Computer Simulations of Li–N–H Systems

Thesis

The Li–N–H system (lithium compounds with nitrogen and/or hydrogen based anions) is a promising family of materials for chemical energy storage applications; the development of efficient energy storage materials is one of the key challenges for global renewable energy utilisation. The Li–N–H system is of importance for both hydrogen storage and in facilitating the storage of energy as ammonia. Lithium hydride and lithium imide, two materials in this family, were investigated in this thesis.

The interactions of ammonia on the surface of lithium hydride represent the hydrogen formation step in the lithium amide-lithium hydride hydrogen storage system. The study of these interactions through DFT and MD simulations has led to new insights from geometric considerations of ammonia into both its movement across the surface and its stable configuration. Molecules on the lithium hydride surface with tetrahedral geometries closely related to ammonia were investigated to better understand the surface interactions due to the distorted tetrahedral structure of ammonia. This lithium hydride surface study is the foundation for future investigations of ammonia on more complex Li–N–H surfaces, namely lithium imide, which is the dehydrogenated form of the hydrogen store, and itself an active catalyst for ammonia decomposition. The structure and dynamics of bulk lithium imide were investigated between 300–700 K with a particular attention to lithium-ion mobility through the structure. The evolution of the structure was observed from ensemble average nitrogen environments of the hydrogen and lithium ions. On heating to 700 K, a monoclinic to cubic phase transition (I112/b →Fm3̄m) was observed with the structural transition around 500 K. Frenkel defect formations with lithium ions displaced from tetrahedra into octahedra were traced through the structure and two different defect arrangements were obtained at low-temperature using different temperature gradients.

Authors: Marks, PL

• DOI: 10.5287/ora-0z5yzy5wm
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Materials Modelling; Energy Materials; Computational Chemistry