Mechanical and Thermoelectric\nProperties of Na-Intercalated\nSnSe<sub>2</sub> Crystals

Recently, two-dimensional (2D) van der Waals (vdW) SnSe<sub>2</sub>-based crystals have attracted great attention due to their\nmetal-like\nplasticity and good thermoelectric (TE) performance at room temperature.\nThe halogen elements have been successfully doped at Se sites in SnSe<sub>2</sub> crystals to greatly improve the TE properties while maintaining\ngood plasticity. As a typical layered material, the space among the\nvdW layers in SnSe<sub>2</sub> is quite large and can be intercalated\nby guest elements to tune various physical properties. In this work,\nwe successfully prepared a series of Na-intercalated SnSe<sub>2</sub> crystals by using the temperature gradient method. The effects of\nintercalating Na into the vdW gaps on the crystal structure and mechanical\nand TE properties are systematically investigated. Intercalating Na\nin SnSe<sub>2</sub> has little influence on the crystal structure\nand band gap. The electrical conductivity is enhanced by Na-intercalation,\nbut the doping efficiency is lower than those of Br and Cl. A maximum\npower factor (PF) of 5.6 μW cm<sup>–1</sup> K<sup>–2</sup> is obtained for Na<sub>0.02</sub>SnSe<sub>2</sub> at 300 K, comparable\nwith most Ag<sub>2</sub>S- and AgCuSe-based plastic inorganic TE materials.\nThe plasticity of SnSe<sub>2</sub> is well maintained when the Na-intercalating\ncontent is below 0.05. This work provides more understanding of SnSe<sub>2</sub>-based plastic TE materials.