NaSICON Prepared by a Solution-Assisted Reaction Shows Enhanced Stability for High-Voltage Aqueous Redox-Flow Batteries

Sodium superionic conductors (NaSICONs) have garnered considerable attention as ion-exchange membranes in aqueous redox-flow batteries because they can eliminate crossover-induced capacity fade. Two challenges to their practical use are microstructural instability in aqueous solutions and low total conductivity (typically ≤1 mS/cm at room temperature), which causes high cell resistance. In this study, we evaluate the potential for NaSICON synthesized via a solution-assisted solid-state reaction (NZSP_SA-SSR) to address these challenges. Upon immersion in a series of neutral pH to strongly alkaline electrolytes, NZSP_SA-SSR pellets show a more stable impedance and microstructure over time than conventional NaSICON pellets synthesized from commercially available powder (NZSP_COMM). We observe prominent etching of the glassy phase in NZSP_COMM, whereas the amount of glassy phase in NZSP_SA-SSR is negligible. NZSP_SA-SSR pellets show no significant change in the microstructure even when exposed to solutions with high (∼1 M) concentrations of K⁺ ions and strongly oxidizing NaMnO₄. We assemble a 1.9 V Zn-MnO₄ flow cell containing a NZSP_SA-SSR membrane, and it demonstrates good cycling stability for close to 100 h. NZSP_SA-SSR also shows promise for accommodating other high-voltage cell chemistries, such as a pH-decoupled NaCrPDTA-NaMnO₄ system with an open-circuit potential of ∼1.7 V.