Codoping of carbon and boron composition in Na3V2(PO4)2F3 affects its sodium storage properties

Abstract

NASICON-type Na₃V₂(PO₄)₂F₃ (NVPF) is a promising cathode material for Na-ion batteries due to its higher discharge capacity, appropriate voltage platform and output energy density. But its poor electronic conductivity should be increased to push its utilization. Here, the in-situ carbon and heteroatom B are introduced into to NVPF to improve the electrochemical performance. When citric acid is used as a carbon source, the free movement of electrons between V and the citric acid group is facilitated by the electrostatic force, causing a change in the valence state of V. It is a mixed valence of + 4 and + 3 for V in the product after annealing. The doping of B at O sites has no effect on ionic bond in solution and the crystal structure of the product. But, it decreases the formation energy, induces the charge redistribution and improves the conductivity. Excellent electrochemical performance is achieved with a B doping of 15 % to NVPF/C. Even after 1000 cycles, a capacity of 47.9 mAh g⁻¹ is retained. The structure of NVPF/C-B is preserved during cycling, but compositional deviation at the electrode surface leads to some degradation. The relationship between the valence of V, doping B into the NVPF, and the degradation mechanism over repeated cycles provide a deep understanding of the complex interplay between metallic ions and carbon source, and the B-doping.