This work, which was conducted by a research group led by Prof. Zhao Bangchuan from Institute of Solid State Physics, Hefei Institutes of Physical Science of Chinese Academy of Sciences, effectively activated the M1 site in the structure of Na₃V₂(PO₄)₃ by reducing the crystallinity of the material, and realized the reversible insertion and removal of three sodium ions.

Na₃V₂(PO₄)₃ has a hexagonal NASICON structure. Sodium ions occupy two unequal Wyckoff sites, 1/3 of which are located at 6b site (M1) and 2/3 at 18e site (M2). From the point of view of thermodynamic equilibrium and kinetics, sodium ions at M1 site are difficult to participate in the redox reaction. There is no electrochemical reactivity during the charging and discharging processes. The reversible insertion and removal of sodium ions only occurs at M2 site. Only two sodium ions participate in the electrochemical reaction through the V⁴⁺/V³⁺redox pair. Therefore, the theoretical specific capacity of Na₃V₂(PO₄)₃ is just 117.6 mAh g^-1. Whether the specific capacity of Na₃V₂(PO₄)₃ can be improved by activating M1 site and using V⁵⁺/V⁴⁺redox reaction is a great challenge in this field.

In this work, Na₃V₂(PO₄)₃ precursor was deposited on carbon foam substrate by electrostatic spray method, and the crystallinity of Na₃V₂(PO₄)₃ was adjusted by controlling the annealing temperature. Two Na₃V₂(PO₄)₃ materials, NVP-E600 and NVP-E700, were obtained. The corresponding annealing temperature was 600 and 700 °C, respectively.

With nanocrystalline and amorphous phase coexisting structure Na₃V₂(PO₄)₃ material as the cathode and the metal sodium sheet as the counter electrode, the coin-type cell was assembled, and its sodium storage performance was evaluated.

The results showed that the nanocrystalline and amorphous phase coexisting structure Na₃V₂(PO₄)₃ material had excellent rate performance and cycle stability due to the reversible de-insertion of three sodium ions. It displayed a specific capacity of 179.6 mAh g^-1 at 0.2 C, and the capacity retention rate is 99.6% after 200 cycles. Even at a high rate of 10 C, it also has a specific capacity of 73.5 mAh g^-1.

The test results of electrochemical impedance spectrum (EIS) and Cyclic voltammetry (CV) curves showed that this disordered Na₃V₂(PO₄)₃ material had strong electrochemical reaction kinetics, and the charge storage was mainly pseudocapacitive, which was very different from the crystalline Na₃V₂(PO₄)₃ with battery-type charge storage behavior.

The extrinsic pseudocapacitance behavior existing in this highly disordered material could be attributed to the introduction of disordered structure changed the interaction between sodium ions in Na₃V₂(PO₄)₃ material, making the charge-discharge process change from the original two-phase reaction to the single-phase reaction. This resulted in the disappearance of the platform in the charge-discharge curves and the rectangle of the CV curves.

This demonstrated the importance of a disorder structure to the three-electron reaction process and extrinsic pseudocapacitance in NVP cathodes for sodium-ion batteries, according to the team.