Balancing thermodynamics, kinetics, and reversibility in Ti-doped Mg(BH4)2: A first-principles assessment of a practical solid-state hydrogen storage material

Hydrogen storage remains a critical bottleneck for the realization of a sustainable hydrogen economy, where solid-state materials must simultaneously satisfy stringent requirements on storage capacity, release thermodynamics, kinetics, and reversibility. Complex borohydrides are attractive due to their high hydrogen density, yet their practical deployment is limited by sluggish hydrogen diffusion and unfavorable desorption enthalpies. In this work, we present a comprehensive first-principles investigation of pristine and Ti-doped Mg(BH 4 ) 2 as a candidate for solid-state hydrogen storage. All calculations were performed for α-Mg(BH 4 ) 2 (space group P6 1 22) based on the experimentally reported structure. Density functional theory calculations reveal that pristine Mg(BH 4 ) 2 possesses an exceptionally high gravimetric hydrogen capacity of approximately 14.9 wt%, but exhibits a relatively high hydrogen desorption enthalpy of about 42 kJ mol −1 H 2 and diffusion barriers near 0.5 eV, which limit low-temperature operation. Ti substitution at the Mg site markedly improves the storage performance without destabilizing the host lattice. The doped system maintains a high hydrogen capacity (∼10.4 wt%) while reducing the desorption enthalpy to approximately 36 kJ mol −1 H 2 , placing it within the optimal thermodynamic window for practical hydrogen release. Nudged elastic band calculations demonstrate a significant reduction in the hydrogen migration barrier to around 0.38 eV, indicating enhanced diffusion kinetics. Phonon and elastic stability analyses confirm that Ti doping preserves dynamic and mechanical stability. Electronic structure analysis reveals that localized, spin-polarized Ti-3d states near the Fermi level weaken rigid B–H bonding and stabilize transitional hydrogen configurations, providing a clear microscopic origin for the improved thermodynamic and kinetic behavior. A van't Hoff analysis further suggests hydrogen release temperatures approaching ambient conditions. These results demonstrate that Ti substitution significantly improves the thermodynamic and kinetic descriptors of Mg(BH 4 ) 2 , supporting its potential as a balanced and reversible hydrogen storage candidate. • Ti-doped MgB 2 H 8 retains ultra-high hydrogen capacity (>10 wt%) while entering the optimal thermodynamic window. • Zero-point and finite-temperature corrections place the desorption enthalpy at ∼36 kJ mol −1 H 2 . • Ti substitution reduces hydrogen diffusion barriers from ∼0.52 eV to ∼0.38 eV. • Phonon and elastic analyses confirm full dynamic and mechanical stability after doping. • Spin-polarized Ti-3d states provide a clear electronic mechanism for enhanced hydrogen mobility.