Understanding trends in lithium binding at two-dimensional materials

Layered structure and peculiar electronic properties of two-dimensional (2D) materials foster the concept of utilizing them as main components of lithium-ion batteries. Understanding basic physical mechanisms governing the interaction of Li with 2D crystals is of key importance to succeeding in a rational design of cathode and anode materials with superior functionalities. In this study density functional theory was applied to reveal the microscopic picture of Li interaction with 15 2D crystals, including several transition metal oxides and dichalcogenides, carbides of Group XIV elements, functionalized graphene, silicene, and germanene, as well as black phosphorus and ${\mathrm{Ti}}_2\mathrm{C}$ MXene. We found that the general trend in Li binding can be estimated from positions of conduction band minima of 2D materials, since the energy of the lowest empty electronic states shows a nice correlation with the strength of Li adsorption. At variance to the majority of studied surfaces where the electron transferred from Li is spread across the substrate, in monolayers of carbides of Group XIV elements the interaction with Li and the charge transfer are well localized. This gives rise to their capability to accommodate Li structures with a nearly constant binding energy of alkaline atoms over Li coverages ranging from well-separated adatoms to a full monolayer.

Figure 1

Top and side views of Li adatom at 2D crystals, classified into the groups with qualitatively the same adsorption geometries: (a) graphane, silicane, germanane or fluorinated graphene; (b) SiC, GeC and SnC monolayers; (c) ${\mathrm{MX}}_2$ monolayers; (d) black phosphorus; (e) ${\mathrm{Ti}}_2\mathrm{C}$; and (f) ${\mathrm{MoO}}_2$ monolayer. Li atoms are presented as small green spheres; the numbers correspond to the enumeration of 2D structures in Table 1.

Figure 2

Correlation between Li atom binding energy (${\mathrm{E}}_{\mathrm{B}}$) and conduction band minimum (CBM) for selected 2D crystals; surfaces without and with midgap state, induced upon Li adsorption, are marked with blue circles and red rectangles, respectively. Arrows indicate energy shifts from CBM to the energies of midgap states. The numbering of 2D materials is the same as in Table 1. The dashed line serves as a guide to eyes.

Figure 3

(a) Total DOS of pristine ${\mathrm{CrO}}_2$ monolayer and the monolayer with adsorbed Li atom; (b) DOS od the ${\mathrm{CrO}}_2$ monolayer with adsorbed Li atom, projected on relevant atomic orbitals.

Figure 4

Electron density induced upon Li adsorption at ($\mathrm{a}){\mathrm{CrO}}_2$ and (b) SnC monolayer; only isocontours corresponding to charge accumulation are plotted.

Figure 5

(a) Total DOS of pristine SnC monolayer calculated for the ideal sheet (brown curve) and deformed monolayer (blue line) where Sn and C atoms are fixed at the positions as in SnC with Li adatom. $\delta \mathrm{E}$ indicates the position of the midgap state relative to the conduction band minimum (CBM) of ideal SnC monolayer; (b) Isosurface plot of Kohn-Sham wavefunction of the midgap state at $\mathrm{\Gamma }$ point. Regions with different signs of the wave function are colored differently. The number indicates displacement (in Å) of Sn from the plane of other Sn atoms, induced by Li adsorbate.

Figure 6

Total DOS of SnC monolayer with adsorbed Li atom.

Figure 7

Atomic structure of (a) Li dimer, (b) Li tetramers and the most stable ordered structures on SnC monolayer at Li coverage, (c) $\mathrm{\Theta }=1/2$ ML, and (d) $\mathrm{\Theta }=1$ ML. Sn, C, and Li atoms are represented by large yellowish, small gray, and green spheres, respectively. The numbers indicate Li binding energy in eV per atom. A full monolayer (ML) represents the coverage where the number of Li and Sn atoms is equal. Surface unit cells of ordered structures are marked with black dashed lines.

Figure 8

Total and Atom projected DOS of (a), (b) ${\mathrm{Li}}_2$ dimer and (c), (d) Li monolayer on SnC.

Figure 9

Atomic structure of (a) Li dimer and the most stable ordered structures at ${\mathrm{CrO}}_2$ monolayer at Li coverage, (b) $\mathrm{\Theta }=1/4$ ML, (c) $\mathrm{\Theta }=1/3$ ML, and (d) $\mathrm{\Theta }=1/2$ ML. The numbers indicate Li binding energy in eV per atom; Cr, O, and Li atoms are represented by light blue, red, and green spheres, respectively. A full monolayer (ML) refers to the coverage where the number of Li and Cr atoms are equal. Surface unit cells are marked with black dashed lines.