$M_{2}X$ Monolayers as Anode Materials for $\mathrm{Li}$ Ion Batteries

Electrochemically efficient electrode materials are required for clean energy storage in $\mathrm{Li}$ ion batteries. We predict two-dimensional hexagonal metal nitrides, borides, and phosphides (${\mathrm{Sc}}_2\mathrm{B}$, ${\mathrm{Sc}}_2\mathrm{N}$, ${\mathrm{Y}}_2\mathrm{B}$, ${\mathrm{Y}}_2\mathrm{N}$, and ${\mathrm{Y}}_2\mathrm{P}$) and evaluate the feasibility of experimental realization. The materials combine excellent metallicity, as required for electrodes, with $\mathrm{Li}$ binding energies providing high storage capacity and a low average open-circuit voltage. In contrast to two-dimensional silicene, borophene, and ${\mathrm{Sn}\mathrm{S}}_2$, we observe negligible structural distortions during $\mathrm{Li}$ adsorption and extraction, which results in high reversibility and a long cycle life. Superionic $\mathrm{Li}$ diffusion enables fast charge or discharge of next-generation $\mathrm{Li}$ ion batteries.

Figure 1

(a) Top and side views of the ${\mathrm{Y}}_2\mathrm{N}$ monolayer (1T structure). Phonon spectra of the $M_{2}X$ monolayers: (b) $\mathrm{Sc}$${}_2$$\mathrm{B}$; (c) $\mathrm{Sc}$${}_2$$\mathrm{N}$; (d) $\mathrm{Y}$${}_2$$\mathrm{B}$; (e) $\mathrm{Y}$${}_2$$\mathrm{N}$; (f) $\mathrm{Y}$${}_2$$\mathrm{P}$.

Figure 2

The temperature (black) and total energy (blue) during the ab initio molecular-dynamics simulations of the (a) ${\mathrm{Sc}}_2\mathrm{N}$ and (b) ${\mathrm{Y}}_2\mathrm{N}$ monolayers at 300 K. The insets show the atomic structures after 6 ps of simulation time.

Figure 3

The electronic band structures (spin-orbit coupling included) of the $M_{2}X$ monolayers: (a) $\mathrm{Sc}$${}_2$$\mathrm{B}$; (b) $\mathrm{Sc}$${}_2$$\mathrm{N}$; (c) $\mathrm{Y}$${}_2$$\mathrm{B}$; (d) $\mathrm{Y}$${}_2$$\mathrm{N}$; (e) $\mathrm{Y}$${}_2$$\mathrm{P}$.

Figure 4

Top and side views of the (a) H and (b) T adsorption sites and the valence-charge-density difference induced by $\mathrm{Li}$ adsorption on the (c) ${\mathrm{Sc}}_2\mathrm{N}$ and (d) ${\mathrm{Y}}_2\mathrm{N}$ monolayers (yellow, charge accumulation; blue, charge depletion; $\mathrm{isovalue}={10}^{-3}\mathrm{electrons}/{\AA }^3$).

Figure 5

The electron localization functions of the (a) ${\mathrm{Sc}}_2\mathrm{N}$ and (b) ${\mathrm{Y}}_2\mathrm{N}$ monolayers with one adsorbed $\mathrm{Li}$ layer.

Figure 6

The total densities of states ($3\times 3\times 1$ supercell, spin-orbit coupling included) of the (a) ${\mathrm{Sc}}_2\mathrm{N}$ and ${\mathrm{Li}}_2{\mathrm{Sc}}_2\mathrm{N}$ and (b) ${\mathrm{Y}}_2\mathrm{N}$ and ${\mathrm{Li}}_2{\mathrm{Y}}_2\mathrm{N}$ monolayers. The Fermi energy is set to 0 eV.

Figure 7

The relative energies for $\mathrm{Li}$ diffusion along the pathways (a) T-H-T and (b) T-T on the ${\mathrm{Sc}}_2\mathrm{N}$ and ${\mathrm{Y}}_2\mathrm{N}$ monolayers. The insets show the trajectories of the pathways in top and side views.