Enhancement of the performance of rechargeable batteries by proposing new materials

Abstract

Nowadays, rechargeable batteries are widely used for many different purposes and they areamong the main clean energy storage systems that are readily available. Electrical energy isstored in the battery through oxidation and reduction and via a charge transfer agent that movesbetween the cathode and anode. Rechargeable batteries commonly use the alkali metal, lithium(Li) as the charge transfer agent. The capacity of the anode material and selection of a chargetransfer agent that is easily adsorbed by the anode material are two important factors whendesigning rechargeable batteries. In this PhD thesis, we propose several types of carbonaceousmaterials that could be potentially used as an anode in rechargeable batteries. These materialsare derivatives of graphene: graphdiyne (GDY), hydrogenated defective graphene (H-DG),edge-functionalised graphene nanoribbons (F-GNRs), and doped graphene. In addition, weselect the charge transfer agents, sodium (Na) and calcium (Ca) on several materials, and wealso consider the use of potassium (K).The first material, GDY, adsorbed Na atoms with a binding energy appropriate for batteryapplications. Adsorption occurs on the 6-membered ring and the pores, with an empiricalformula of NaC2.57, which is equivalent to an electrical capacity of 497 mA h g-1. If expansionis allowed for the bulk layered GDY, this loading will occur on each layer of bulk GDY, withan expansion of 28% of the interlayer spacing compared with the unloaded material.Furthermore, the bulk layered GDY in an AB-2 stacking has a barrier of energy against Namovement, parallel to the layers of 0.84 eV and as low as 0.12 eV for vertical movement withina slit pore. These values were even lower for the bulk material with AB-3 stacking.A hydrogenated mono-vacancy repeated four times within a graphene sheet supercell with 68carbon atoms, 4(H1-MVG), has been considered as another suitable material for the NIB andCIBs. It also provided a model hydrogenated graphene that could be compared with graphene.Compared with graphene, this material strengthened the binding of Na by an energy of 0.72eV. A similar strengthening occurred for Ca binding. This material can bind up to 16 Na and14 Ca atoms per 4(H1-MVG) supercell on a single layer. Although the bulk layered 4(H1-MVG) did not strongly bind Na within the layers, Ca effectively intercalated within the layers,with binding energies varying between -2.05 to -2.79 eV, which is strong in comparison withCa binding to graphene (-0.82 eV). Finally, we found that up to 16 Ca atoms could intercalate within the bulk layered material, which is equivalent to an electrical capacity of 591.2 m A hg-1 and results in 29.3% for the interlayer expansion.Edge-functionalised graphene nanoribbons have been proposed as the third material for NIBsand CIBs. Nanoribbons with zigzag and armchair edges are functionalised with the selectedoxygen-containing functional groups hydroxyl (HO-), carbonyl (O=), and carboxyl (HOOC-).These groups are found abundantly in reduced graphene oxide (r-GO), and we intended tosimulate the strength of Na and Ca adsorption over these groups. According to the results, thehydrogen passivated and edge-functionalised ribbons improved the binding of Na and Ca byabout 1.1 to 1.9 eV compared with the bindings over the graphene sheet, respectively. Thisimprovement is due to electronic structure of the nanoribbons as well as the direct interactionwith the oxygen functional groups. The improvement in binding compared with graphene wasmore significant over the edge of the ribbons with carbonyl and carboxyl groups. In addition,each carbonyl and carboxyl group could bind as many as five Na or Ca adatoms around it.DFT calculations for the binding of Na atoms to the nitrogen-doped graphene (graphitic N) andnitrogen-containing functional groups within the graphene sheet showed that pyridinic N,amine and amide groups bound Na more strongly than the Na cohesive energy. We also carriedout DFT calculations to determine the binding of Na, Ca and K over the oxygen groups foundin the basal plane of the r-GO sheet. When considering Ca adsorption over the hydroxyl group,the Ca bound to the OH group in all cases due to the high charge of Ca as a multivalent metal.However, K did not form KOH, except in one case. Therefore, K is a better choice than Na andCa in binding over the oxygen groups in the r-GO. Finally, we considered the binding of Kover HO-, O-, and HOOC- groups at the edge of GNRs. We found that HO- and O-GNRsbound K with energies that are suitable for use in rechargeable batteries. We completed thisstudy to investigate the effect of oxygen groups found in the cellulose derivative materialapplicable in potassium ion batteries.