According to a research published in Advanced Science recently, scientists successfully improved the specific capacity of Na3V2(PO4)3, a new electrode material. They also found its relatively exotic extrinsic pseudocapacitance behavior.
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.
Graphic figure: Origin of three-electron reaction mechanism and extrinsic pseudocapacitance behavior for the new Na3V2(PO4)3 material (Image by MA Hongyang)
Structure and morphology characterization results of Na3V2(PO4)3 materials with different crystallinity (Image by MA Hongyang)
Electrochemical performance measurements of Na3V2(PO4)3 material with nanocrystalline and amorphous phase coexistence structure. (Image by MA Hongyang)