A Theoretical Study On Chemically Doped Graphdiyne For Supercapacitors And Lithium Ion Batteries

Energy shortage is one of the most severe crises human beings are facing with.It is urgent to exploit new and clear energy resources.,and the development of energy storage and conversion devices is the key for future utilization of these resources.One of these devices,supercapacitor,has been extensively studied due to its high power density,long cycle life and environmental friendly properties.However,the low energy density of supercapacitor can’t meet the practical needs,which severely limits its further applications.Another device,lithium ion battery,which has no memory effect,renewable charging and discharging,as well as the high energy density,has also attracted considerable interest.However,lithium ion batteries suffer from low power density that leads to slow rate of charging.These advantages/disadvantages are directly related to the properties of electrode materials,either for supercapacitors or lithium ion batteries.Consequently,to find appropriate electrode materials becomes the focus of study to their performance of devices.Currently,supercapacitors and lithium ion batteries mainly include carbon-based materials as the electrodes.Graphdiyne,as an allotrope of graphene,exhibits many outstanding properties due to its unique structure,including high specific surface area,carrier mobility and the semiconducting feature.In this thesis,by firstprinciples calculations based on density function theory,we systematically investigated the properties of graphdiyne as the electrode materials of supercapacitors and lithium ion batteries in the context of energy storage and conversion.The main results are shown as follows,(1)First,we systematically investigated the structural,stability,electronic and capacitance properties of Boron/Nitrogen-doped graphdiyne.Results show that Boron/Nitrogen doping has low formation energies in graphdiyne,indicating facile experimental synthesis.Interestingly,Boron/Nitrogen doping transforms graphdiyne from a direct-band-gap semiconductor to an excellent metal,originating from the shift of Fermi level into the valence/conduction bands.By evaluating the quantum capacitance of graphdiyne with different doping concentrations,we demonstrate that the capacitance of Boron(Nitrogen)-doped graphdiyne as anode(cathode)can reach a record high value of4317 F/g(6150 F/g),significantly outperforming other 2D carbon electrodes.With high specific surface area,unique porous structure and tunable electronic properties,we expect graphdiyne have great potential for supercapacitors and other energy storage devices.(2)Second,based on the results of the first work,we have also exploited the performance of Boron/Nitrogen doping graphdiyne as the electrode materials of lithium ion batteries.It was found that the introduction of Boron/Nitrogen does not change the most stable adsorption sites of lithium on the graphdiyne surface.Boron also increases the adsorption energy of lithium form-2.34 e V(with our dopant)to-3.64 e V,while Nitrogen dopant has little influence on adsorption energy.The calculated migration barriers of lithium on the electrode surfaces suggest that Boron/Nitrogen drastically decrease the migration barriers,indicating facile ion transportation.By gradually increasing the number of lithium ions on the surface,we found that-Boron and Nitrogen can improve the adsorption strength of multiple number of lithium ions.Based on these results,we believe that Boron and Nitrogen doping should have significant effects on enhancing the performance of lithium ions batteries by using graphdiyne as electrode.