NaSICON Prepared\nby a Solution-Assisted Reaction Shows\nEnhanced Stability for High-Voltage Aqueous Redox-Flow Batteries

Sodium superionic conductors (NaSICONs) have garnered\nconsiderable\nattention as ion-exchange membranes in aqueous redox-flow batteries\nbecause they can eliminate crossover-induced capacity fade. Two challenges\nto their practical use are microstructural instability in aqueous\nsolutions and low total conductivity (typically ≤1 mS/cm at\nroom temperature), which causes high cell resistance. In this study,\nwe evaluate the potential for NaSICON synthesized via a solution-assisted\nsolid-state reaction (NZSP<sub>SA‑SSR</sub>) to address these\nchallenges. Upon immersion in a series of neutral pH to strongly alkaline\nelectrolytes, NZSP<sub>SA‑SSR</sub> pellets show a more stable\nimpedance and microstructure over time than conventional NaSICON pellets\nsynthesized from commercially available powder (NZSP<sub>COMM</sub>). We observe prominent etching of the glassy phase in NZSP<sub>COMM</sub>, whereas the amount of glassy phase in NZSP<sub>SA‑SSR</sub> is negligible. NZSP<sub>SA‑SSR</sub> pellets show no significant\nchange in the microstructure even when exposed to solutions with high\n(∼1 M) concentrations of K<sup>+</sup> ions and strongly oxidizing\nNaMnO<sub>4</sub>. We assemble a 1.9 V Zn-MnO<sub>4</sub> flow cell\ncontaining a NZSP<sub>SA‑SSR</sub> membrane, and it demonstrates\ngood cycling stability for close to 100 h. NZSP<sub>SA‑SSR</sub> also shows promise for accommodating other high-voltage cell chemistries,\nsuch as a pH-decoupled NaCrPDTA-NaMnO<sub>4</sub> system with an open-circuit\npotential of ∼1.7 V.