Structural Design Of Three-dimensional Self-supported Electrodes With Fast Energy Storage

Three-dimensional(3D)self-supported materials have the characteristics of fast ion and electron conduction,good mechanical properties as well as simple preparation processes,and hence are widely used in energy storage devices(such as lithium-ion batteries and zinc-air,etc.).However,the structure is easily collapsed at high current densities,which results in poor cycle stability.Therefore,it is still a challenge to develop a3D self-supported electrode that can meet the structural stability at high current density.In this dissertation,we have designed a class of 3D multilayer coaxial nanoarray electrode composed of heteroatom-doped carbon materials and transition metal oxides(TMO)and defect-rich nickel-cobalt bimetallic sulfide nanotube arrays with fast energy storage.The relationship between the structures,compositions and electrochemical properties is systematically discussed.The main results and discussions are described as follows:1)Structural design of N,O-C/Sn O₂/N,O-C multilayer coaxial nanotube arrays with fast energy storageWe have proposed a self-template method and chemical vapor deposition(CVD)method to successfully synthesize N,O-C/Sn O₂and N,O-C/Sn O₂/N,O-C multilayer coaxial nanotube arrays using urea as carbon.Electrochemical characterizations reveal that N,O-C/Sn O₂/N,O-C three-layer structure as the negative electrode for lithium-ion battery shows the specific capacity of 3.4 m Ah cm^-2at 5 m A cm^-2(2.0 A g^-1),higher than 3.2 m Ah cm^-2for N,O-C/Sn O₂two-layer structure.More importantly,after 1,000 repetitive cycles at5 m A cm^-2,59.2%of the specific capacity is retained for N,O-C/Sn O₂/N,O-C electrode,while it's only 39.8%for N,O-C/Sn O₂electrode.The improved specific capacity and stability performances are contributed by that the outer and inner carbon layers not only synergistically inhibit the agglomeration of Sn O₂and maintain the structural stability during the lithiation and de-lithiation process,but also accelerate fast lithium-ion diffusion and charge transfer.2)Construction of full-cell lithium-ion battery based on Sn O₂and V₂O₅multilayer coaxial nanotube arraysWe have also successfully prepared N,O-C/V₂O₅/N,O-C three-layer coaxial nano-array electrodes using the above methods.Electrochemical studies have further shown that the three-layer coaxial nano-array structure can exhibit fast energy storage behavior at high current density.N,O-C/V₂O₅/N,O-C three-layer structure as the positive electrode of lithium-ion battery delivered the specific capacity of 0.60 m Ah cm^-2at 5 m A cm^-2and retained 78%of the initial specific capacity after 150 cycles,while The initial specific capacity of the two-layer structure N,O-C/V₂O₅is 0.55 m Ah cm^-2,and only 36%of the initial specific capacity is maintained after 150 cycles.Moreover,N,O-C/V₂O₅/N,O-C and N,O-C/Sn O₂/N,O-C three-layer coaxial nano-array electrodes have successfully assembled into soft package lithium-ion batteries.It delivered good stability at the different bending degree(0-180^o).The initial specific capacity is as high as 1.96 m Ah cm^-2at 5 m A cm^-2,and56%of the initial specific capacity is retained after 150 cycles.3)Design Defect-rich Ni-Co Bimetal Sulfide Nano-Array Electrodes for Li-Ion and Zinc-Air BatteriesWe have proposed a simple self-template method and thermal reduction method to successfully prepare defect-rich nickel-cobalt bimetal sulfide three-dimensional nanoarray electrode for lithium-ion and Zn-air batteries.It's found that the defects generated during the thermal reduction process provided large electrochemically active surface area and improved electrical conductivity,and the metal sulfides/metal heterostructure enhanced the intrinsic activities towards oxygen evolution reaction(OER)and oxygen reduction reaction(ORR).The most optimized defect-rich nickel-cobalt bimetal sulfide three-dimensional nano-array electrode showed the ORR oxygen reduction peak potential of 0.89 V vs.RHE and the OER overpotential of 178 m V at 10 m A cm^-2in O₂-saturated 1.0 M KOH solution,much better than that of defect-free nickel-cobalt bimetal sulfide electrode.Moreover,we also propose that defect-rich free nickel-cobalt bimetal sulfide nanoarray is directly grown on the gas diffusion layer as the air electrode of Zn-air batteries,which significantly improved the powder density 65 m W cm^-2to 95 m W cm^-2.Moreover,after 240 h continuously operating,the discharge potential almost had no decay.Defect-rich nickel-cobalt sulfide nanoarrays as the negative electrode of lithium-ion batteries also showed the initial specific capacity of 4.5 m Ah cm^-2at 2 m A cm^-2,and retained 79%of the initial specific capacity after 500 cycles,which are much higher than that 56%of the defect-free electrode under the same experimental conditions.