Energy Storage Properties Of Niobium-based Transition Metal Carbides(MXene/MAX)

With the rapid development of the world economy,great changes have taken place in people's life style,and the pollution to the environment is becoming more and more serious.The application and development of clean energy can not only meet the energy needs of human beings in production and life,but also reduce the impact on the environment,which is the main strategic policy for the future development of the world.Lithium-ion batteries and supercapacitors are not only environmentally friendly,but also have high energy/power density,long life and high operability,so they are considered as the most promising energy storage and conversion devices.Electrode materials is the main factor in the development of electrochemical energy storage device,therefore,it is an important way for China's energy development to seek the electrode materials which are environment-friendly,low-cost,safe and non-toxic,with long life and excellent electrochemical performance.The appearance of two-dimensional laminar transition metal carbide MXene has brought new vigor and vitality for the application and development of electrochemical energy storage,because they have larger interlayer spacing,high electrical conductivity,exercellent specific surface area,no need for adhesive and conductive additives etc.No matters from the cost of production or its superior electrochemical performance,MXene are ideal electrode materials.In this thesis,Nb-based transition metal carbide(Nb₄C₃T_x/Nb₂SnC)as the main research object,according to the improvement of the traditional experimental process,structural regulation,we improve and explore the electrochemical properties and reaction mechanism of materials in a variety of ion capacitors and lithium ion batteries,the research results obtained in this paper are as follows:First,the precursor material Nb₄AlC₃was prepared by solid-phase sintering method.After chemical etching in HF solution,Mxene material Nb₄C₃T_xwas obtained.Then,organic solvent TMAOH was used to delaminate Nb₄C₃T_xto obtain a"paper"flexible film.The layer spacing of the flexible membrane is about 17.7(?),and like most MXene materials,oxygen-containing functional groups on its surface.Nb₄C₃T_xis very stable,the degassed Nb₄C₃T_xsolution can be stored for more than 30 days when refrigerated,and the layer spacing of the flexible film can be maintained at 16.6(?)at 500? and vocume.Nb₄C₃T_xshowed good electrochemical performance in supercapacitors.In 1 M H₂SO₄electrolyte,the volumetric capacitance was 1075F/cm³at the scan rate of 5 m V/s.Due to the extremely small amount of TMA⁺in the middle of the Mxene layers,the interlayer spacing of Nb₄C₃T_xis large enough(21(?))to accommodate H⁺/Mg²⁺insertion without causing lattice changes.In addition,in order to further explore and improve the capacitance performance of Nb₄C₃T_xfilm in the capacitor,we adjusted the reaction time of etching solvent HF with MAX phase Nb₄AlC₃to introduce micropore to the surface of the sheet.The capacitive performance of the Mxene films with holes in lithium ion and sodium ion capacitors is obviously improved.In 1 M Li₂SO₄electrolyte,the capacitance retention increased from 18.4%to 44.1%.In 1 M Na₂SO₄electrolyte,the capacitance retention increased from 13.3%to 56.8%,which is about 4.3 times that of Nb₄C₃T_xwithout holes.This is because the introduction of holes can not only promote the diffusion of ions in the electrolyte between the electrolyte and Mxene interface,but also effectively shorten the transport path,which is crucial to improve the electrochemical performance,especially the rate capability.However,excessive etching can make the material brittle and affect the structural stability.Subsequently,based on the previous work,we used TBAOH as the delamination solvent to improve the monolayer yield of Mxene and the conductivity of Nb₄C₃T_xfilm.We investigated the effects of TBAOH and TMAOH on the Zeta potential,particle size and electrochemical performance of Nb₄C₃T_xsolution.Compared with TMAOH-Nb₄C₃T_x,the conductivity and electrochemical performance of TBAOH-Nb₄C₃T_xfilm was significantly improved.At the scan rate of 5 m V/s,in the electrolytes of 1 M H₂SO₄,1 M KOH and 1 M Mg SO₄,the mass capacitance of TBAOH-Nb₄C₃T_xfilm was 292 F/g 309 F/g and 143 F/g,respectiviely.At the current density of 2 A/g,the capacitance retention increased from 58.56%of TMAOH-Nb₄C₃T_xto 93%of TBAOH-Nb₄C₃T_xfilm after 5000 charge-discharge cycles in the electrolyte of 1 M Mg SO₄.Both cycle life and rate capability are significantly improved.According to the dynamics analysis of the material in the different ion capacitors,the improved performance may come from the faster ion diffusion between the electrolyte and electrode materials interface,and the increase of the conductivity of the flexible Nb₄C₃T_xmembrane itself delaminated by TBAOH.Finally,given the Nb₄C₃T_xfilm with excellent performance in the field of energy storage,we use solid phase sintering method to prepare the MAX phase Nb₂SnC with"Sn"elemen as"A"layer,and investigate the electrochemical performance and reaction mechanism in a lithium battery as anode material for the first time.At the current density of 50 m A/g and 500 m A/g,the capacity is 234 m Ah/g and 151 m Ah/g,respectively,and the discharge capacity increased with the increasing of cycle number.This is because Li_xSn,the alloy product generated by the reaction of Li⁺with Sn,has obvious volume expansion,which can gradually exfoliate Nb₂SnC into several layers or single Nb₂C sheets,making Sn fall off from the structure and decompose MAX phase particles into smaller and more electrochemically active particles,promoting the pseudocapacitance reaction and gradually increasing the discharge capacity.The MAX phase Nb₂SnC,which is used as the anode of the battery,combines the advantages of the layered material and alloying element,showing a longer cycle life than most nanomaterials containing Sn nanoparticles.In this thesis,we improve the capacitance performance of Nb₄C₃T_xfilms in a variety of ionic capacitors by adjusting the experimental details in the etching and delamination process to regulate the conductivity and structure.The capacity and reaction mechanism of MAX phase material Nb₂SnC in lithium battery were investigated.It provides experimental basis and reference for the application and development of other kinds of MXene and anode materials.