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 NaMnO 4 . We assemble a 1.9 V Zn-MnO 4 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-NaMnO 4 system with an open-circuit potential of ∼1.7 V.