Constructing p-type substitution induced by Ca in defective NaVCa(PO)/C wrapped with conductive CNTs for high-performance sodium-ion batteries.

NaV(PO) (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 a facile sol-gel method. The addition of partial Ca with low valence at the V 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 Ca with a larger ionic radius (1.03 Å 0.64 Å of V) can have a pillar effect to support the cell structure, improving the structural stability of NVP. Meanwhile, the larger radius of Ca 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 at 0.1 C, while that of NVP is only 69.4 mA h g. Moreover, it delivers an initial capacity of 110.1 mA h g 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 while that of NVP is only 75.9 mA h g. Interestingly, it exhibits excellent cycling stability at 50 C; the initial capacity is 75.7 mA h g and the capacity retention is around 99% after 4000 cycles.