Structural, electronic and diffusion properties of new two-dimensional materials: Li3CrMnX4 (X = S, O)

Exploring high energy density electrode material and superionic conductors are of great significance in fields such as lithium-ion batteries (LIBs). Here, two new 2D materials, i.e., Li₃CrMnS₄ and Li₃CrMnO₄, are proposed. Theoretical capacities of Li₃CrMnS₄ and Li₃CrMnO₄ are 314 mAh/g and 419 mAh/g, respectively. The calculations of phonon spectra show that both the materials are dynamical stable. The first-principles molecular dynamics simulations also show that they have thermodynamic stability at room temperature. The calculations on the electronic structures suggest that both materials are semiconductors, and their band gaps are 1.33 eV and 1.67 eV, respectively. The ground states of Li₃CrMnS₄ and Li₃CrMnO₄ are ferromagnetic and antiferromagnetic, respectively. In order to explore the possibility of these two materials as superionic conductors, the diffusion properties of Li ions are emphasized. The diffusion coefficients of Li ions in both materials reach 10⁻⁵ cm²s⁻¹, for the Li₃CrMnS₄, two Li-ions concerted migration has the highest diffusion coefficient. The minimum migration energy barriers of Li ions in Li₃CrMnS₄ and Li₃CrMnO₄ are 0.16 eV and 0.12 eV, respectively. The Li ions migration is dominated by the Li ions between the octahedral layers.