Based on first-principles calculations, the diffusion properties of Na+ in bilayer MoS2 and the MoS2-WS2 heterojunction were investigated. Owing to the similar structures, the diffusion properties of Na are almost identical in both systems. On perfect surfaces, the diffusion energy barrier is smaller than 0.1 eV. The diffusion characteristics of Na+ ions between layers are related to the size of the system. For a small size (
