Exploring the potential of MB2 MBene family as promising anodes for Li-ion batteries

Abstract

In recent years, finding high-performance energy storage materials has become a major challenge for Li-ion batteries. B-based two-dimensional materials have become the focus of attention because of their abundant reserves and non-toxic characteristics. A series of two-dimensional transition metal borides (MBenes) are reported and their electrochemical properties as anode materials for Li-ion batteries are investigated by density functional theory (DFT) calculations. The surface of MB₂ possesses medium adsorption strength and diffusion energy barrier for Li atoms, which are conducive to the insertion and extraction of Li-ions during the charge/discharge process of Li-ion batteries. Herein, we explore the potential of MB₂ (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn) as the anode material for LIBs. Excitingly, the Li atom can be stably adsorbed on the surface of MB₂ (M = Sc, Ti, V, Nb, Mo, W) monolayers, and the theoretical capacity of the MB₂ monolayer is high (521.77-1610.20 mA h g^-1). The average open circuit voltage range is within 0.10-1.00 V (vs. Li/Li⁺). The relationship between the p-band center of the B atom and the adsorption energy of Li on the surface of MB₂ is also investigated. Furthermore, it is found that the charge transfer of Li atom and metallic center in the most stable position is strongly related to the corresponding value of diffusion energy barrier. These results confirm that MB₂ monolayers are promising 2D anode materials for Li-ion batteries, demonstrating the application prospects of B-based 2D materials.

Fig. 1. (a) Top and (b) side views of MB₂ monolayers (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn), site 1 to 6 are different adsorption sites, where purple and green colors denote the transition metal and boron atoms, respectively.

Fig. 2. (a) The adsorption energy of Li atom on the B atomic side of MB₂ monolayers; (b) top and side views of differential charge density diagram of Li atom adsorbed on TiB₂ monolayer, where the green region indicates charge depletion and yellow region indicates charge accumulation and blue, orange, and green colors denote the Ti, Li and B atoms, respectively.

Fig. 3. The relationship between the p-band center and the adsorption energy of Li atom on the B atomic side of MB₂ monolayers and p-orbital projected density of states (PDOS). The curve in the inset diagram is the PDOS curve of the B atom in TiB₂.

Fig. 4. (a) Diffusion energy barrier and diffusion path of Li atom on the B atomic side; (b) adsorption energy and diffusion energy barrier of Li atom on the B atomic side; the diffusion energy barrier of Li atom along path II with the amount of charge transfer of (c) Li atom and (d) metal atom at the most stable position, respectively.

Fig. 5. (a) The structural configuration of four layers of Li atoms adsorbed on the surface of MB₂; (b) the change in average adsorption energy and (c) the open circuit voltage of Li_xMB₂ (M = Sc, Ti, V, Nb, Mo, W and Ta) as a function of Li concentration.

Fig. 6. The theoretical specific capacity of different MB₂ monolayers with respect to (a) diffusion energy barrier and (b) adsorption energy of Li atoms on B atomic side.

Fig. 7. (a–f) Energy band structures and PDOS of the Li₄MB₂ (M = Sc, Ti, V, Nb, Mo, and W) monolayers. The horizontal black dashed lines represent Fermi levels.