Ab Initio Study of Intercalated TMDCs and their Superlattices

Abstract

In this thesis, the structures and properties of layered transition metal dichalogenide (TMDC) materials are investigated at the atomic scale using ab initio density functional theory methods, with a focus on determining their suitability for intercalation electrodes in lithium-ion and beyond-lithium-ion batteries. Layered materials with a van der Waals spacing have already demonstrated a lot of success as intercalation electrodes, owing to the natural channels through which foreign ions can move during cell cycling and be stored in when fully charged or discharged. Asa result, the fundamental function and working of batteries have changed little since the 1970s and 1980s. The TMDCs represent a particularly broad family of such layered materials that have received a lot of attention in a wide range of applications, but only selected materials have been considered as intercalation electrodes. We therefore provide a comprehensive study of these materials and several key electrode properties, including the volume expansion, the voltage, and the reversible intercalation capacity. From this, we conclude TMDC sulfides to be the best in general for lithium intercalation, highlighting the Group IV, V, and VI in particular for their low volumetric expansion, moderate intercalation voltages, and high stability against conversion reactions. TMDCs composed of early transition metals are also shown to offer the best performance for magnesium intercalation. We extend this investigation to consider how the elastic and mechanical properties change with intercalation. Such properties are particularly important for electrode modelling beyond the atomic scale, and we find that the introduction of an intercalant reduces elastic anisotropy but increases the bulk, shear, and Young’s moduli of the host material. Out of these broad studies, we identify ScS2 in particular to be a promising material for consideration as a cathode due to its high voltages and high intercalation stability, though it has received little attention previously. Consequently, we present a more thorough study of this material, employing a mix of machine learning and ab initio techniques, and consider other beyond-lithium intercalants. Ultimately, we find that ScS2 is able to compete with current market leaders and that the introduction of scandium into the structure of other cathodes could be used to improve their performance. Finally, we consider how the formation of TMDC superlattices affects the properties of the TMDCs. From of a study of 50 pairings, we are able to show that, in general, many key of the key electrode properties of van der Waals superlattice structures can be well approximated with the average value of the equivalent property for the component layers. Thus, we conclude that superlattice formation can be used to improve material properties through tuning of intercalation voltages towards specific values, and by increasing the stability of conversion-susceptible materials.