Controllable Synthesis Of Transition Metal Carbides And Application Of Lithium-Ion Capacitors

Transition metal carbides exhibit unique physicochemical properties by combining the properties of noble metals,ionic crystals and atomic crystals,which are widely used in the field of energy storage and conversion.However,the synthesis of transition metal carbides usually involves high-temperature carbon reduction processes or use of reducing agents（e.g.,H₂,Mg,etc.）.There are problems such as fast reaction process,difficult control,harsh synthetic conditions,and large particles of synthetic materials.To this end,we used the carbonization method of organic-inorganic hybrid materials in this thesis,with V,Mo,W and Nb-based carbides as research objects,and studying its carbonization mechanism and synthesis differences by regulating the carbonization temperature,carbonization time and carbon source addition.The electrochemical properties of V,Mo,W and Nb-based carbides were further studied,and their energy storage mechanism was clarified by Ex-situ XRD characterization.In addition,lithium ion capacitors were constructed with biomass-derived carbon or activated carbon.This thesis conducted research on the above issues.The main research content is as follows:1.Controllable synthesis of carbide.The V₈C₇-carbon microsphere composite（V₈C₇@CMs）was prepared by the combination of hydrothermal treatment and high-temperature in-situ carbonization,in which the carbon microspheres acted as a supporting framework to effectively buffer the volume change of V₈C₇ during the electrochemical reaction and to prevent the large amount of V₈C₇ agglomeration.However,there was no in-depth study on the optimal content of carbon microspheres,so the role a nd optimal content of free carbon were deeply analyzed during the synthesis and performance study of Mo₂C.The Na Cl cubes covered with C₆H₁₂O₆-Na₂Mo O₄composite film on the surface were prepared by sol-gel method,which were stacked with each other and self-assembled into a three-dimensional framework.The precursor was carbonized at high temperature in Ar atmosphere to obtain Mo₂C/carbon composite（MC-X）,and the regulation of free carbon content in MC-X was achieved by regulating the amount of salt template.There are many component phases of Mo-base carbides.The experimental scheme was further improved by grinding Mo O ₃with dicyandiamide（C₂H₄N₄）and then carbonizing it at high temperature in Ar atmosphere.The experimental parameters were adjusted to re alize the controllable synthesis of Mo₂C/carbon composites（Mo₂C/C）and Mo C/carbon composites（Mo C/C）component phases.It was found that the amount of carbon source added was the key factor to realize the controllable component phases.The synthesis method was extended to the synthesis of W and Nb-based carbides,and studied the difference between the corresponding carbide synthesis when the type of transition metal is different.In the process of synthesis W-based carbides,the amount of carbon source was the key factor to realize the selective synthesis of WN and WC/C composites.In addition,metal W was easily generated during carbonization,and it was difficult to realize the phase control of W ₂C and WC_1-xcomponents.In the experimental regulation of Nb-based carbides,the carbonization temperature and carbonization time were the key factors to achieve the selective synthesis of Nb N/carbon composites（Nb N/C）and Nb C/carbon composites（Nb C/C）.It was found that the synthesis temperature of nitrides was lower than those of carbides during the regulation of Mo,W,and Nb-based carbide synthesis.2.Performance evaluation of lithium storage.At 0.1 A g^-1,the specific capacity all electrode materials showed a trend of increasing with the increase of cycle times.Among them,V₈C₇@CMs had the highest specific capacity(774 m Ah g^-1),followed by Mo C/C(380 m Ah g^-1),and the specific capacities of MC-2,Mo₂C/C,WN and Nb C/C were similar(about 300 m Ah g^-1),and the specific capacity of WC/C was the lowest(182 m Ah g^-1).V₈C₇@CMs and MC-2 had excellent cycle stability under different current densities.The kinetic process analysis showed that Mo₂C/C,Mo C/C,WN,WC/C,Nb N/C and Nb C/C electrode materials had high pseudocapacitance contributions at different scanning rates(75.85%,74.69%,73.74%,80.74%,83.43%and 84.23%at 2 m V s^-1,respectively).3.Research on energy storage mechanism.The microstructure of V₈C₇@CMs and MC-2 after cycling was characterized by Ex-situ SEM.The results showed that the electrode material structures were powdered after cycling.The energy storage mechanism of Mo₂C/C,Mo C/C,WN,WC/C,Nb N/C and Nb C/C materials during charging and discharging was investigated by Ex-situ XRD characterization.The results showed that only a shift of the diffraction peaks occurred in the materials except for WN,which had a partial phase transition during Li⁺intercalation and deintercalation,and there was no phase transition in the rest of the materials.4.Preparation of biomass-derived carbon materials,construction of Lithium-ion capacitors and research of their electrochemical performance.Nitrogen-doped grapefruit peel biochar（NGPB）were synthesized by introducing CH₄N₂O to induce nitrogen doping followed by KOH activation using grapefruit peel as the raw material,and regulating the mass ratio of the raw material to KOH,with the specific surface area as high as 3883.2m² g^-1 at a mass ratio of 1:2.The high specific surface area and C-N network structure enabled NGPB-2 to have high specific capacity and excellent rate performance(0.1 A g^-1-125 m Ah g^-1,5 A g^-1-115 m Ah g^-1).In addition,Badam shells,which have naturally abundant pore structures,were used as raw materials,and badam shell biochar（BSBs）were prepared by KOH activation with the maximum specific surface area(3730.0 m² g^-1)at a mass ratio of 1:3.The higher specific surface area enabled BSB-3 to have a specific capacity of up to 250 m Ah g^-1 and excellent cycling stability(2 A g^-1-10,000 cycles-91 m Ah g^-1).Finally,V₈C₇@CMs//NGPB-2,MC-2//BSB-3,M（M=Mo₂C/C,Mo C/C,WN,WC/C,Nb N/C,Nb C/C）//AC devices were constructed using transition metal carbides and nitrides as anode and biomass-derived carbon/commercial activated carbon as anode.The Mo C/C//AC device exhibited the highest energy density(138 Wh kg^-1)at a low power density of 195 W kg^-1.The V₈C₇@CMs//NGPB-2 device exhibited the highest energy density(63.69 Wh kg^-1)at a high power density of 9.36k W kg^-1.The capacity retention rate of MC-2//BSB-3 device was 85%after9000 cycles at 1 A g^-1.The above results showed that it was not the case that the higher the specific capacity of a single electrode,the higher the energy density of the device at low power density.In this thesis,the organic-inorganic hybrid material carbonization method was used to achieve the controllable synthesis of V,Mo,W,and Nb-based carbides by regulating the addition of carbon source,carbonization temperature,and carbonization time,and the Lithium storage mechanism was clarified by the Ex-situ XRD characterization.In addition,high-performance biomass-derived carbon materials were prepared by KOH activation.The electrochemical performance of the constructed lithium-ion capacitor was determined by the positive and negative electrodes using transition metal carbides as the negative electrode and biomass-derived carbon or activated carbon as the positive electrode.