1 Center for Atomic-scale Materials Design, Center, Technical University of Denmark2 Department of Energy Conversion and Storage, Technical University of Denmark3 Atomic scale modelling and materials, Department of Energy Conversion and Storage, Technical University of Denmark4 Department of Physics, Technical University of Denmark
This thesis is dedicated to the investigation and design of new functional materials for energy storage. The focus of the presented work is on components for the successful Li-ion and the promising Li-air batteries. First principle density function theory calculations are applied to screening studies for new materials, as well as to more detailed investigations on interesting properties of different battery components. In the screening studies simple predictors are used to search for desired material properties and reduce the number of materials that need to be studied in more detail. Solid electrolytes are believed to increase safety in Li based batteries as they would prevent metallic growth in the electrolyte. LiBH4 has a solid superionic conducting HT phase that is stable above 390 K. The HT phase can be stabilized at room temperature with substitution of I into the LiBH4 structure. Here we show how the stabilization of the HT phase can be described with calculations of relative structural stability. Relative structural stability is then used as a descriptor in a screening study, searching for new stable solid ionic conductors that could be used in an all solid state battery. The mechanisms for ionic conduction in the HT phase of LiBH4 is studied with the nudged elastic band method and harmonic transition state theory. The results show that the high ionic conductivity originates from formation of Li interstitial and vacancy defects and the high mobility of the defects in the HT phase. It is hoped that high energy dense Li-air batteries will be able to replace Li-ion batteries in the future. There are however number of challenge that need to be solved before that can happen. We have studied the growth and decomposition of Li2O2, which is the main discharge product of Li-O2batteries. The results show that both growth during discharge and decomposition during charge can take place at low overpotentials. The energy of formation and the diffusion rate of VLiin bulk Li2O2are calculated and the effect the vacancies have on electronic conductance in the semiconducting Li2O2is discussed. Finally we have studied adsorption of S and SO2on Li2O2(1100) and (0001) surfaces. We show that SO2binds preferably to step (1100) sites and makes discharge growth close to the step less favorable. The possibility of using selective poisoning with SO2to control the growth of Li2O2is discussed.