Construction of superior performance Na3V2-xCrx(PO4)2F3/C cathode by homovalent doping strategy toward enhanced sodium ion storage

Abstract

Na3V2(PO4)2F3, with robust 3D structural framework and high operating potential, is regarded as a promising cathode candidate for sodium-ion batteries. However, the low electrical conductivity caused by poly-anionic polyhedron leads to slow kinetics and poor rate performances. In this work, Cr3+ doped Na3V2(PO4)2F3/C is prepared by using chromium as homovalent dopant ion. The synergistic effect of Cr3+ doping and carbon coating promotes the rapid transportation of electrons in Na3V2(PO4)2F3/C particles. The Rietveld refinements show that Cr3+ doping can adjust the crystal structure and accelerate the diffusion kinetics of Na+. The Cr3+ doping improves the electrical conductivity of Na3V2-xCrx(PO4)2F3/C. In-situ electrochemical impedance spectroscopy measurements show that Cr3+ doping effectively reduces the charge transfer impedance and plays an important role in improving the charge transfer kinetics. Consequently, the optimized Na3V1.98Cr0.02(PO4)2F3/C exhibits excellent rate capability of 91.2 mAh·g−1 at 30 C and long cyclic stability, in which the capacity decay per cycle is 0.0029% over 1000 cycles at 10 C. The results provide a facile strategy and beneficial way to design advanced Na3V2(PO4)2F3 cathode toward sodium-ion batteries.