Ultracapacity Properties of the Refined Structure in Na-Rich Na3.4V2(PO4)3/C as Sodium-Ion Battery Cathodes by Tapping the Na-Vacancy Potential

Na3V2(PO4)3 has been attracting great interest from scholars owing to its high voltage platform and high energy storage capacity. However, its poor electronic conductivity and weak ion diffusion ability seriously restrict the application of its actual industrialization. In view of the above defects, Na3+xV2(PO4)3/C (x = 0, 0.2, 0.4, 0.6) cathode materials for sodium-ion batteries (SIBs) are prepared through a solid-phase method in this paper. The X-ray diffraction (XRD) results show that the Na-rich amount of x = 0.4 attains the upper limit of the solid solution of the Na-rich Na3V2(PO4)3, and the "ultracapacity" effect reaches the maximum at this x value; the capacity is as high as 132.4 mAh/g, with remarkable cycle stability (96% capacity retention after 300 cycles). The results of density functional theory (DFT) calculations clearly explain that the reason for the excess sodium occupying the electrochemically active Na2 site is the reason for the ultracapacity. It is found through the electron paramagnetic resonance (EPR) test that excessive sodium caused some high-valent V to be reduced to low-valent V, which maintained the electrical balance of the crystal and the stability of the Na-rich solid solution structure. Through the X-ray absorption near edge structure (XANES) of the V element, it is found that the change of V valence during the charge and discharge process of the Na-rich Na3V2(PO4)3 solid solution is consistent with that of Na3V2(PO4)3. Refined structural characterization by spherical aberration electron microscopy and ex situ XRD also prove that the Na-rich Na3V2(PO4)3 solid solution undergoes a phase transition during the charge–discharge process and can be reversibly recovered. These findings further prove that it is feasible to synthesize a new Na-rich Na3V2(PO4)3 solid solution with a stable structure, which makes Na3V2(PO4)3 a practical cathode material for SIBs having great potential in the future.