Abstract: The effects of P vacancies of black phosphorus (BP) on the adsorption and diffusion of sodium (Na) at the interface of BP-NaZr₂(PO₄)₃ heterojunction (BP-NZP) were studied by first-principles calculations. The results indicate that Na atoms are more inclined to adsorb at the interface region of BP-NZP heterojunction, and Na adsorption has little effect on the dielectric properties of NZP and the electronic properties of BP, which is crucial for high-performance sodium ion batteries. In addition, P vacancies provide low energy barrier pathways, facilitating Na diffusion from the interface into the BP layer. The CI-NEB and electrostatic potential distribution analysis further confirmed that P vacancies significantly improved the migration of Na from the interface to the BP layer.