing chemically stable, high-conductivity NaSICON, paving the way for its application in both sodium SSBs and aqueous-based RFBs.