Phase Composition and Electronic Modulation of Na‐V‐PO ₄ Enabling a 3.8 V Higher‐Voltage Plateau for High‐Energy Sodium‐Ion Batteries

Two redox reactions, V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺, are accepted in Na₃V₂(PO₄)₃ (NVP) during the charge/discharge process. The discharge capacity at the higher-voltage plateau ≈4.0 V varies with the heteroatom doping. To clarify this behavior, the V/PO₄ ^3- ratio is tuned in this work. The results reveal that the composition of 3Na-0.9V-3PO₄ is composed of the Na₇V₄(P₂O₇)₄PO₄ (NVPP) phase. Owing to the increased electron density around V donated by citrate ions, the apparent valence state approaches +3, resulting in a poor EPR signal. In the 3Na-xV-3PO₄/C(x = 0.9-1.5) composites, the proportion of rod/tubular-like NVPP regularly decreases, while the NVP phase becomes increasingly dominant with rising V content. This evolution is clearly reflected in the CV curves. NVPP/C phase delivers a discharge capacity of ≈40 mAh g^-1 at 117 mA g^-1 with a voltage plateau near 3.8 V. As the V content increases, this high-voltage plateau becomes shorter, while that plateau associated with NVP/C extends, leading to the increase in discharge capacity. Crystal structure and the morphology of these composites remain stable, as well as the peak of V 2p_1/2 shifts during charging/discharging, while the electron signal at the V 2p_3/2 orbital nearly disappears. These finding offers a strategy to enhance high-voltage plateaus in sodium-ion batteries.