Doped ${\mathrm{VS}}_2$ as a high-performance electrode material for rechargeable $\mathrm{Mg}$-ion batteries

Two-dimensional (2D) vanadium disulfide (${\mathrm{VS}}_2$) can serve as a universal host for reversible intercalation and deintercalation of alkali and alkaline earth metal ions. However, its practical application in rechargeable metal-ion batteries is limited by its low energy density (559 Wh ${\mathrm{g}}^{-1}$). Herein, the effects of $\mathrm{O}$ doping and $\mathrm{C},\mathrm{O}$ codoping on the electrochemical performance of 2D ${\mathrm{VS}}_2$ used as anode for magnesium-ion batteries (MIBs) are investigated by first-principles calculations. Values of both the energy density and specific capacity increase with increasing $\mathrm{O}$-doping concentration, and those of $\mathrm{VSO}$ are 2.17 times and 1.16 times higher than those of ${\mathrm{VS}}_2$, respectively. However, due to the strong bond between $\mathrm{O}$ and $\mathrm{Mg}$, the diffusion barrier of $\mathrm{Mg}$ atoms on 2D $\mathrm{VSO}$ is relatively high (1.02 eV). Further introduction of $\mathrm{C}$ (${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$) can reduce the diffusion barrier of $\mathrm{Mg}$ atoms (0.80 eV) to a level comparable to that of ${\mathrm{VS}}_2$. Meanwhile, the values of energy density and specific capacity of ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$ are 1.53 times and 1.17 times those of ${\mathrm{VS}}_2$. Our results suggest that $\mathrm{O}$ doping and $\mathrm{C},\mathrm{O}$ codoping of 2D ${\mathrm{VS}}_2$ are effective strategies to improve the overall performance of MIBs and it should be possible to generalize such doping strategies to other rechargeable MIBs based on 2D transition metal dichalcogenides.

Figure 1

Top and side views of the structure of ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$, and possible adsorption sites of $\mathrm{Mg}$ atoms on the monolayer ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$. ($\mathrm{V},\mathrm{S},\mathrm{O}$, and $\mathrm{C}$ atoms are marked with purple, green, pink, and brown colors, respectively). The adsorption positions on the top and bottom surfaces are indicated by “top” and “bottom,” respectively. For example, 1-top, 2-top, and 3-top are the top sites on $\mathrm{S}$, in the hollow, and on $\mathrm{V}$ of the top surface, respectively. (a), (b), (c), (d), (e), and (f) correspond to ${\mathrm{VS}}_2$, ${\mathrm{VS}}_{1.75}{\mathrm{O}}_{0.25}$, ${\mathrm{VS}}_{1.5}{\mathrm{O}}_{0.5}$, ${\mathrm{VS}}_{1.25}{\mathrm{O}}_{0.75}$, $\mathrm{VSO}$, and ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$, respectively.

Figure 2

The most stable adsorption sites on the top surfaces of ${\mathrm{VS}}_2$ (a), ${\mathrm{VS}}_{1.75}{\mathrm{O}}_{0.25}$ (b), ${\mathrm{VS}}_{1.5}\mathrm{O}0.5$ (c), ${\mathrm{VS}}_{1.25}{\mathrm{O}}_{0.75}$ (d), $\mathrm{VSO}$ (e), and ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$ (f).

Figure 3

Structure of the maximum adsorption of $\mathrm{Mg}$ atoms on ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$: ${\mathrm{VS}}_2$ (a), ${\mathrm{VS}}_{1.75}{\mathrm{O}}_{0.25}$ (b), ${\mathrm{VS}}_{1.5}{\mathrm{O}}_{0.5}$ (c), ${\mathrm{VS}}_{1.25}{\mathrm{O}}_{0.75}$ (d), $\mathrm{VSO}$ (e), and ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$ (f).

Figure 4

(a) Adsorption energies of $\mathrm{Mg}$ on ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$ with the number of $\mathrm{Mg}$ atoms. (b) OCV curves of ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$. (c) Average OCV of ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$. (d) Theoretical specific capacity of ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$. (e) The energy densities of ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$.

Figure 5

The possible diffusion paths and their corresponding barriers on the top surface for ${\mathrm{VS}}_2$ (a),(g), ${\mathrm{VS}}_{01.75}{\mathrm{O}}_{0.25}$ (b),(h), ${\mathrm{VS}}_{1.5}{\mathrm{O}}_{0.5}$(c),(i), ${\mathrm{VS}}_{1.25}{\mathrm{O}}_{0.75}$ (d),(j), $\mathrm{VSO}$ (e),(k), and ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$ (f),(l), respectively.

Figure 6

The integrated performance diagram of ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$. Clockwise from the top: charge transfer (e⁻), the number of charges transferred at the most stable adsorption site; the average open circuit voltage; the theoretical capacity; the metallicity; the maximum adsorption of $\mathrm{Mg}$ atoms; the energy density; and the inverse of diffusion energy.

Figure 7

(a),(b),(c) PDOS of ${\mathrm{VS}}_2$, $\mathrm{VSO}$, and ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$, respectively. (d),(e),(f) PDOS of ${\mathrm{VS}}_2$, $\mathrm{VSO}$, and ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$ after $\mathrm{Mg}$ is adsorbed on 5-top site, respectively.

Figure 8

(a),(b) The charge density difference isosurface for ${\mathrm{VS}}_2$ before and after an $\mathrm{O}$ substitution of a $\mathrm{S}$, respectively. (c),(d) The charge density difference for $\mathrm{VSO}$ before and after a $\mathrm{C}$ substitution of an $\mathrm{O}$, respectively. Green and pink are electron-rich and electron-deficient regions, respectively. The isosurface level is set to 0.0025 e/ų.

Figure 9

Charge density isosurface configurations for $\mathrm{Mg}$ adsorbed on ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$ monolayer: (a) on ${\mathrm{VS}}_2$, (b) on ${\mathrm{VS}}_{1.75}{\mathrm{O}}_{0.25}$, (c) on ${\mathrm{VS}}_{1.5}{\mathrm{O}}_{0.5}$, (d) on ${\mathrm{VS}}_{1.25}{\mathrm{O}}_{0.75}$, (e) on $\mathrm{VSO}$, (f) on ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$, and (g) on ${\mathrm{VSO}}_{0.75}{\mathrm{C}}_{0.25}$ both at the 5-top and 5-bottom sites. Green and pink are electron-rich and electron-deficient regions, respectively. The isosurface level is set to 0.0025 e/ų.

Figure 10

Electron localization function isosurfaces (0.8) of the (110) slice of ${\mathrm{VS}}_x{\mathrm{O}}_y{\mathrm{C}}_z$ monolayers without and with four $\mathrm{Mg}$ adsorption layers: (a),(g) ${\mathrm{VS}}_2$, (b),(h) ${\mathrm{VS}}_{1.75}{\mathrm{O}}_{0.25}$, (c),(i) ${\mathrm{VS}}_{1.5}{\mathrm{O}}_{0.5}$, (d),(j) ${\mathrm{VS}}_{1.25}{\mathrm{O}}_{0.75}$, (e),(k) $\mathrm{VSO}$, and (f),(l) ${\mathrm{VSO}}_{0.75}{\mathrm{O}}_{0.25}$, respectively.