Boosting Grain Boundary Ionic Conductivity in NASICON‐Type Solid Electrolytes Through Vanadium Doping for Next‐Generation Sodium Batteries

All‐solid‐state batteries have been the focus of current research to address challenges related to economic, environmental, and safety concerns associated with state‐of‐the‐art lithium‐ion and sodium‐ion batteries (SIBs). Despite the emergence of all solid‐state SIBs , the subpar room temperature sodium‐ion conductivity of typical solid‐state electrolytes is a major hurdle in their commercialization. Few material options exist, with NASICON‐type Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 (NZSP) being one of them. In this work, the synthesis of NZSP with vanadium (V) partially substituting zirconium (Zr), using a solid‐state reaction method, is reported. Incorporating V 4+ in the NZSP matrix improves conduction across grain boundaries, facilitated by the percolation of ion‐conductive precipitants. The bulk ionic conductivity is measured to be ≈2.34 mS cm −1 , a twofold increase over pure NZSP (≈1.16 mS cm −1 ). Subsequently, symmetric cells assembled with the V‐doped NZSP (Na || NZVSPx || Na) operate stably for 100 cycles at 0.1 mA cm −2 while also demonstrating good electrochemical compatibility with sodium metal, evidenced by a critical current density of 0.8 mA cm −2 . The applicability of this electrolyte in a quasi‐solid‐state battery configuration with Na 3 V 2 (PO 4 ) 3 as the cathode is also conducted in this study, highlighting its potential in next‐generation SIBs.