Thesis presented June 26, 2024

Abstract:
As humanity grapples with the pressing challenge of greenhouse gas emissions, the significance of sustainable energy solutions becomes increasingly evident. Lithium-ion (Li-ion) batteries, hailed as a promising avenue for electricity energy storage,which is critical for embedded electronics, electric transportation, and irregular production from renewable sources such as wind, geothermal, solar. e.t.c. However, their widespread adoption hinges on two main critical factors such as the non-availability of Li in the Earth’s crust and its difficulty in extraction. Hence, its supply may lead to future conflicts. Apart from these, Li-ion batteries are required to store more energy, that is, have better capacity and also charge quickly. Perhaps, the high capacity requirement of Li-ion batteries could possibly be met by investigating into the key components of Li-ion, specifically the Anode (negative) and Cathode (positive) electrodes. These electrodes host the Li-ions that move in opposite direction to electric current during charge and discharge. Within this framework, the study of graphite intercalation compounds (GICs) emerges as a pivotal field, offering insights into enhancing the capacity of specifically the Anode electrode where graphite is the host material, hence the name GIC.
Basically, GIC which belongs to layered materials, involves the regular insertion of guest atoms, ions, or molecule between the layers of graphite. In the context of GIC, both theoretical and experimental work have been carried out in a bid to understand and tackle the challenges faced with Li-ion batteries. For instance, researchers have tried to explore the use other Alkali Metals (AM) which are readily available such as Na, and K as substitutes for Li. However, the formers seems to have reduced capacity, particularly in the case Na, where fully Sodiated compound has been known not to form. Furthermore, while fully Lithiated materials of Li-GIC have been well studied and characterized, phenomena at dilute or low concentration regime remains elusive. Similar to the case of Li, little or no information about the dilute regime has been known for K-GIC. In fact, K has been reported to occupy graphite gallery in a disordered manner without any established stoichiometry between C and K. Furthermore in this regime, questions like (i) the local environment evolution of AM as a function of concentration, (ii) the AM content at which pristine graphite stacking (AB or Bernal) transit to the fully lithiated (AA or hexagonal) stacking during lithiation, (iii) the mechanism driving intercalation, and many more are still open questions in the field of Alkali Metal Graphite Intercalation Compound (AM-GIC).
phases using the Density Functional Theory (DFT) formalism. The aim of this work is to understand the intercalation of AM (Li, Na, and K) into graphite with a particular emphasis on the dilute regime. Although with our DFT tool, we realized that not much calculations could be performed with Na due to its computational cost. Therefore, we focused on Li and K for which different behavior is reported in experiments. Hence pointing to different mechanisms at the atomic scale that we aim to capture with our approach. Using the DFT tool, we have shown that the interaction between Li and K in the graphite gallery is not merely electrostatic as assumed so far. Furthermore in the dilute regime, AM locally deforms the graphite sheet to avoid an over-compression by C atoms. This structural deformation is different in AB and AA graphite. We have used this observed structural difference between AB and AA graphite to substantiate the transition from AB to AA stacking during Li intercalation based on the total energy calculations from DFT.

Keywords:
Solid electrode Interface, Intercalation, staging, Graphites, Stacking