Ionic conduction and interfacial stability in Na1<b>+</b>xZr2SixP3<b>−</b>xO12 solid electrolytes: Past, present, and future perspectives

While the development of new solid electrolytes (SEs) is crucial for advancing energy storage technologies, revisiting existing materials with significantly improved knowledge of their physical properties and synthesis control offers significant opportunities for breakthroughs. Na1+xZr2SixP3−xO12 (NaSICON) SEs have recently regained attention for applications in both solid-state and aqueous redox flow batteries due to their improved electrochemical and mechanical properties, along with their inherent electrochemical stability, air robustness, and low manufacturing cost. Recent improvements in NaSICON have primarily targeted macroscopic property enhancements and synthesis techniques. To enable further breakthroughs in the performance of NaSICON SEs, future efforts should focus on understanding how modified synthesis conditions influence atomic and microscopic-scale features, such as conduction channels, electronic structures, phase distributions, and grain boundaries. These features ultimately control ion conductivity, mechanical properties, and electrochemical stability of NaSICON and its interfaces. Here, we review the current understanding of the structure-chemistry-property relationships of NaSICON SEs, focusing on atomic and microscopic levels. First, we introduce the proposed ionic conduction mechanisms in NaSICON crystallites. Then, we explore experimental investigations at phase and grain boundaries to assess ionic conduction and interfacial stability. We also examine strategies to address interfacial challenges such as high resistance and chemical reactions between SEs and electrodes, highlighting the difficulties in analyzing interfaces at the nano/atomic scale. Finally, we provide an outlook on advancing microscopy and spectroscopy techniques to enhance insights into NaSICON SEs ionic conduction and interfacial stability, supporting the development of improved long-duration energy storage devices.