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Title: Lithium-Nitrogen Based Compound as Lithium Ionic Conductor and Chemical Hydride for Hydrogen Storage: Experimental and First-Principles Investigation
Authors: LI WEN
Keywords: Lithium ionic conductor, Chemical hydride, Hydrogen storage, Density functional theory, First-principles
Issue Date: 7-Aug-2011
Citation: LI WEN (2011-08-07). Lithium-Nitrogen Based Compound as Lithium Ionic Conductor and Chemical Hydride for Hydrogen Storage: Experimental and First-Principles Investigation. ScholarBank@NUS Repository.
Abstract: The development of solid-state materials with superionic conductivities is critical to solid-state battery technologies. Due to the relatively smaller ionic radius, Li+ ion (0.60 ?) is a common conducting species in ionic conductors. Lithium nitride (Li3N) attracts particular attention as a lithium solid electrolyte in the last century because of its superior ionic conductivity at room temperature. More recently, Li3N has been a focus again since the discovery of Li3N-Li2NH-LiNH2 (Li-N-H) system for hydrogen storage. Li-Mg-N-H and Li-Ca-N-H systems were further developed to improve the Li-N-H system. In this thesis, both experimental ionic conductivity measurements and first-principles calculations are employed to investigate the Li+ ionic conduction properties and diffusion mechanisms in above systems, i.e., Li3N (both a and ? phases), Li2NH and LiNH2, Li2Mg(NH2)2 and Li2Ca(NH2)2. The experimental results show that Li+ ions present superionic conduction in the order of ~ 10?4 S?cm?1 for Li3N (both a and ? phases) and Li2NH at ambient temperature; Li2Ca(NH2)2 exhibits moderate Li+ ionic conductivity of ~ 10?6 S?cm?1; while LiNH2 and Li2Mg(NH2)2 are almost insulators. The first-principles simulations reveal that a-Li3N and ?-Li3N have distinct Li+ ion diffusion mechanisms. In Li2NH, the Li+ ion diffusion is more likely to occur via interstitialcy mechanism or vacancy-mediated jumps between octahedral and tetrahedral sites. In LiNH2, however, Li defects are difficult to be created to mediate Li+ ion diffusion, leading to low concentration of charge carriers and poor Li+ ion conduction at lower temperatures. Although the involvement of Mg/Ca cations creates favorable conditions for hydrogen storage, Li+ conduction could be blocked by Mg cations in Li2Mg(NH2)2 or unfixed N-H bonds orientation in Li2Ca(NH2)2, resulting in relatively poor Li+ conduction properties compared to Li2NH. These studies contribute to the understanding of the role of Li+ ion transport in the hydrogenation/dehydrogenation pathways of Li-based metal-N-H complex hydrides for hydrogen storage. More importantly, it promotes the development of complex hydrides as novel solid lithium ion conductors for battery applications. In addition to store hydrogen in above Li-N based compounds, another option is to bond hydrogen with both B and N in chemical hydrides. The representative is ammonia borane (NH3BH3, AB), which has attracted considerable attention because of its high hydrogen storage capacity (19.6 wt. %); however, the relatively poor kinetics and issues with energetically undesirable regeneration of used fuel are still big challenges for the practical application of AB. To improve the performance of AB, a series of derivatives have recently been developed such as metal amidoboranes (LiAB, NaAB, CaAB), metal amidoborane-ammonia borane (LiAB?AB), metal amidoborane ammoniates (CaAB?2NH3, MgAB?NH3) and multi-cation amidoborane (LiNaAB). In this study, in-depth theoretical investigations have been carried out to understand the improved dehydrogenation properties of these chemical hydrides for hydrogen storage. Furthermore, the first-step dehydrogenation mechanism is proposed for each system on the basis of solid-phase simulations. The findings stimulate the attempts to look for new hydrogen storage materials with optimized thermodynamics and kinetics; moreover, the conclusion could also provide instructive guidelines for the experimental synthesis and processing of multi-component chemistry hydrides for hydrogen storage.
Appears in Collections:Ph.D Theses (Open)

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