Transition Metal Oxides Nanostructures: Controllable Synthesis and Electrochemical Performance

Author:Zhang Ji Jun

Supervisor:chen ze xiang


Degree Year:2019





The development of society has put forward higher request to energy storage devices.It is an important trend to develop new energy storage devices with high safety,high specific capacity,high rate capability and good cyclic stability.The existing energy storage devices have lots of problems for now,such as slow charging speed,low power density and poor safety performance and so on.In order to solve these problems fundamentally,it is necessary to deal with the materials of energy storage electrodes.The research in this paper is based on the transition metal oxide energy storage materials to synthesize the nanostructure with controllable morphology and prepare new electrode materials for high electrochemical performance.In particular,graphene,NiAl-LDH materials,Mn doping combined with transition metal oxides with controllable morphology are used to form composite materials with specific structure.The structures of the composite materials are characterized and the electrochemical properties of the electrodes of energy storage device are analyzed.In the aspect of high-performance cathode materials,the main research objects in this paper are CoNiO2 materials with 3d nano-network structure,rGO/CoNiO2 composites and CoNiO2@NiAl-LDH composites.In the aspect of high-performance anode materials,the main research objects in this paper are the materials with hollow sphericalα-Fe2O3 and Mn dopedα-Fe2O3@rGO composite.The contents are divided into five parts shown as follows.(1)Study on the controllable synthesis and electrochemical properties of 3d nano-network structure CoNiO2 materials.The CoNiO2 material with 3d nano-network structure was prepared and the synthesized CoNiO2 electrode material was characterized and analyzed.The growth mechanism,morphological evolution mechanism and the formation mechanism of the three-dimensional nano-network CoNiO2 electrode materials were studied by different hydrothermal reaction time,combining with the microstructure and electrochemical properties.The 3d nanostructures and the electrochemical performance of CoNiO2 were optimized by adjusting the concentration of Poly(sodium-p-styrenesulfonate)and the PH of the reactants.The electrochemical tests were performed on the CoNiO2 electrode materials prepared under optimal preparation conditions.By galvanostatic charge-discharge tests with different current densities,the specific capacity can reach 1462 F/g when the current density is 1 A/g.Even at the high-current density of32 A/g,the specific capacity can still maintain 68.4%.The cycle stability of 800 cycles shows its performance is 87.55%of the initial stage which was carried out by galvanostatic charge-discharge tests with the current density of 4 A/g.(2)On the basis of preparing CoNiO2 materials,graphene was introduced to prepare rGO/CoNiO2 materials and their electrochemical properties were also studied.The flake-like graphene oxide in the solution had rich functional groups which was conducive to the growth of composite materials.The CoNiO2 material with three-dimensional network structure was designed to grow on the graphene oxide sheet for higher rate capability.Compared with CoNiO2 material,the rGO/CoNiO2 material has better hierarchical structure and more electron transmission channels which prevents the agglomeration and collapse of the structure during the redox reaction and increases electrical conductivity for the higher rates of cyclic stability.At the high current density of 32 A/g,the specific capacitance is 72.7%compared with that at 1 A/g.Under the relatively high current density of 8 A/g,with 2000 cycles of cyclic test,94.2%of performance was retained,indicating that the prepared rGO/CoNiO2 electrode material has high rate capability and excellent cyclic stability even at high current density.(3)On the basis of the prepared CoNiO2,the NiAl-LDH was introduced to prepare the CoNiO2@NiAl-LDH and its electrochemical properties were studied.Taking advantage of its good rate capability,high cycle stability and large specific capacity,the CoNiO2 is used as the skeleton structure and the NiAl-LDH with high specific capacitance grows on it.This special structure can avoid the agglomeration of NiAl-LDH nanosheets and increase the contact area between the active materials and the electrolyte.Meanwhile,the interlaced CoNiO2 skeleton also increases the electron transport channel between the NiAl-LDH nanosheets.After electrochemical tests,it can be concluded that the CoNiO2@NiAl-LDH can have high specific capacitance,good rate capability and excellent cyclic stability.(4)The hollow sphericalα-Fe2O3 was controllable synthesized and its electrochemical properties were studied.The hollow sphereα-Fe2O3 scheme was determined by the cyclic life tests of Fe2O3 materials synthesized by different schemes,and then the formation mechanism of hollow sphereα-Fe2O3 was proposed by combining hydrothermal reaction.Through the characterization and analysis,it is found that this unique structure enables the electrolyte to enter the inner part of the hollow sphere relatively easily,and the active materials can get sufficient contact with the electrolyte,shorten the distance of ion transmission,and increased the effective active area in the redox reaction.At the high current density of 16 A/g,the specific capacitance was retained 70.3%compared with that at 1 A/g,indicating that it has a very good rate capability.At the current density of 1 A/g,the properties ofα-Fe2O3 retained 68.7%after1000 cycles of test,indicating that the material has good cycling stability.Two solid electrolyte supercapacitors in series were assembled by using hollow sphericalα-Fe2O3as the anode material and CoNiO2@NiAl-LDH as the cathode material,and PVA&KOH as the solid electrolyte and separator.Through the characterization of its electrochemical properties,it is found that the solid electrolyte improves the cycling stability compared with the liquid electrolyte.This may be mainly due to the fact that the solid electrolyte binds the volume change ofα-Fe2O3 and prevents the dissolution and diffusion of iron in the electrochemical reaction,thus giving it a better cycle life curve.(5)Fe-Mn-O@rGO was prepared based on hollow sphericalα-Fe2O3,which was doped with Mn and coated with graphene.The electrochemical properties of Fe-Mn-O@rGO was studied as anode materials of supercapacitor.The formation mechanism of Fe-Mn-O@rGO was proposed according to its chemical reaction in hydrothermal process.The Mn doping improves the stability and conductivity of Fe(OH)2 formed in the redox process.Meanwhile,the rich functional groups on the surface of graphene oxide were used to make the graphene easily wrap in Fe-Mn-O outside,which improves the stability and electrical conductivity,and also improves the electron transfer performance between the ellipsoids.In the aspect of electrochemical performance tests,it is found that from 1A/g to 16 A/g,the specific capacitance can still retain 64.4%of its original value.After1,000 cycles of life,its performance only attenuated to 83.4%of the original maximum value.A sandwich pseudocapacitance supercapacitor with Fe-Mn-O@rGO as the anode electrode and NiAl-LDH as the cathode electrode was assembled.In 1000 cycle life test,it can still retain 81.2%performance.At the scan rate of 10 mV/s,the power density is3.8 kW/kg,and the corresponding energy density can reach 85.3 Wh/ 100 mV/s,the power density reaches 12.9 kW/kg,while the energy density can remain at 40 Wh/kg.