Structural Characteristics Of Tubular Halloysite And Its Application In The Field Of Electrochemistry

As an emerging one-dimensional（1D）nanomaterial,tubular halloysites（HNTs）are widely used in many fields such as ceramic,sustained release,carrier,adsorption,filler,template,catalyst,and energy and nanoscience.HNTs possess high aspect ratio,different charge and chemical properties,and abundant hydroxyl groups among the various clay minerals.HNTs can be functionalized by various modification strategies to enhance the interaction between HNTs and other materials.At present,most of the research on HNTs focuses on the fields such as adsorption,sustained release and inorganic/organic composites around the world,while overlooking the high ionic conductivity and hydrophilicity of HNTs and underestimating the application potential of HNTs in the field of electrochemical energy storage.In order to take advantage of the unique characteristics of HNTs and deeply explore the application value of HNTs-based composite systems,in the present work,the isothermal heating evaporation-induced self-assembly（IHES）method was used to fabricate functionalized HNTs-based orientated membrane（HOM）and the ion transport property of HOM was systematically studied,and then optimizing the structure of the HNTs-based composite material system to maximize the electrochemical properties and applications in the field of energy storage.The main results and understanding obtained are as follows:（1）The mineralogical characteristics of HNTs used in the present work were studied by various characterization methods.HNTs possess high crystallinity and purity,complete crystal form,excellent dispersibility and uniform particle size distribution.HNTs have a structure with open ends,uniform diameter lumen,high aspect ratio,and smooth charged surface,resulting in an ideal ion transport in various electrolytes and a promising applications in electrochemistry.（2）The structure of HNTs functionalized with sodium hexametaphosphate was optimized by IHES method and the HOM with large-scale ion transport channels were developed.The conditions for the efficient,rapid and high-quality reconstruction of the HNTs were clarified.It is proved that the size and shape controllable HOM has a continuous,dense and extremely well-aligned structure on a large-scale.The mechanism of HNTs orientation on a large scale is revealed,and then the ion transport properties of HOM in various electrolyte solutions are characterized.The nanofluidic ion channels in HOM show a strong surface-charge-governed ion transport behavior in various electrolyte solutions.It is determined that the unique tubular structure of HNTs can effectively transport various types of ions,which has significant advantages in the field of electrochemical energy storage,especially in secondary ion batteries.（3）A continuous and dense polypyrrole（PPy）conductive layer was successfully polymerized on the outer surface of HNTs by an efficient one-step in situ polymerization method.The hydrodynamic radius of HNTs coated with PPy increased from 440.2 nm to555.8 nm and zeta potential of HNTs transferred from-29.6 m V to+29.8 m V.The electrochemical performance of HNTs-PPy as cathode material for aqueous zinc ion batteries（ZIBs）was investigated.The ZIBs with HNTs-PPy cathode exihibit high capacity and excellent electrochemical performance.The HNTs-PPy full cells show satisfactory capacity retention of 87.4%after 500 cycles.This time-saving and cost-efficient approach is expected to provides a new strategy of large-scale producting cathode material for energy storage devices.（4）A time-efficient and operation-easy approach was used to prepare a dense HNTs coating on the surface of the Zn anode relying on electrophoretic deposition.The components of the deposition solution for effectively preparing HNTs coating were determined:the dispersing medium is ethanol,the binder is polyvinyl butyral,the surface smoothing agent is dibutyl phosphate,and the conducting agent is KOH.HNTs coating has a uniform thickness,smooth surface,and homogeneous distribution on the Zn foil.The relationship between the electrophoretic deposition time and the mass density of the HNTs coating was clarified.The mass density of the HNTs coating is the highest when the deposition time is 8 min.The HNTs-8 coating has the most suitable electrochemical performance for ZIBs through various electrochemical tests.The Zn/Zn symmetric cells based on HNTs-Zn anodes maintain a low overpotential at all current densities.HNTs-Zn is still maintained a low overpotential at 68 m V after 650 h at 0.5 m A cm^-2.The protection of HNTs coating extended the Zn/Mn O₂ full cells cycling performance up to 400 cycles with the capacity retention of 79%at 3 C.（5）The excellent corrosion resistance mechanism of HNTs coatings was elucidated.The uniform deposition of Zn²⁺can be induced to limit the growth of nanoscale Zn cores and reduce the electrode polarization owing to the excellent ion transport ability of HNTs and the high porosity of the 3D network formed by the stacking of HNTs.In addition,the negatively charged HNTs repel the SO₄^2-and attract Zn²⁺,resulting in the inhibition of the formation of basic zinc sulfate.The HNTs coating acts as a physical barrier and ion transport,which can effectively regulate the migration of Zn²⁺.Meanwhile,the HNTs coating can avoid the short circuit of the cells caused by the Zn dendrites and supress the formation of by-products.（6）A high-strength and ultra-stable nanocomposite hydrogel was prepared with high ionic conductivity based on 3-Methacry-loxypropyltrimethoxysilane（MPS）modified HNTs（M-HNTs）-cross-linked polyacrylamide（PAM）hydrogel.MPS was successfully grafted onto the outer surface of HNTs by covalent bonding with the hydroxyl groups on the surface of HNTs.The C=C double bond in MPS grafted on the outer surface of HNTs is opend by the initiator underwent in situ radical polymerization with AM to form a hydrogel.Meanwhile,M-HNTs entangle with PAM chains by hydrogen bonding to form an intricate three-dimensional network structure and HNTs act as an inorganic framework to support the entire hydrogel network.M-HNTs/PAM hydrogels have high flexibility,smooth surface,uniform thickness,translucent state,porous structure and large pores.（7）M-HNTs/PAM hydrogels have ideal mechanical properties and ionic conductivity and can withstand a maximum tensile strain of 1200%.The M-HNTs/PAM hydrogels have an obvious hysteresis loop under different tensile strains and exhibit excellent elasticity and fast recovery properties.（8）The flexible Zn/Zn symmetric cell based on M-HNTs/PAM hydrogel electrolyte can operate for 1200 h at 4.4 m A cm^-2 and maintain a polarization voltage below 100 m V.Meanwhile,the flexible Zn/Mn O₂ full cells based on M-HNTs/PAM hydrogel electrolyte exhibits excellent cycling stability and the capacity retention is around 92.7%at 10 C after 1000 cycles.The corresponding Coulombic efficiencies remained in the range of98.5-100%.（9）The flexible ZIBs based on M-HNTs/PAM hydrogel electrolytes exhibit excellent durability and flexibility to suffer from harsh mechanical environments such as bending,folding,hammering,and poking.The M-HNTs/PAM hydrogel electrolyte reveals potential applications in the field of stretchable batteries owing to the outstanding flexibility and promotes the development of flexible ZIBs in the future.