Heterointerface effects of lithium intercalation and diffusion in van der Waals heterostructures

The van der Waals gap of two-dimensional (2D) materials provides natural ion diffusion channels for lithium. However, individual 2D materials (such as graphene, ${\mathrm{MoS}}_2$, and MXene) cannot offer all the properties needed to maximize the performance for energy storage. Heterostructures, by stacking different 2D materials, are promising for combining the advantages and eliminating the disadvantages of individual 2D materials. ${\mathrm{MoS}}_2$-based heterostructures have been prepared for lithium storage, while intercalation and diffusion mechanisms have yet to be elucidated. In the current work, the heterointerface effects on lithium intercalation and diffusion of seven $H$- and distorted $T$ $\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$-based heterostructures are systematically studied by density functional theory calculations, and highlight the atomistic origin of differences between $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$/graphene, $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2/H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$, and $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$/MXene hetero- and homostructures. From the energetic point of view, the heterointerface makes for an easier phase transition of ${\mathrm{MoS}}_2$ to the $\mathit{dT}$ phase in the ${\mathrm{MoS}}_2$/graphene heterostructure. Then we discuss the general idea of a rational design of the van der Waals gap for efficient lithium storage and diffusion. Our work opens a new avenue for optimizing the ion diffusion properties of 2D material heterostructures by designing the van der Waals channel for ion diffusion.

Figure 1

Side views of the hetero-/homointerfaces of $H$- and $dT\text{−}{\mathrm{MoS}}_2$-based structures without lithium adsorption. (a) and (b) $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2$/graphene heterointerfaces. (c)–(e) Three $H$- and $dT\text{−}{\mathrm{MoS}}_2$ homo-heterointerfaces. (f) and (g) $H$$\left(\mathit{dT}\right)$-${\mathrm{MoS}}_2/{\mathrm{Ti}}_2{\mathrm{CO}}_2$ heterointerfaces. S(i) and S(o) denote the inner and outer sulfur atoms of $dT\text{−}{\mathrm{MoS}}_2$ that close to the vdW interface.

Figure 2

Critical lithium concentration for the transition from the $H$- to $\mathit{dT}$-phase of ${\mathrm{MoS}}_2$ in the ${\mathrm{MoS}}_2/\mathrm{graphene}$ and ${\mathrm{MoS}}_2$ monolayer. Note the black line for ${\mathrm{MoS}}_2/\mathrm{graphene}$ and the red line for the ${\mathrm{MoS}}_2$ monolayer, respectively. $E_{dT/\mathrm{Gr}}$ ($E_{H/\mathrm{Gr}})$ is the energy of $\mathit{dT}\left(H\right)\text{−}{\mathrm{MoS}}_2/\mathrm{graphene}$ heterostructures. $E_{\mathrm{Corr}.}$ is the energy correction from the lattice mismatch. $E_{dT}$ ($E_H$) is the energy of the ${\mathrm{MoS}}_2$ monolayer. $\mathrm{\Delta }E$ is the energy difference per formular unit of ${\mathrm{MoS}}_2$.

Figure 3

Interface diffusion barriers and diffusion paths of lithium in ${\mathrm{MoS}}_2/\mathrm{Gr}$ and bilayer ${\mathrm{MoS}}_2$ structures for ${\mathrm{MoS}}_2$ in $H$- and $\mathit{dT}$-phases. Note that the vertical axes in (a) for ${\mathrm{MoS}}_2/\mathrm{Gr}$ and (b) for bilayer ${\mathrm{MoS}}_2$ structures are scaled differently. Top and side views of the diffusion paths in (c) and (e) for $H\text{−}{\mathrm{MoS}}_2/\mathrm{Gr}$ and in (d) and (f) for $dT\text{−}{\mathrm{MoS}}_2/\mathrm{Gr}$. The two diffusion paths in (c) are equivalent and hence only one is shown in the left panel of (a).

Figure 4

Interface diffusion path analysis of lithium in bilayer $H\text{−}{\mathrm{MoS}}_2, H\text{−}/dT\text{−}{\mathrm{MoS}}_2$ and $H\text{−}{\mathrm{MoS}}_2/\mathrm{Gr}$. Pink and purple lithium atoms represent the transition state (TS) and the initial state (IS), respectively. Three-dimensional illustration of the local structure around lithium in (${\mathrm{a}}_3$) $H/H$ and (${\mathrm{b}}_3$) $H/\mathit{dT}$ interlayers. Side and top views in (${\mathrm{a}}_2$) and (${\mathrm{a}}_4$) for bilayer $H\text{−}{\mathrm{MoS}}_2$, (${\mathrm{b}}_2$) and (${\mathrm{b}}_4$) for $H$-/$dT\text{−}{\mathrm{MoS}}_2$, and (${\mathrm{c}}_2$) and (${\mathrm{c}}_3$) for $H\text{−}{\mathrm{MoS}}_2/\mathrm{Gr}$. S(i) and S(o) denote inner and outer sulfur atoms in $dT\text{−}{\mathrm{MoS}}_2$ that close to the vdW interface.

Figure 5

Interface diffusion barriers and diffusion paths of lithium in ${\mathrm{MoS}}_2/{\mathrm{Ti}}_2{\mathrm{CO}}_2$ structures for ${\mathrm{MoS}}_2$ in the $H$-phase (a) and the $\mathit{dT}$-phase (b). (c) and (d) Interface diffusion path analysis of lithium in ${\mathrm{MoS}}_2/{\mathrm{Ti}}_2{\mathrm{CO}}_2$ structures. Pink and purple lithium atoms represent the transition state (TS) and the initial state (IS), respectively. Three-dimensional illustration of the local structure around lithium in the (${\mathrm{c}}_3$) $H\text{−}{\mathrm{MoS}}_2/{\mathrm{Ti}}_2{\mathrm{CO}}_2$ and (${\mathrm{d}}_3$) $dT\text{−}{\mathrm{MoS}}_2/{\mathrm{Ti}}_2{\mathrm{CO}}_2$ interlayers. Side and top views in (${\mathrm{c}}_2$) and (${\mathrm{c}}_4$) for $H\text{−}{\mathrm{MoS}}_2/{\mathrm{Ti}}_2{\mathrm{CO}}_2$, (${\mathrm{d}}_2$) and (${\mathrm{d}}_4$) for $dT\text{−}{\mathrm{MoS}}_2/{\mathrm{Ti}}_2{\mathrm{CO}}_2$, respectively. S(i) and S(o) denote inner and outer sulfur atoms in $dT\text{−}{\mathrm{MoS}}_2$ that close to the vdW interface.

Figure 6

Side views of other 2D materials that surface atoms with intrinsic different heights or large lateral separations [57, 58, 59, 60, 61, 62, 63, 64].