Hydrogen Storage Capacity and Optoelectronic Response of Mechanically and Thermally Stable Lithium-Based Tetrahydrates (LiXH4, X = B, Al, Mn), a DFT Approach

In this work, density functional theory (DFT)-based first-principles investigations are performed by Generalized Gradient Approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional in the CASTEP code. These simulations provide the insights of the structural, electronic, optical, thermodynamic, mechanical and hydrogen storage gravimetric ratios of lithium-based tetrahydrides (LiBH4, LiAlH4, and LiMnH4) for hydrogen storage and photovoltaic (PV) applications. All these structures crystallize in orthorhombic Cmcm (No. 63) geometry with different lattice parameters and bonding strengths. Thermodynamic stabilities of hydrides are obtained by dispersion of phonons and phonon density of states. The measured band gaps of hydrides are 3.81 eV (LiBH4), 4.60 eV (LiAlH4), and 0.53 eV (LiMnH4), which are calculated by GGA-PBE approach. Moreover, the optical characteristics with strong optical activity are observed from visible to ultraviolet (2 eV to 6 eV) regions. High dielectric response between 6 and 8 and absorption coefficient up to 105 cm−1 for hydrides are observed. Debye temperature has exceeded from 300 K to 600 K for all hydrides and saturation occurred closer to Dulong–Petit limit ~75 J mol−1 K−1. Mechanical stability in hydrides has been observed by Born-Hung mechanical stability criterion, demonstrating ductile nature. These natural hydrides have shown exceptional hydrogen storage capacities, as 18.5 wt% for LiBH4, 10.6 wt% for LiAlH4, and 6.1 wt% for LiMnH4, are measured; these values have exceeded the U.S department of energy (DOE) targets (5.5 wt% H2). These analyses prove that LiXH4 (X = B, Al, Mn) hydrides are promising candidates for solid state hydrogen storage materials.