High‐Voltage Off‐Stoichiometric Vanadium‐Based Pyrophosphate Na_6.51V_3.16(P₂O₇)₄ for Rechargeable Sodium‐Ion Batteries

Abstract The accessibility of high‐performance sodium‐ion batteries (SIBs) hinges upon cathodes with high voltage and large reversible capacity. Among the contenders, vanadium‐based polyanionic compound with strong inductive effect from anionic group and multiple accessible oxidation states of vanadium emerged as a formidable competitor, however, only limited candidates in this field were reported so far. This manuscript delineated the rational synthesis of a novel off‐stoichiometric pyrophosphate Na_6.51V_3.16(P₂O₇)₄. It delivers a specific capacity of 102 mAh g⁻¹ at the current density of 20 mA g⁻¹, as well as a high discharge median voltage of 3.87 V vs. Na⁺/Na, rendering it a rare sodium‐ion storage host with multi‐redox couples (V^5+/4+/3+/2+). The experimental approach to phase adjustment in this article offers guidelines for exploiting of new cathode materials for high‐performance SIBs.