Constructing p-type substitution induced by Ca²⁺ in defective Na₃V_2-Ca(PO₄)₃/C wrapped with conductive CNTs for high-performance sodium-ion batteries

Na₃V₂(PO₄)₃ (NVP), with a high tap density, is considered a prospective cathode material due to its low cost, ideal specific capacity and cycling stability. However, its low ionic/electronic conductivity has become the main factor hindering its development. In the current work, a dual modification strategy has been proposed to optimize NVP, which is successfully achieved via a facile sol-gel method. The addition of partial Ca²⁺ with low valence at the V³⁺ site produces favorable p-type substitution in the pristine NVP bulk, generating beneficial hole carriers in the electronic structure to accelerate the migration rate of Na⁺. Moreover, the doped Ca²⁺ with a larger ionic radius (1.03 Å vs. 0.64 Å of V³⁺) can have a pillar effect to support the cell structure, improving the structural stability of NVP. Meanwhile, the larger radius of Ca²⁺ contributes to the expansion of the lattice spacing, significantly facilitating the diffusion efficiency of Na⁺ to optimize the diffusion kinetics. Besides, the evenly coated carbon layers derived from the excess carbon resources combine with the enwrapped carbon nanotubes to construct a highly conductive network to enhance the transportation of electrons. Notably, the modified Ca0.04-NVP@CNTs electrode exhibits a high capacity of 117.4 mA h g^-1 at 0.1 C, while that of NVP is only 69.4 mA h g^-1. Moreover, it delivers an initial capacity of 110.1 mA h g^-1 at 1 C and the mass loss rate per lap is only 0.01%. At 5 C, the initial capacity of Ca0.04-NVP@CNTs is 104.3 mA h g^-1 while that of NVP is only 75.9 mA h g^-1. Interestingly, it exhibits excellent cycling stability at 50 C; the initial capacity is 75.7 mA h g^-1 and the capacity retention is around 99% after 4000 cycles.