| Global warming and the exhaustion of fossil fuels have been alarming a bell to the sustainable development of humankind.In response to these challenges,a great deal of research effort has been dedicated to exploring and utilizing the energy from wind,solar and other alternative and renewable energy sources.On account of the intermittent feature of these energy resources,reliable energy storage systems,storing and delivering the acquired power in a stable and controlled manner,are in urgent demand.Flywheel energy storage,compressed air storage,pumped storage,and electrochemical energy storage systems,as the key and major member among the numerous energy storage systerms,have attracted escalating attention.Rechargeable batteries and supercapacitors are two important subclasses of electrochemical energy storage systems and have been widely used.Although other types of rechargeable batteries,such as sodium ion batteries,magnesium ion batteries and aluminum ion batteries,are also growing,their widespread commercializations are restricted by its poor security,lack of duration and operability and other issues.As a result,the current market for rechargeable batteries is still dominated by lithium-ion batteries.Lithium-ion batteries and supercapacitors follow different working principles in the electrochemical processes,and thus display different energy storage properties.Lithium-ion batteries rely on the difihsion-controlled Faradaic reaction occurring in the bulk electrodes,and thus the process is slow.The bulk energy storage mechanism of lithium-ion batteries make them possess high energy densities(up to 180 Wh kg-1)at the expense of limited power density and shorter life(usually only a few hundred cycles).Unlike lithium-ion batteries,supercapacitors store energy by rapid reversible adsorption on the surface of the electrode/electrolyte or by rapid oxidation-reduction on the surface/near surface.Thus,the supercapacitors have the advantage of higher power density(>10 kW kg-1),long cycle life(>105 cycles)and good reliability.However,the energy density of supercapacitors is much lower than that of lithium-ion batteries,and commercial supercapacitors are usually less than 10 Wh kg-1.To meet the ever-growing requirements of future electronic equipment,there is an urgent need to boost storage performance of the current lithium-ion batteries and supercapacitors.Transition metal oxides are considered to be the most promising electrode materials for lithium ion batteries and supercapacitors due to their low cost,ease of synthesis and environmental friendliness.However,transition metal oxides of single component suffer from quickly fading capacity(capacitance),poor conductivity,severe volume expansion and large voltage hysteresis,which severely baffled their commercialization.The preparation of composites based on transition metal oxides with two or multiple components and well-defined shape is of great significance for high performance lithium ion batteries and supercapacitors.In this dissertation,composites with micro/nanoscale,large specific surface area,good electrical conductivity and structural stability,and rapid ion/electron transport were successfully designed and fabricated by using various strategies.The special structure of composites enables excellent electrochemical properties than single component,such as high specific capacity(capacitance),long cycle life and high rate performance,which is mainly attributed to the synergetic effect from multiple components.The main contents of this paper are as follows:(1)The three-dimensional porous Co3O4@a-TiO2 core-shell micro/nanostructures were successfully constructed through the combination solvothermal and sol-gel processes.Noteworthy,by adjusting the volume ratio of ethanol to water in the sol-gel process,a-TiO2 shell with controllable pore size can be obtained.This is the first to use core-shell Co3O4@a-TiO2 micro/nanostructures as the anode electrode materials for lithium ion batteries.The electrochemical performance of the composites is closely related to their structure.Due to the excellent stability of the a-TiO2 shell,the high specific capacity of the Co3O4 core and optimized pore size in composites,these novel Co3O4@a-TiO2 core-shell structures display high specific capacity,long cycle life and good rate performance as advanced electrode materials for lithium ion battery.A high reversible specific capacity of 800 mAh g-1 was obtained at a current density of 0.5 A g-1,which well-maintained after 60 charge/discharge cycles.(2)Despite the preparation of hierarchical structures of transition metal oxides has been studied in depth,the synthesis of hierarchical multi-component transition metal oxides still has great challenges.Herein,a general method is proposed to fabricate a three-component transition metal oxide,namely MnO2@NiO/NiMoO4 nanowires@nanosheets hierarchical porous composite(MnO2@NiO/NiMoO4 HPCSs).Through a chemical solution method and subsequent calcination,the MnOOH@NiMo precursor is transformed into MnO2@NiO/NiMoO4 HPCSs without significant structural changes.Ultrathin NiO/NiMoO4 nanosheets are interconnected into honeycomb structures with abundant mesopores.The experimental results show that hexamethylenetetramine(HMT)and the solution system play an important role in the formation of MnOOH@NiMo precursors.When used as electrode materials for supercapacitors,MnO2@NiO/NiMoO4 HPCSs with a mass loading up to 5 mg cm-2,provide a specific capacitance of 918 F g-1 at a current density of 1 A g-1 and maintained excellent cycle stability,which exhibits better electrochemical performance than electrode materials consisting of a single component.Moreover,a high-voltage asymmetric supercapacitor based on MnO2@NiO/NiMoO4 HPCSs(positive)and activated carbon(negative)shows superior cycle stability,high energy density(26.5 Wh kg-1)and power density(401 W kg-1).These results confirm the superiority of the multi-component hierarchical structure as an advanced supercapacitor electrode material.(3)Metal organic clusters are zero-dimensional,free-standing and discrete molecules which have more than one metal center bridging with anions and outer shell organic ligands into a whole.No attempts have been made to create micro/nanocomposites from such kind of precursors and their unique structure-depended properties are also a virgin land.Herein,we reported a convenient preparation of MnO@Mn3O4 core-shell nanoparticles embedded in the nitrogen-doped porous carbon frameworks(MnO@Mn3O4/NPCFs)by pyrolyzing the mixed-valent octanuclear Mn cluster at 500 ? under argon atmosphere.Moreover,the potential applications for lithium ion batteries are also studied.Due to the unique structure and composition characteristics,these novel MnO@Mn3O4/NPCFs manifest superior lithium storage ability:ultrahigh specific capacity,long cycling stability and excellent rate performance,which also solved the issues of pulverization,slow ion/electron kinetics and particle agglomeration.This work opens a new way to design and construct anode materials for next generation lithium ion batteries.(4)We reported a simple two-step method to synthesize the MoO2/graphene composites(MoO2 NP/rGO)with high specific capacity,where MoO2 nanoparticles are uniformly anchored on the rGO nanosheets.In the two-dimensional nanocomposites,rGO can be used as a favorable support for the loading of electrochemically active MoO2 nanoparticles.Meanwhile,MoO2 nanoparticles can effectively prevent the stacking of the rGO.The effective combination of MoO2 nanoparticles and rGO nanosheets furnish additional electrochemically active sites for lithium ion storage.The MoO2 NP/rGO nanocomposites exhibit excellent cycle and rate performances as an anode material for lithium ion batteries.The resulting samples displays a high specific capacity of 1516.4 mAh g-1 after 150 cycles at a current density of 0.2 A g-1.After 350 charge/discharge cycles at 1.0 A g-1,the specific capacity can still maintain at 641 mAh g-1.This general strategy can be extended to construct other transition metal oxide/graphene-based composites for high performance lithium ion battery. |
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