Density Functional Study of Ionic Adsorption and Diffusion on Two-Dimensional Materials

Abstract

Search for efficient functional nanomaterials for the future nanotechnology is one of the main focus of the material scientists and engineers. Computational methods have been playing a leading role in the atomistic understandings of structure and properties analysis of nanomaterials for the last two decades. The central theme of this thesis is to use density functional theory calculations to introduce new nanomaterials with boosted efficiency and understand the physics and chemistry behind it. Rechargeable Li ion batteries (LIBs) are playing a crucial role in the development of portable electronic technology. On the other hand, to avoid global warming and reduce the dependence on the limited energy fossil fuel, sustainable energy resources should be utilized. Since the production of energy from such resources fluctuates highly, short time storage of energy is needed. In this context, we studied different anode materials for LIBs and possible alternatives to boost their efficiency, lower the cost and toxicity. Experimentally possible nanosheets of the TiS2, W2C, and unzipped graphene oxide (UGO) have been considered for metal ion adsorption and diffusion. These materials are found to have high metal ion storage capacity, high electronic and ionic conductivity and suitable anodic open circuit voltage. Monolayer SnS2 has high Na storage capacity, but its high expansion under sodiation and poor electrical conductivity are barriers to be used as anode. On the other hand, graphene is metallic and mechanically very strong, but its Na binding energy is not enough to be used as an anode. A heterostructure of graphene and SnS2 becomes a perfect anode material where all the problems are solved adequately. Similarly, the stable monolayer MoS2 has a high band gap and low Li/Na storage capacity while monolayer VS2 is metallic and has a high Li/Na storage capacity, but it is chemically unstable. Suspended single-layer of VS2 is not grown in the laboratory till today. Because of low lattice mismatch and similar geometry, it is easy to make VS2/MoS2 heterostructure. The heterostructure stabilizes of the VS2 layer and boosts the Li/Na capacity of MoS2 simultaneously. The enhancement in stability and electrochemical performance of the VS2/MoS2 nanocomposite is attributed to the charge redistribution in the formation of the nanocomposite. Besides new anode materials for rechargeable LIBs, we also report on the possible existence of new single-layer materials, namely MoC, WC, WS, and WSe. These materials have spontaneous switchable out-of-plane polarization. An in-plane uniaxial or biaxial strain can change the magnitude of the out-of-plane polarization while to produce the in-plane piezoelectric effect, a uniaxial strain along the armchair direction is necessary.