Li₄B₈H₈ as a reversible hydrogen storage unit in free and graphene-anchored forms

Hydrogen storage remains a major bottleneck for the deployment of hydrogen-based energy technologies, as conventional approaches such as high-pressure compression and cryogenic liquefaction impose high energy penalties, safety risks, and demanding infrastructure. Solid-state materials offer a safer and more reversible alternative, yet no existing system simultaneously satisfies the U.S. DOE targets of moderate adsorption enthalpy (-0.1 to -0.6 eV per H₂), fast kinetics, and gravimetric capacities above 5 wt%. Here, we propose the Li₄B₈H₈ cluster as a lightweight, electronically robust hydrogen-storage unit based on a nido-B₈H₈^4- borane framework functionalized by four exposed Li⁺ centers. Global potential-energy-surface searches and ab initio molecular dynamics identify a C_2v global minimum for the isolated Li₄B₈H₈ cluster. Extended-Lagrangian ADMP simulations reveal that the cluster is kinetically persistent up to 800 K, with the nido-B₈H₈ backbone remaining structurally intact. Beyond this temperature, at 1000 K, the high mobility and rearrangement of the Li centers compromise the structural integrity of the borane framework. Stepwise adsorption calculations reveal reversible coordination of up to 20H₂ molecules with average adsorption energies BSSE-corrected of -0.17 to -0.09 eV per H₂, yielding a gravimetric capacity of 24.8 wt% in the free cluster. When immobilized on graphene, Li₄B₈H₈ maintains efficient storage performance, accommodating 16, 20, and 24H₂ molecules at 5.9, 7.4, and 8.7 wt%, respectively, with adsorption energies consistently within -0.13 to -0.10 eV per H₂, and retaining fully physisorptive character as confirmed by IGMH interaction analysis. These results establish Li-decorated nido-boranes as a previously overlooked class of molecular hydrogen sorbents, extending cluster-based design strategies beyond conventional B₆-derived motifs and offering a promising route toward high-capacity, intermediate-binding solid-state hydrogen-storage media.