Theoretical Prediction of Two-Dimensional SnP<sub>3</sub> 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 SnP<sub>3</sub> 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 SnP<sub>3</sub> by exfoliation of bulk crystal could be feasible due to the moderate cleavage energy (∼1.10 J/m<sup>2</sup>). Because of the weak π–π interaction and Jahn–Teller effect, the single-layer SnP<sub>3</sub> 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 10<sup>3</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, which is comparable to that of black phosphorene. More importantly, monolayer SnP<sub>3</sub> experiences indirect–direct band gap and semiconductor–metal transitions under biaxial strain. Furthermore, we explore SnP<sub>3</sub> as an anode for Na-ion batteries. Upon Na adsorption, the semiconducting SnP<sub>3</sub> transforms to a metallic state, ensuring good electrical conductivity. Specially, the ultralow energy barrier (0.03 eV) of Na diffusion on monolayer SnP<sub>3</sub> 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<sup>–1</sup>) and moderate average electron potential (1.29 V) are in between those of commercial anodes, graphite and TiO<sub>2</sub>. Moreover, bilayer SnP<sub>3</sub> greatly improves the Na binding strength. Meanwhile, the diffusion of Na on the outside surface of bilayer SnP<sub>3</sub> 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 SnP<sub>3</sub> as a potential anode for Na-ion batteries.