High‐Voltage Off‐Stoichiometric Vanadium‐Based Pyrophosphate Na6.51V3.16(P2O7)4 for Rechargeable Sodium‐Ion Batteries

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 Na6.51V3.16(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 (V5+/4+/3+/2+). The experimental approach to phase adjustment in this article offers guidelines for exploiting of new cathode materials for high-performance SIBs.