Hollow Li20B60 Cage: Stability and Hydrogen Storage

A stable hollow Li20B60 cage with D2 symmetry has been identified using first-principles density functional theory studies. The results of vibrational frequency analysis and molecular dynamics simulations demonstrate that this Li20B60 cage is exceptionally stable. The feasibility of functionalizing Li20B60 cage for hydrogen storage was explored theoretically. Our calculated results show that the Li20B60 molecule can adsorb a maximum of 28 hydrogen molecules. With a hydrogen uptake of 8.190 wt% and an average binding energy of 0.336 eV/H2, Li20B60 is a remarkable high-capacity storage medium.

Scientific RepoRts | 6:24500 | DOI: 10.1038/srep24500 On each "face-centered" site, there exists a Li 2 dimer with an average Li-Li distance of 2.635 Å. Four Li atoms are located on the top sites of the "truncated face" formed by the boron hexagons. The remaining 4 Li atoms are located on the top sites of the boron triangles. We refer to these atoms below as Li I , Li II , and Li III , respectively, as shown in Fig. 2. The relative stability of this Li 20 B 60 cage was discussed by comparing with other structures, which arose during the high-temperature dynamic simulations, but no lower-energy structures were found (see Figures  S2 and S3 in the Supplementary Information).
The stability of Li 20 B 60 was further checked using vibrational frequency analysis and molecular dynamics (MD) simulations. The vibrational frequency analysis of the Li 20 B 60 cage indicates no imaginary frequencies and the highest intensity frequency was 687.9 cm −1 . For more details see Section IV of the Supplementary Information, as well as some low-frequency modes. Therefore, the Li 20 B 60 cage is kinetically stable. We also carried out ab initio molecular dynamics simulations with the constant-temperature, constant-volume (NVT) ensemble in a Massive Nosè-Hoover thermostat. The total simulation time was set to be 1.0 ps with 1000 dynamics steps. It was found that the structure of the D 2 -Li 20 B 60 cage was not disrupted up to a temperature of ~600 K. These results indicate that D 2 -Li 20 B 60 cage has good thermodynamic stability.
It is natural to explore the electronic structure of the Li 20 B 60 cage. To this end, we calculated the deformation electron density, partial density of states (PDOS), and frontier molecular orbitals, including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as shown in Figs 2(b) and 3. From the deformation electron density, one can see an alternation of three-center and two-center bonds on each of the six centered-faces of the "truncated octahedron". On the "truncated faces" formed by the boron hexagons, there is obvious sp 2 -like bonding between B atoms, while on the "truncated faces" formed by the boron triangles, the Li atoms contribute parts of their 2s electrons to the neighboring B atoms. From the point of view of doping, the three-center triangular regions could be regarded as donors and the two-center hexagonal regions could be  regarded as acceptors 14 . Thus it is the mixing of the two-center and three-center bonding that promotes the stability of the D 2 -Li 20 B 60 cage. As for the case of the HOMO and LUMO orbitals, it was found that the HOMO orbitals are mostly localized on the B atoms and have a predominantly s-p hybridization characteristic. The LUMO orbitals are also mostly localized on the B atoms and the hybridization is also predominantly of s-p character, which is consistent with the hybridization of Li-B sheet 15 . The energy level features show that the LUMO orbital is doubly degenerate. Close examination of the PDOS (see Fig. 3) further confirms the hybridization characteristics of the HOMO and LUMO orbitals.
We next investigated the interaction between the Li 20 B 60 cage and hydrogen molecules. It was found that hydrogen can bind to the Li sites with a binding strength reflecting typical van der Waals interactions. The hydrogen binding energy (E b ) for Li 20 B 60 is defined as 20 B 60 (H 2 ) n ]}/n. We first added one H 2 molecule near each Li atom. After energy minimization, it was found that the H 2 molecule tends to occupy a position above the Li atom and with its axis parallel to the boron hexagonal or triangle plane. The average distance of the H 2 molecule from the Li atom is 2.270 Å indicating a van der Waals interaction between the H 2 molecules and the Li 20 Supplementary Information). This hydrogen storage capacity is in excess of 6 wt%, the U. S. Department of Energy target and is comparable to some similar systems, such as the alkali-metal(Li, Na, K)-doped B 80 fullerenes 8 , the Li-doped boron sheet 13,15 and boron nanotubes 13,15 . All these results suggest that the Li-B cage is a potential candidate for hydrogen storage.
In summary, our first-principles studies have identified a stable Li 20 B 60 molecule. The results of vibrational frequency analysis and molecular dynamics simulations demonstrate that this Li 20 B 60 cage is exceptionally stable. The Li 20 B 60 cage can adsorb a maximum of 28 H 2 molecules, resulting in a hydrogen gravimetric density of 8.190 wt% with an average adsorption energy of 0.336 eV/H 2 . This is a remarkable result indicating another application for the Li-B cage as a potential high-capacity storage medium.

Methods
Our calculations were carried out with the exchange-correlation potential described by the Perdew-Burke-Ernzerhof version (PBE) of the general gradient approximation (GGA) 16 , as implemented in the DMol 3 package 17 . The double-numerical basis plus polarized functions (DNP) was chosen. When discussing the adsorption of hydrogen molecules onto the Li 20 B 60 cage, the van der Waals (vdW) interactions 18 , which are crucial for the formation, stability, and function of molecules were taken into account. Here, the hybrid semi empirical dispersion-correction approach of Tkatchenko and Scheffler (TS) scheme 19 , was used in the process of structure optimization. Some previous studies 13,15,20 have investigated the hydrogen capacity of metal-decorated 2D sheets or 1D nanotubes using the semi-empirical dispersion-correction approach. All structures were fully relaxed and geometric optimizations were performed with convergence thresholds of 10 −5 hartree (Ha) for the energy, 2 × 10 −3 Ha/Å for forces, and 5 × 10 −3 Å for the atomic displacements. In the self-consistent field calculations, the convergence threshold was set to 10 −6 Ha on the total energy. Geometry optimizations were performed with unrestricted spin and without any symmetry constraints.