A novel two-dimensional monolayer tetragonal boron carbide as a promising anode for Mg-ion.

Secondary Mg-ion batteries (MIBs) have attracted significant interest due to the abundant supply of magnesium and their potential for high energy density. However, their development is hindered by a lack of suitable electrode materials. In this study, density functional theory (DFT) calculations were used to investigate, for the first time, a two-dimensional monolayer of tetragonal boron carbide (T-BC) as a potential anode for MIBs. The results show that T-BC exhibits a low Mg adsorption energy, which prevents metal clustering, stabilizes surface adsorption, and ensures overall electrode stability. A high theoretical specific capacity of 2346 mA h g⁻1 was obtained, together with a mean open-circuit voltage of 0.20 V, which is moderate but favorable for energy density. Electronic structure analysis reveals that increasing Mg concentration enhances electrical conductivity while maintaining metallic character after adsorption. Although the diffusion energy barriers are slightly elevated, they remain comparable to those of conventional two-dimensional anode materials. Finally, the light-shading property of T-BC is proposed as a practical screening criterion for identifying promising negative electrode materials for MIBs. DFT calculations were performed using the B3LYP/6-311 + G(d) level of theory with Grimme's DFT-D3 dispersion correction, as implemented in Gaussian 03. Phonon dispersion and CI-NEB calculations were carried out using Quantum ESPRESSO to evaluate dynamical stability and Mg diffusion barriers, respectively. AIMD simulations at 400 K were also performed to confirm thermal stability at maximum Mg loading.