Solid State Reactions in Energy Materials
On the boundary between physics and chemistry solid state reactions occupy a complex and active research area. In our research we focus on energy materials used in solid state hydrogen storage and battery materials.
Complex hydrides
In this research the thermal decomposition of the complex hydrides, borohydrides and alanates, is being studied using computational methods. Borohydrides, especially Mg(BH4)2 and LiBH4, show high promise for hydrogen storage due to their high gravimetric hydrogen content. However, in order to make this promise come true, a catalyst is needed to lower the hydrogen desorption temperature. To develop the insights that are needed to find a catalyst, the complex, solid state desorption reactions must first be understood. The complexity of these materials and that of their decomposition reactions makes this a challenging and interesting class material to study from a fundamental point of view as well.
The research is conducted by state-of-the-art computational methods and is be closely coupled with high quality experimental work. A part of this research is the identification of the intermediate reaction products in the decomposition reactions and calculating the thermodynamics of all the reaction steps. Secondly, the reaction mechanisms are investigated. Finally, possible catalysts, and the influence of nano-structuring is explored. During all steps on one hand the input from experiments and on the other hand developing ideas for further experimental work is strongly enhances the efficiency.
Ion diffusion in battery materials
Many of the solid state reactions taking place in the charging and discharging of battery system are perfectly suited to be studied by first principles methods. On can think of the intercalation process, the nucleation of phases, diffusion of ions and phase stability. Both the thermodynamics and the kinetics of these processes can be studied quantitatively and compared to the experimental results. Such comparisons will enable to distinguish between various atomic scale processes and hence gain insight into the fundamental microscopic processes.
We employ density functional theory (DFT) methods. According to the specific problem at hand, these are performed using either a periodic or a cluster based code. The thermodynamics of intercalation reactions is studied by firstly searching for optimal nucleation sites using molecular dynamics techniques and secondly comparing temperature dependent free energies for the phases. The kinetics of diffusion reactions is studied in a tree step approach. In a first step molecular dynamics simulations are used to identify possible reaction mechanisms. For mechanisms found, reaction paths are investigated using elastic band methods in order to find the transition states. Finally transition state theory can be employed to calculate the reaction rates of the diffusion reactions.