First principles design and investigation of 2-dimensional carbon lattices and their hetero-atom doped analogues for alkali ion batteries

Abstract

Continuous increase in global population and technology dependence has led to extensive exploitation of non-renewable sources of energy like fossil fuels. Adverse consequences on environment due to burning of fossil fuels calls for a transition of the energy sources from non-renewable to renewable accelerating the sustainable development. Inexhaustible energy sources such as sun, wind, etc. are present in ample amount but are not being e ectively used due to their intermittent nature. To build a bridge between the demand and supply of energy, batteries have attracted much interest as energy storage devices. Presently, the commercial market is dominated by Li ion batteries, but the high cost and rare availability of Li in addition to dendrite formation at electrode-electrolyte surface limits its usage in foreseeable future. The need to overcome these shortcomings, paves path to non-Li alkali ion batteries like Na/K ion batteries owing to the similar electrochemical working mechanism to Li ion batteries. The practical realization of these alternates is hindered due to low energy density of electrode materials and sluggish kinetics owing to the large Na/K ion radius and thus, continues to grab the interest of scienti c community to develop e cient electrode materials. Lack of e cient anode materials for non-Li ion batteries drives the objective of the presented thesis to design and investigate 2D materials by electronic structure calculations employing density functional theory for their application as anode in Na/K ion batteries. We present the rst principles study to design di erent 2D carbon lattices where the carbon atoms are arranged in varied conformations and exist in di erent hybridization states as sp, sp2 and sp3. Additionally, Si and N are doped in the carbon lattice which opens a new domain of materials modulating the characteristic properties of parent lattice. Two di erent Si doped sp2 hybridized carbon geometeries called Siligraphene (SiC7) and Si doped T-graphene (t-SiC3) have been systematically studied for structural, geometric, dynamic and thermal stabilities and established as e ective anode materials in Na ion batteries. In addition to Si doped T-graphene, the T-graphene lattice has been doped by N atom to obtain a novel 2D material, N doped T-graphene, existing as orthorhombic crystal system having Pmm2 space group. Hetero-atom doping into the carbon lattice imparts competitive adsorption sites on monolayer surface, which has been thoroughly studied and explored for the favourable adsorption behaviour. Additionally, C-silicyne, a 2D lattice comprising of sp hybridized C atoms has also been reported as e ective anode material for both Na and K ion battery. The adsorption interaction of metal ions with the electron rich surface is governed by the cation- interaction called Dougherty e ect. Moreover, a detailed comparative study of t-SiC3 for Na and Li ion batteries illustrate the e cient charge/discharge kinetics of the monolayer for Li/Na ion battery. The dynamic and thermal stability of the proposed materials has been validated by phonon dispersion spectra and ab initio molecular dynamics. The metal ions adsorb on the monolayer surfaces by weak ion interactions, hence dispersion corrections by Grimme have been incorporated in the calculations to account for the failure of DFT while considering long-range interactions. The Br nsted-Evans-Polanyi linear relation governs the adsorption and desorption behaviour of metal ions on monolayer surfaces. Furthermore, quantitative and qualitative charge analysis by Bader charges and charge density di erence plots illustrates redistribution of electron charge density owing to e ective binding of metal ions. The electronic character analysis signi es the semi-conducting nature of SiC7 monolayer having a small band gap of 0.76 eV while all other studied materials are metallic in electronic behaviour. For practical application of the monolayers as anode in Na/K ion batteries, high storage capacities and low di usion barriers are achieved. The open circuit voltage for the proposed materials lie in the ideal voltage range of 0.1-1 V for metal ion batteries. Based on these comprehensive set of ab initio, and thermodynamic calculations, aforementioned hetero-atom doped 2D carbon lattices are proven to be effcient anode materials in metal ion batteries. At the end of the thesis, work has been summarized along with concluding remarks and future directions followed by a bibliography.