Studies On Advanced 2D MXene Based Composites As Anode Materials For Lithium-Ion Batteries

In recent years,with the ever-increasing environmental problems and the depletion of fossil energy,the development of efficient and clean energy storage and conversion devices is urgently needed.Lithium-ion batteries(LIBs)are the leading energy storage devices that have attracted a lot of attention due to their practical application in electric vehicles and portable devices.On the other hand,LIBs have several drawbacks i.e.,short cycle life,low power,and energy densities.Several anode materials,in particular,have a high capacity but are unable to perform fast charge/discharge cycles and have a limited cycle life,therefore the development of novel anode materials with high performance is crucial.Graphene-like layered transition metal carbide titanium carbide(Ti3C2Tx)and vanadium carbide(V2CTx)MXenes,as a new type of two-dimensional(2D)nanomaterial,have shown application potential in LIB anode materials.However,due to the easy restacking nature of Ti3C2Tx nanosheets,the specific capacity of pure Ti3C2Tx as anode materials in LIBs is not high.In response to the above problems,and further improving the electrochemical performance of Ti3C2Tx and V2CTx,this thesis uses a synergistic effect to adopt different synthesis methods to combine multilayered V2CTx and ultra-thin Ti3C2Tx nanosheets with Si nanoparticles and metal oxide and sulfide,respectively.The specific research content includes the following three parts:(1)Si@V2CTx composite anode materialSilicon(Si)is abundant in nature and has a high theoretical specific capacity(4200 mAh g-1).However,in the process of lithium-ion(Li+)intercalation/deintercalation,Si faces robust volume expansion,resulting in the destruction of the electrode structure and the rapid decline of capacity.The electrochemical performance of the Si@V2CTx electrode was improved by nanostructure strategy,using commercial Si nanoparticles as raw materials.Si@V2CTx nanocomposite was synthesized by a facile one-step sonication process.The powder was characterized by XRD,XPS,and TEM techniques.The results show that the Si nanoparticles were homogeneously anchored on the surface and interlayers of MXene.The Si nanoparticles have the highest first discharge capacity of 3309.2 mAh g-1 and over 150 cycles,the reversible capacity was 439.7 mAh g-1 at 200 mA g-1,which is about 4-5 times the specific capacity of pure V2CTx.The in-situ analysis of EIS and GITT of pristine Si and Si@V2CTx nanocomposites reveals that the interaction between pristine Si and Si@V2CTx overcomes the rate-determined Li+diffusion coefficient(DLi+)for fast and stable Li+extraction.(2)Bi2S3@Ti3C2Tx composite anode materialTi3C2Tx is used as a good conductive matrix to combine with Bi2S3 nanoflower to synthesize a composite Bi2S3@Ti3C2Tx anode material through a one-step hydrothermal method.In comparison to agglomerated bare Bi2S3 nanoflowers,Bi2S3@Ti3C2Tx nanocomposite results in high electrical conductivity and high cycling stability.Electrochemical performance shows that the composite anode material has high reversibility,and excellent electrochemical efficiency,with a cycling capacity of 390 mAh g-1 at a current density of 200 mA g-1 over 100 charge/discharge cycles.The promise of Bi2S3@Ti3C2Tx as high-performance anode materials for LIBs is demonstrated by its unique structural features,as well as a good rate capability and substantially improved cycling stability.(3)V2O5@V2CTx composite anode materialVanadium pentoxide(V2O5)is one of the most attractive anode materials.It has been widely studied because of its low cost,high abundance,easy synthesis,good safety,and relatively high theoretical capacity of 1472 mAh g-1,which is much higher than the graphite anode(372 mAh g-1).The controlled synthesis of V2O5@V2CTx nanohybrids through a one-step hydrothermal procedure at various temperatures(160℃,180℃,200℃),which are termed as V-160℃,V-180℃,V-200℃ described in this study.Our findings show that V-180℃ nanohybrids anode has better cycling stability,rate capability,and electrochemical reversible reactivity.V-180℃ nanohybrids anode achieves a reversible capacity of 528 mAh g-1 with a coulomb efficiency of 99.8%over 100 cycles at 500 mA g1,which is considerably better than the pure V2O5(132 mAh g-1)and V2CTx(189 mAh g-1)anodes.The improved reversible capacity and rate capability are credited to the synergic effect between V2O5 and multi-layered V2CTx sheets.The multilayers of high conductive V2CTx MXene are electrostatically well connected with V2O5 to ameliorate the rate performance for V2O5@V2CTx nanohybrids anode material.In this thesis,I briefly presented a recent insight on prospects of electrochemistry in energy storage.I have summarized my recent three review papers and presented a recent insight on future prospects of electrochemistry in energy storage i.e.,(a)Rise of 3D MXene beyond LIBs,(b)Ambiguity about terminating groups of various MXenes,and(c)Recent advances in LIBs.