Theoretical atomic-scale modelling of perovskite-type photo-battery materials

The coupling of a solar cell with a lithium ion battery is a promising approach for the development of miniaturised energy harvesting-storage devices. As hopeful candidates, halide perovskites have not only good photovoltaic efficiency, but also have good ionic conductivity and lithium storage potential, as reported recently. This allows in principle the combination of both functions, photovoltaic energy conversion and electrochemical energy storage, in a single device. Since the realization of both functions depends largely on the behavior of Lithium ions inside perovskite crystals, this involves fast diffusion and high concentration of Lithium, and moderate changes in the electronic structure of the crystal after Li insertion. This thesis aims to advance the understanding of intercalation and diffusion of interstitial Li ions in the halide perovskites by means of first-principles methods. As first step, we deal with the diversity and complexity of hybrid (organic-inorganic) halide-perovskite
structures. The perovskites have a wide variety of two-dimensional and three-dimensional crystal structures. The elements in these structures can even be selectively substituted to meet device design demands. Typically, perovskite structures are considered to have more or less intact cubic symmetry. However, our results indicate that even at room temperature, the cubic symmetry of such halide perovskites is only an average in space and time, and the low-symmetry structures with tilted octahedra are more stable.
Then, we investigate the stability of interstitial positions for Li ions in hybrid perovskites. We use two crystal models, i.e. a high-symmetry cubic α structure and a low-symmetry pseudo-cubic γ’ structure. We find for the inorganic perovskite CsPbI3 that the α structure does energetically not allow Li uptake, whereas in the γ’ structure, adding Li is energetically favorable. We also extend the study to the organic perovskite Methylammonium (MA) Lead Iodide, MAPbI3. The directionality of the MA cation provides more structural degrees of freedom, which leads to a different order of stability of interstitial positions for Li ions to the inorganic perovskite CsPbI3. Next, we study the migration of interstitial Li ions in CsPbI3. We consider two scenarios: the fast-ion limit of Li in the α structure and the slow-ion limit of Li in the γ’ structure. Migration of fast Li ions requires to overcome lower energy barriers along simple migration pathways in the α structure. In the γ’ structure, there are several non-degenerate interstitial sites and the network of migration pathways becomes complicated and (locally) anisotropic, due to the lower symmetry of the structure. Nevertheless, the effective III barriers for the long-range diffusion are only moderately different for the two scenarios. Hence we conclude that CsPbI3 can be a suitable material for a Li-ion battery in terms of the Li-ion diffusivity. Last, we examine the electronic structure of LixCsPbI3. Interstitial Li has three effects on the electronic structure. There are in descending order of magnitude: the induced structural distortion, the changed electronic screening by the additional electron coming with the Li ion in the conduction band, and Li defect states in the band structure of CsPbI3. In the realistic structure of LixCsPbI3, the presence of Li in even moderate concentration increases the band gap significantly.