Uncover the Structural, Hydrogen Storage Mechanism, and Comprehensive Properties of NaAlH 4 Hydride

ABSTRACT To explore the intrinsic relationship between the crystal structure and hydrogen storage mechanism of NaAlH 4 hydride, we apply the first‐principles method to study the structural stability, mechanical, electronic, and optical properties of the four NaAlH 4 phases. The results indicate that all four phases exhibit thermodynamic stability. Essentially, these crystal structures are based on the [AlH 4 ] − tetrahedral anion as the fundamental building block, with Na + ions filling the interstitial site to form a stable ionic network. The high gravimetric hydrogen storage capacity originates from the material's high hydrogen concentration, while the Al─H bonds in the [AlH 4 ] units provide the structural foundation for stable hydrogen storage. The coordination environment of Na and its connection to multiple [AlH 4 ] units are key factors in regulating structural stability. A high Na coordination number and bridging bonds enhance lattice compactness and thermodynamic stability, but may also increase the hydrogen dissociation energy barrier, which inhibits hydrogen release. The calculated electronic structure and optical properties further corroborate this trade‐off mechanism, and all four NaAlH 4 hydrides exhibit a wide band gap semiconductor feature, with their optical absorption peaks located in the ultraviolet region.