Abstract: The effects of P vacancies in black phosphorus (BP) on the adsorption and diffusion of sodium (Na) at the interface of the BP–NaZr₂(PO₄)₃ heterojunction (BP–NZP) have been studied by first-principles calculations. The results indicate that Na atoms are more inclined to adsorb at the interface region of the BP–NZP heterojunction, and Na adsorption has little effect on the dielectric properties of NZP or 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 climbing image nudged elastic band(CI–NEB) and electrostatic potential distribution analysis further confirmed that P vacancies significantly improve the migration of Na from the interface to the BP layer.