Study Of Micro/nano Structure Fabrication Methods For Micro Energy Storage Devices

As an efficient energy storage system,supercapacitors have been widely used in the start-up system of electric vehicles,electric buses,smart grid,portable electronic devices and other systems due to their high power density.However,supercapacitors still suffer from relatively low energy density compared to lithium battery,which limit their further commercial applications.To improve the energy density of supercapacitors without sacrificing power density and cycle life is still challenging.Owing to the high surface area,high conductivity,superior structural stability and porosity brought by the scale effects,fabricating mirco/nano structures could be an efficient way to improve the energy density of energy storage deives.In this thesis,a variety of micro/nano fabrication methods were applied to fabricate micro/nano-scale structures.The formation process,physical and chemical properties and energy storage properties of the as-synthesized nanomaterials were studied.All-solid-state supercapacitors based on these nano materials were assembled to further explore their electrochemical energy storage properties.The main contents of this thesis include:?1?The energy density of commercial carbon-base supercapacitor is relatively low,in order to improve the energy density of activated carbon-based supercapacitors,in this work,a micro/nano fabrication method was proposed to fabrication carbon-based porous composite nanostructures.The lotus seedpod was used as the carbon source to fabricate mesoporous three-dimensional activated porous carbon structure.Then,the porous carbon structures were integrated with transition metal sulfides to fabricate activated carbon/NiCo₂S₄ composite structure.Owing to the synergetic effect of the carbon material and the ternary-transition metal sulfides,the supercapacitor electrode based on the composite nanostructures shows a very high specific capacitance and an excellent cycling stability.An asymmetric all-solid-state supercapacitor based on this structure was assembled.The as-assembled supercapacitor exhibits a very high energy density(49.7 Wh kg^-1).?2?To improve the performance of electode in flexible energy storage device,a multi-scale fabrication method was proposed.Combing the flexible carbon substrate and mesoporous nano materials,several free-standing hierarchical electrodes have been fabricated.1.A hollow polyhedron-like CuO nanostructure was fabricated on flexible carbon cloth using metal organic framework as template,the CuO polyhedron structure shows better electrochemical performance than the nanoarray structures.2.An anion exchange method was used to transform the carbon cloth supported CuCo precursor into porous CuCo₂S₄ nanorod arrays.BET results show that the CuCo₂S₄ nanorod arrays have a very high surface area.When applied as supcapacitor electrode material,the CuCo₂S₄nanorod arrays show a very high specific capacitance(1536.9 F g^-1 at 1 A g^-1)and a very good cycling stability?only 12%loss after 10000 cycles?.A flexible all-solid-state supercapacitor was assembled using porous CuCo₂S₄ nanorod can deliver high energy density(56.96 Wh kg^-1)and power density(4800 W kg^-1).?3?In order to further explore the approches for improving the performance of the electrode materials in energy storage devices,the nanomaterials loaded on flexible substrate was further modified with a nano-scale fabrication method.A NiCo₂S₄@NiCo₂O₄ core-shell structure was successfully integrated on the flexible conductive carbon cloth by a step-by-step method.The supercapacitor electrode based on the core-shell structures can deliver a very high specific capacitance of 2258.9 F g^-1 at the current density of 0.5 A g^-1.In addition,the capacitance of the electrode can be maintained at 87.8%3000 charge-discharge cycles,indicating that the core-shell structure can also improve the electrochemical stability of electrode.A flexible all-solid-state supercapacitor was assembled based on the NiCo₂S₄@NiCo₂O₄ composite structure,the assembled all-solid-state supercapacitor exhibits high energy density(44.06 Wh kg^-1),high power density(6400 W kg^-1),and excellent cycling stability?92.5%after 6000 cycles?.The almost unchanged electrochemical performance of the all-solid-state supercapacitor under bending condition indicate that the as-assembled supercapacitor can be applied for flexible applications.?4?Considering the miniaturization and scalable fabrication of energy storage device,a porous and uniform carbon aerogel based composite structures were fabricated.The carbon aerogels derived from organic aerogels show good structural stability,hence the overall three-dimensional porous structure can be maintained after laser cutting.Subsequently,the Ni?OH?₂ nanostructures were uniformly loaded on carbon aerogels with electrochemical deposition.When used as supercapacitor electrode,the porous carbon aerogel/Ni?OH?₂ composite structure shows a high area specific capacitance of 2.25 F cm^-2.The as-assembled all-solid-state asymmetric supercapacitor based on the composite structure also exhibit a high volumetric energy density(3.2 mWh cm^-3),higher than commercial carbon-based supercapacitors.In addition,the capacitance of carbon aerogel/Ni?OH?₂//carbon aerogel asymmetric supercapacitor can be maintained at 85.6%of the initial value after 6,000 cycles,indicating that the as-prepared supercapacitor has a good cycle life.This work provides a new idea for the preparation of supercapacitor electrodes in a scalable way.