Preparation Of Integrated Electrodes Based On Layered Double Hydroxides Towards High-performance Supercapacitors

Supercapacitors（SCs）have a great significance in the advanced energy storage devices owing to their fast charge/discharge rates,high power density and long service life,but the low energy density restricts their development in the future.Therefore,the construction of advanced SCs with high energy density and high power density is highly desirable.Due to its unique layered structure,large specific surface areas and adjustable the host layers and the interlayer negative ions,layered double hydroxides（LDHs）show excellent performance in various energy storage fields.In this dissertation,a systematic study on the electrode structural design and surface regulation has been performed based on typical pseudocapacitor materials（LDHs）,so as to achieve high mass-loading LDHs-based electrodes and obtain largely enhanced electrochemical performance.Firstly,the hierarchically integrated electrodes with high mass-loading of LDHs were constructed based on a multi-stage conductive network.The highly-exposed active sites significantly improve the specific capacity and energy density of electrode.Secondly,a fine regulation of surface defects in the LDHs-based electrodes was carried out by the electrochemical activation process;consequently,the electrochemically activated Co Ni-LDH（ECA-Co Ni-LDH）exhibits a widened potential window and promoted metal-ion storage properties in neutral electrolytes.Finally,the energy density and potential window of the as-prepared supercapacitor device are significantly improved by coupling with suitable positive electrodes.In addition,the influences of LDHs chemical composition,molar ratio,interlayer anions on electrochemical properties of LDHs-based electrodes were also investigated.This work provides a theoretical basis and practical exploration for developing supercapacitor materials and advanced energy storage devices with high performance.Main research and findings are as follows:1.A strategy to fabricate a LDHs nanosheet array in situ on the two-dimensional carbon sheet（CNs）array was developed.A hierarchically integrated electrode with using the carbon nanosheet array as core and Co Ni-LDH as shell（CNs@LDHs）,has been successfully prepared,for the purpose of increasing the mass-loading and electrical conductivity and thus enhancing the supercapacitor performance.A well-defined Co-based zeolitic-imidazolate framework（ZIF-67）nanosheet array was firstly synthesized on carbon cloth（CC）through a facile liquid-phase deposition method,followed by a pyrolysis process to obtain the carbon nanosheet array.Subsequently,a dense layer of Co Ni-LDH array was directly electrodeposited on the surface of as-prepared CNs to achieve a hierarchical structure.The mass-loading of Co Ni-LDH can be regulated by adjusting the deposition time,and the mass-loading of LDH reaches up to 11.8 mg cm^-2.Due to the improved high specific surface area and enhanced conductivity,the resulting electrode materials exhibit a high specific capacitance(198.4 m Ah g^-1)and an excellent cycle stability（93.8%capacitance retention after 20000 cycles）.The hierarchical electrode delivers a high areal capacitance even with a high mass-loading of more than 10 mg cm^-2.Furthermore,the structure evolution and supercapacitor storage mechanism of hierarchical electrode were deeply investigated via a variety of ex-situ techniques.In the first charge process,the valence state of Co in CC/CNs@Co Ni-LDH increases from Co²⁺to Co³⁺,and H atom detaches from the hydroxy group of Co Ni-LDH.This process is accompanied with the removal of interlayer anion.In the subsequent discharge process,the valence state of Co recovers from Co³⁺to Co²⁺,and OH^–simultaneously enters into interlayer region of Co Ni-LDH.In the subsequent discharge/charge process,Co Ni-LDH delivers a reversible change of the Co valence state,indicating the highly reversible storage of OH^-.Finally,a flexible quasi-solid-state ASC device based on CC/CNs@Co Ni-LDH cathode was constructed,which delivered a superior mass energy density of 69.8 Wh kg^-1 and a power density of 4.3 k W kg^-1.Moreover,the capacitance remains 91.1%even after 10000 cycles,which is superior to previously reported LDHs-based supercapacitors.This work provides a facile route to construct integrated electrode with high-mass-loading,which is important for the application of high-performance supercapacitors.2.The potential window of LDHs-based aqueous supercapacitor materials is usually lower than 0.5 V due to the oxygen evolution reaction（OER）,which leads to a relative low energy density.To solve this issue,a hierarchically integrated array electrode based on the defect enriched LDHs was fabricated,in which the potential window is extended to 0-1 V and the energy density increases largely.Firstly,various Co-based LDHs were prepared via a hydrothermal method,followed by an electrochemical activation to tuen the surface defects.During the electrochemical activation process,LDHs material undergoes a phase transformation to construct hydrogen vacancies,which is accompanied by the detachment of hydrogen and the exposure of oxygen on the LDHs host layers.The relationship between the ion storage capacity of electrochemically activated LDH（ECA-LDH）and the Co content presents a"volcanic"curve;and the maximum capacitance of LDHs is achieved in the ratio of Co to other metal within 2:1-5:1.To further improve the electrochemical performance of LDHs,an integrated Co Ni-LDH nanosheet array was prepared by in-situ growth of metal organic framework compounds（MOF）,followed by sacrificing templates to enhance active sites.Surprisingly,the as-prepared Co Ni-LDH nanosheet array with an optimized architecture exhibits an overwhelming metal-ion storage after the electrochemical activation.The ECA-Co Ni-LDH nanosheet can serve as a host material for the intercalation of various metal cations(e.g.Li⁺,Na⁺,Mg²⁺,Ca²⁺and Zn²⁺).The specific capacitance is 3.5,9,14,5 and 24 times higher that of unactivated Co Ni-LDH arrays,respectively.Moreover,the potential window of ECA-Co Ni-LDH（0-1 V）in the neutral aqueous electrolyte is the widest among LDH-type materials（<0.5 V）.Furthermore,an asymmetric device based on ECA-Co Ni-LDH cathode and Fe₂O₃ anode delivers an energy density of 184.4 Wh kg^-1 and a capacitance of 120 m Ah g^-1 with an excellent cycle stability（without decline after 10000 cycles）,which is comparable to those of commercial lithium-ion batteries.This work provides a new strategy for the development of high-performance supercapacitors materials.