Abstract:

Silicene, a two-dimensional material, is a promising anode material for lithium-ion batteries. However, it struggles to exist stably alone. Its structural stability can be effectively improved by elemental doping. Germanium (Ge) not only shares the same valence electron configuration as Silicon (Si), but germanene also boasts higher electronic conductivity and better electrochemical properties. This study investigates the impact of Ge doping on the lithium storage behavior of silicene through first-principles calculations based on density functional theory (DFT). The structural stability, adsorption capacity, diffusion behavior, theoretical specific capacity, open-circuit voltage (OCV), and electronic conductivity of Si₁₇Ge were calculated and analyzed. These results demonstrate that Si17Ge maintains good structural stability after Ge doping without exhibiting structural protrusions, depressions, or planar states. This indicates that Ge doping does not alter the two-dimensional warped structure of silicene, distinguishing it from Ge-doped graphene. The adsorption and diffusion energy barriers indicate that Ge doping enhances the lithium adsorption and diffusion capacity of silicone in horizontal and vertical directions. In the horizontal direction, the diffusion step spans 4.63 ? and requires overcoming an energy barrier of 0.18 eV, which is substantially lower than that of Li atoms on graphene (0.31 eV) and silicene (0.22 eV). Conversely, in the vertical direction, the diffusion energy barrier is 1.14 eV, higher than that on the surface, indicating increased difficulty in vertical diffusion of Li atoms in Si₁₇Ge. Nevertheless, this value is lower than the vertical diffusion barrier of Li atoms in pure silicene (1.67 eV) and graphene (10.02 eV). Through OCV and adsorption energy calculations, it is estimated that Si₁₇Ge can adsorb a maximum of 18 Li atoms and has a theoretical specific capacity as high as 876.85 mAh/g. It exhibits a higher theoretical specific capacity and lower diffusion energy barrier than existing two-dimensional materials. Density of state (DOS) analysis reveals that when Si₁₇Ge adsorbs a lower Li concentration, the Fermi level DOS is enhanced, and the system exhibits metallicity. When Si₁₇Ge absorbs a higher concentration of Li, a noticeable bandgap appears at the Fermi level, causing the system to transition from a conductor to a semiconductor. During the process of Si₁₇Ge adsorbing Li atoms, the density of states near the Fermi level is primarily attributed to Si orbitals. In contrast, the Ge orbitals contribute minimally to the density of states. This study offers crucial theoretical guidance for the design of two-dimensional Si-based anode materials and other two-dimensional materials.

Key words: first principles, silicene, lithium storage behavior, doping