Preparation and Application of Layered Graphene and MoSe2 Based Materials in Electrochemical Energy Storage

Author:Xiong Dong Bin

Supervisor:bai zhi min li xi fei

Database:Doctor

Degree Year:2019

Download:50

Pages:135

Size:7157K

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In recent years,graphene and related two-dimensional(2D)materials have long been the hotspots in the field of electrochemical energy storage due to their unique physical,electronic and chemical properties.Owing to their strong in-plane chemical bonds and weak van der Waals force between adjacent layers,their inherent properties differ from those of their bulk lamellar systems.In particular,the well-maintained in-plane integrity contributes to exceptional mechanical properties,such as flexibility and processability.Meanwhile,the high specific surface area and highly exposed surface atoms is also conducive to effective regulation of physical and chemical properties of materials by the introduction of defects,lattice strains,and phase transformation.Herein,we propose a novel spin-coating technique for fabricating flexible,freestanding,and mechanically robust graphene film(GF)for supercapacitors(SCs).Flexible electrodes with different nanostructures based on GF were fabricated,and the interrelation between the microstructure and electrochemical properties were analyzed and discussed.In addition,2D transitional metal dichalcogenides(TMDs)—MoSe2 as efficient sulfur hosts were synthesized to overcome the barriers of lithium–sulfur(Li–S)batteries.The effect of MoSe2 with tunable phase and morphology on the conversion kinetics of lithium polysulfide was systematically investigated.(1)The GF with high flexibility and high conductivity was fabricated via a novel spin-coating technique with subsequent thermal reduction.It shows the significant advantages of simple operation,easy scale-up,and low cost compared to the conventional filtration approach.More importantly,the GF can be readily processed to films with desired sizes,thicknesses and shapes.After fabricating to thin films with suitable sizes,mechanically tough GF electrodes without binder and collector were feasibly obtained.Additionally,the proposed GF can be used as flexible structural support to prepare composite films by compositing with metal,metal oxides/hydroxides and metal sulfides.(2)With graphene films(GF)as robust substrates,a series of flexible composite film electrodes of high-performance SCs were obtained by directly growing 3D nanostructured arrays(Co3O4,MnCo2O4 and Co9S8)on the graphene films.Originating from the high mass loading of the pseudocapacitance-activated materials,the highly conductive GFs,and the 3D arrays in the composite film configuration.The fabricated free-standing composite film electrodes deliver outstanding electrochemical properties without current collectors or binders.For example,the unique architecture of the flexible Co9S8/GF film exhibited high specific capacitance of 653 F/g at 0.5 A/g,superior rate capability(469 F/g at 10 A/g),as well as good cycling life with 91.8%capacitance retained over 2500 cycles.The main reason is that the Co9S8 nanoarrays are directly grown on GF with robust adhesion,ensures an intimate contact and effective electron transport between the highly conductive GF substrates and the Co9S8nanotubes.Besides,the vertical Co9S8 nanotubes with thin walls shorten the ion and electron diffusion paths,and the hollow structure with enough void spaces among Co9S8nanotubes allows more electrochemical active sites exposed to the electrolyte for effective ion transport.The influence mechanism of the structure and composition of the composite film on the SCs performance was also discussed,aiming to supply theory foundation toward the realization of high performance flexible SCs.(3)2D TMDs—MoSe2 was synthesized as advanced sulfur hosts for boosting the performance of Li–S batteries.Since phase structure has a profound influence on properties such as conductivity and chemical stability,the effect of MoSe2 phase structure(2H and 1T)on the electrochemical properties of Li–S batteries was investigated for the first time,and the kinetic mechanism of MoSe2 phase structure on the performance of Li–S batteries was revealed.Careful structural characterization and electrochemical measurements show that the formation of the 1T phase MoSe2 through phosphorus doping can effectively improve the intrinsic activity and conductivity.When employed as sulfur host,the 1T phase MoSe2 can provide strong chemical bonding towards polysulfides,and show high catalytic effect to facilitate the polysulfides conversion reaction,effectively suppressing the“shuttle effect”.Additionally,the 3D MoSe2 structure enables fast and continuous electron transportation and accommodates large volumetric change during the charge/discharge processes.As a result,the 1T phase MoSe2 cathode based Li-S battery exhibits significantly improved rate capability and cycling stability,including an excellent retained discharge specific capacity of 479 mAh/g at 2 C,and outstanding cycling stability with reversible specific capacity of 698 mAh/g after 100 cycles at 0.1 C.Significantly,the enhanced Li-S battery performance provide significant insight for realizing high performance Li-S batteries by tuning the phase and morphology of 2D TMDs as sulfur hosts.