Computational Studies of Energy Storage Materials under High Pressures
Abstract
Materials subjected to high pressure can form new phases with novel properties. Computational investigations are important for understanding these behaviors under extreme conditions, particularly when experimental methods are limited in fully characterizing new phases or revealing the mechanisms driving their transformations. This thesis employs first-principles calculations to investigate the high-pressure behavior of three energy storage materials: hydrazine borane (N2H4BH3), trimethylamine borane (TMAB, (CH3)3N·BH3), and sodium amide (NaNH2). By combining electronic structure calculations with advanced structure search methods, this work addresses the challenges of experimentally identifying and analyzing high-pressure phases. The results provide detailed insights into the pressure-induced phase transitions, structural stability, bonding characteristics, electronic properties, and charge density changes, revealing the mechanisms behind phase transitions.
In the study of N2H4BH3, computational methods were employed to predict and reveal two phase transitions at 15 and 25 GPa, respectively. Detailed theoretical analyses showed that these pressure-induced phase transitions are driven by the evolution of dihydrogen bonding frameworks, together with changes in compressibility, and enthalpy differences between polymorphs. The computational study on TMAB revealed its phase transition around 9 GPa, driven by the evolution of C−H…H−B dihydrogen bonding. Further theoretical analyses revealed that the unprecedented structural stability of TMAB is attributed to the absence of N−H…H−B dihydrogen bonds, which are more sensitive to external pressure. Detailed examination of TMAB’s pressure-dependent crystal lattice parameters, unit-cell volumes and bulk moduli highlights its compressibility, while changes in its electronic band gap energy suggest a trend toward metallization at higher pressures. These two studies highlight the significant role of dihydrogen bonding in driving phase transitions in ammonia-borane-like molecular crystals under high pressure. In the study of NaNH2, computational methods were employed to determine the crystal structure of a new high-pressure phase above 2.5 GPa, which was confirmed by the experimental XRD. Theoretical investigations into changes in charge density and atomic rearrangements revealed these as key factors driving the phase transitions and contributing to the robust stability of the high-pressure phase. These findings provide a deeper understanding of material behavior under extreme conditions and support their potential applications in hydrogen storage and other energy-related technologies.