Investigation of carbon-based two-dimensional materials for use in lithium and lithium-sulfur batteries

Two-dimensional materials exhibit different and unique properties compared to three-dimensional structures due to the emergence of dominant quantum mechanical effects. These properties lead to significant changes in the physical and chemical behavior of materials. The motivation for this thesis is that there are studies in the literature showing that the use of low-dimensional materials in various fields such as gas sensors, water desalination, hydrogen energy storage and battery applications yields more efficient results due to the increase in surface/volume ratio compared to their three-dimensional state. Therefore, within the scope of this thesis, the changes in the electronic and magnetic properties of some two-dimensional materials under external effects such as foreign atom/molecule doping, strain application and increase in the number of layers and the usability of carbon-based two-dimensional materials in lithium, lithium-sulfur batteries were investigated with the help of density functional theory (DFT) based numerical methods. In this thesis, it is theoretically proved that Cr2TaC2 and Cr2TaC2O2 MXene structures retain their structural stability, electronically metallic and magnetically ferromagnetic character under strain of -3% - 9%. In two-dimensional materials such as FeN4 and FeP4, it has been shown that the application of strain and the formation of double layers can significantly change the electronic and magnetic properties of the materials. For example, the monolayer FeN4 structure is a Neel-type antiferromagnetic semiconductor with an indirect bandgap of 1.290 eV in the equilibrium state, whereas under 5% and 6% tensile strain it transforms into a ferromagnetic semiconductor with a direct bandgap. The FeP4 structure was found to exhibit ferromagnetic properties in the monolayer state and interlayer antiferromagnetic properties in the bilayer state. Finally, the effects of point defects and gas adsorption on the BeN4 structure were investigated in detail, showing that these structures have the potential to be used in applications such as H2 storage and gas sensing. The usability of carbon structures with sp and/or sp2 hybridizations in two dimensions (Graphyne, Dodecagonal, Haecklites (5-7), GY-5, Kagome Graphene, Porous Graphene, T-Graphene) as anode materials in Li-ion batteries was investigated. The trapping energy of a single lithium atom on the structures was calculated and then the maximum lithium storage capacity of the structures was theoretically determined by gradually increasing the number of lithium atoms on the layers. Accordingly, the Li atom storage capacities of the structures were found to be in the order of magnitude of Graphyne > T-Graphene > Dodecagonal > Haecklites (5-7) > Kagome Graphene and were calculated to be three to fourteen times higher than graphite (372 mAhg-1), the bulk form of Graphene. The GY-5 carbon structure was found to be strongly bound by a single Li atom, but after a second Li atom was attached to the structure, the structure deformed into a different crystal, Ψ-Graphene. However, it was revealed that Li atom cannot be used as an anode material in lithium ion batteries due to its weak binding energy of -1.260 eV to Porous Graphene structure. In this thesis, the interaction of Graphene, Graphyne and Graphdiyne monolayers with lithium polysulfides (LixSy; 1< x<2, 1