Exploring X 2 BH 6 (X = Li, Na, and K) Double Perovskite Hydrides for Solid‐State Hydrogen Storage Applications

ABSTRACT Solid‐state hydrogen storage in lightweight hydrides represents a promising pathway toward a sustainable hydrogen economy. In this work, we employ density functional theory to systematically investigate the structural, electronic, mechanical, dynamical, and hydrogen storage properties of X 2 BH 6 (X = Li, Na, and K) hydrides. All hydrides exhibit negative formation energies, confirming their thermodynamic stability. The gravimetric hydrogen storage capacities are remarkably high: 19.71, 9.64, and 6.37 wt% for Li 2 BH 6 , Na 2 BH 6 , and K 2 BH 6 , respectively, all surpassing the 2025 U.S. Department of Energy target of 5.5 wt%. Electronic band structure analysis reveals metallic character for all hydrides. Evaluation of Born mechanical stability criteria reveals that Li 2 BH 6 and K 2 BH 6 are mechanically stable, whereas Na 2 BH 6 is mechanically unstable. Phonon dispersion calculations demonstrate that only K 2 BH 6 is dynamically stable, while Li 2 BH 6 and Na 2 BH 6 exhibit imaginary phonon modes. Combining all stability criteria, K 2 BH 6 emerges as the most promising candidate, possessing simultaneous thermodynamic, mechanical, and dynamical stability alongside a gravimetric capacity of 6.37 wt% and a hydrogen desorption temperature of 308.00 K that falls within the DOE operating target. These findings suggest K 2 BH 6 as a potential candidate for solid‐state hydrogen storage and provide theoretical guidance for future experimental synthesis.