Computational and experimental understanding of Al-doped Na3V2-xAlx(PO4)3 cathode material for sodium ion batteries: Electronic structure, ion dynamics and electrochemical properties

Abstract First principles calculations are employed to understand the effects of Al substitution on the electronic structure, ion dynamics properties, as well as structural stability of Na3V2-xAlx (PO4)3 cathode material. The results reveal that Al doping decreases the band gap energy and transforms the material from indirect to direct band gap electronic structure. Several Na-ion diffusion routes in the crystal structure are proposed and the one with lowest migration energy is identified, which shows one-dimensional transport characteristics along c-axis direction via a curved trajectory. The Al doping increases the Na ion migration energy and thus unfavorable for Na ion diffusion in the crystal. The opposite effects of Al doping on electronic and Na ionic conduction lead to the occurrence of an optimal Al doping level, at which the best electrode reaction kinetics can be expected. The experimental works of the synthesized Na3V2-xAlx (PO4)3 with x = 0–0.4 confirm the theoretical predictions. The sample Na3V1.8Al0.2(PO4)3 with appropriate Al doping exhibits the smallest electrode polarization and superior rate performance. Because of the enhanced binding energy with Al doping as indicated by computation work, the Na3V1.8Al0.2(PO4)3 material displays much stable long-term cycling performance compared to the pristine sample at different temperatures.