Theoretical Prediction of Two-Dimensional SnP₃ as a Promising Anode Material for Na-Ion Batteries

Tin (Sn) as a cheaper and more environment-friendly alternative to lead (Pb) has been widely used in the field of green energy. Especially, Sn-based nanomaterials have attracted tremendous attention in Na-ion batteries. Interestingly, the layered bulk structure of SnP3 has been experimentally synthesized, and it is metallic and stable at room temperature. On the basis of first-principles calculations, we demonstrate that the production of monolayer SnP3 by exfoliation of bulk crystal could be feasible due to the moderate cleavage energy (∼1.10 J/m2). Because of the weak π–π interaction and Jahn–Teller effect, the single-layer SnP3 has a high buckling height with an indirect band gap (0.68 eV) responding to ultraviolet–visible–near-infrared wavelength lights. The hole mobility is up to 103 cm2 V–1 s–1, which is comparable to that of black phosphorene. More importantly, monolayer SnP3 experiences indirect–direct band gap and semiconductor–metal transitions under biaxial strain. Furthermore, we explore SnP3 as an anode for Na-ion batteries. Upon Na adsorption, the semiconducting SnP3 transforms to a metallic state, ensuring good electrical conductivity. Specially, the ultralow energy barrier (0.03 eV) of Na diffusion on monolayer SnP3 indicates a fast diffusivity. During the Na adsorption process, the slight volume variations (<1.0%) suggest a good cycling reversibility. The theoretical specific capacity (253.31 mA h g–1) and moderate average electron potential (1.29 V) are in between those of commercial anodes, graphite and TiO2. Moreover, bilayer SnP3 greatly improves the Na binding strength. Meanwhile, the diffusion of Na on the outside surface of bilayer SnP3 is extremely anisotropic. The diffusion along the armchair direction (0.38 eV) is energetically favorable. However, the high energy barrier (1.83 eV) along the zigzag direction leads to a nearly forbidden diffusion. These results are helpful to deepen the understanding of SnP3 as a potential anode for Na-ion batteries.