Monolayer Ge2S: An auxetic semiconductor with high carrier mobility for metal (Na, K, Mg)-ion battery anodes

Abstract

Using first-principles calculations, we propose a new two-dimensional Ge2S (space group P21212) with unique mechanical and electronic properties. Monolayer Ge2S has excellent thermal, mechanical, and dynamic stabilities, exhibiting a semiconducting behavior with an indirect bandgap and anisotropic carrier mobility. The uniaxial strain along the zigzag direction can induce an indirect-to-direct bandgap transition. Remarkably, Ge2S possesses large in-plane negative Poisson's ratios, comparable with that of well-known penta-graphene. Moreover, we identify Ge2S as a high-performance anode material for metal-ion batteries. It shows metallic features after adsorbing Na, K, and Mg, providing good electrical conductivity during the charge/discharge process. The diffusion of metal ions on Ge2S is anisotropic with modest energy barriers in the armchair direction of 0.12, 0.39, and 0.76 eV for Na, K, and Mg, respectively. Ge2S can adsorb metal atoms up to a stoichiometric ratio of 1:1, which yields storage capacities of 151.17, 151.17, and 302.35 mA h g−1 for Na, K, and Mg, respectively. The volume of Ge2S shrinks slightly upon the adsorption of metal ions even at high concentrations, ensuring a good cyclic stability. Besides, the average open circuit voltage (0.30–0.70 V) falls within the acceptable range (0.1–1.0 V) of the anode materials. These results make Ge2S a promising anode material for the design of future metal-ion batteries.