Realizing Highly Reversible Nb 5+ /Nb 4+ /Nb 3+ Redox Reactions in Bulk NASICON‐NaNbAl(PO 4 ) 3 Anode Under Higher Current Rates

ABSTRACT Natrium SuperIonic CONductor (NASICON)‐Nb 2 (PO 4 ) 3 is regarded as a potential anode for sodium‐ion batteries due to its higher storage capacity (∼150 mAh g − 1 ) stemming from two‐electron Nb 5+ /Nb 4+ /Nb 3+ redox. However, the reversibility of Nb 5+ /Nb 4+ /Nb 3+ redox is limited by its structural degradation. Herein, we unveil highly reversible Nb 5+ /Nb 4+ /Nb 3+ redox reactions in bulk NASICON‐NaNbAl(PO 4 ) 3 (NaNbAl) anode. The introduction of Na‐ions via Al 3+ substitution stabilizes the NASICON framework and activates distinct Nb redox couples with Na (de)intercalation, as demonstrated via our density functional theory‐based calculations. This bulk anode delivers capacities up to 90 mAh g − 1 at 5C with 86% retention after 1000 cycles. More significantly, we capture rapid sodium ion (de)intercalation in NaNbAl at high current rates using operando synchrotron X‐ray diffraction which is in agreement with the calculated low migration barriers associated with Na motion within the structure. A full Na‐ion cell (Na 4 V 2 (PO 4 ) 3 ||NaNbAl) achieves an energy density of 201 Wh kg − 1 (based on cathode mass) and retains 84% capacity over 200 cycles at 1C. This study opens new avenues for realizing reversible Nb 5+ /Nb 4+ /Nb 3+ redox in bulk NASICON anodes with suitable chemical substitutions.