Surface And Interface Engineering Of Transition-metal Compounds For Electrochemical Energy Storage

To alleviate the growing energy and environmental crisis,electrochemical energy storage devices,like supercapacitor and lithium-ion batteries,play more and more important roles in our daily life,such as consumer electronics,hybrid electric vehicle,grid-scale energy storage and so on.Electrode materials are the important parts of electrochemical energy storage devices,whose capacitance,cycling stability and rate performance are critical factors for device performances.To improve these electrochemical performances,plenty of researchers focus on the methods of materials modification.The surface and interface properties of materials will affect the electrode reaction process through electrochemical reaction activity,electrons and ions transportation,resulting in the influence on electrochemical properties.To solve the above problems,we will work on surface and interface modification to improve energy storage ability of electrode materials.Firstly,surface electrochemical properties are relative to crystal facet structures.Different crystal facets with different surface atomic configurations and physical/chemical properties,will have distinct electrochemical performances during their surface/near-surface redox reactions,and it’s important to realize controllable synthesis of high active surfaces for electrode materials.Herein,using first-principle calculations,the electrochemical performances of different surfaces for β-MnO₂ were investigated.Higher surface adsorption pseudocapacitance and lower ion diffusion barrier from surface to near-surface,make {001} surface of β-MnO₂ superior to other surfaces when acting as electrode material.On the other hand,F-termination will decrease the surface energy of {001} surface while suppressing the growth of {110} surface,which was demonstrated as the surface with much lower electrochemical performances.As a result,β-MnO₂ with large percentage of {001} surface was predicted to be obtained through surface F terminating.Secondly,oxygen vacancies are effective to improve the conductivity and surface electrochemical reaction activity of electrode materials.To conquer the weakness of low conductivity and activity for transition metal oxides,we proposed a strategy of lower valence-state doping for electrode materials to facilitate the formation of oxygen vacancies.Using first-principle calculations,we found out that the formation energies of oxygen vacancies both in the bulk and on the surface of R-MnO₂ were decreased apparently after Zn doping.Notably,the surface oxygen vacancies could form spontaneously without any other impetus.In addition,the bulk Zn dopants will provide the enhanced electrons diffusion to the surface,and the positive surface oxygen vacancies will draw the electrons to the reaction sites.In the reaction sites,the oxygen vacancies and reduced Mn ions will improve the activity of the electrode reactions.Thirdly,it’s important of ions transportation within interlayer spacing on energy storage for two-dimensional materials.Restacking of two-dimensional materials reduces the accessibility of electrolyte ions,leading to the hindrance of their potential on energy storage;hence,organic molecules can be used to modify the surface structure and keep the interlayer open.Here,we studied the interaction between MXene,Ti₃C₂T_x,and amino acids via combined theoretical and experimental investigations.From first principle calculations,we presented the functionalization of glycine on the MXene surface,evidenced by the shared electrons between Ti and N atoms.To experimentally validate our predictions,we synthesized flexible freestanding films of Ti₃C₂T_x/glycine hybrids.X-ray diffraction and X-ray photoelectron spectroscopy confirmed the increased interlayer spacing and possible Ti-N bonding,respectively,which agree with our theoretical predictions.The Ti₃C₂T_x/glycine hybrid films exhibited an improved rate and cycling performances for supercapacitor compared to pristine Ti₃C₂T_x,possibly due to better protons diffusion within expanded Ti₃C₂T_x.Fourthly,to prevent the restaking of two-dimensional sheets,except for surface modification by small organic molecule,we also could rebuild their interface structure by polymer,which could benefit for ions diffusion.We achieved in-situ polymerization of 3,4-ethylenedioxythiophene（EDOT）on the surface of Ti₃C₂T_x MXene flakes without using any oxidant,by a simple mixture of their aqueous solutions.Polymerization of EDOT was confirmed by Raman and FTIR spectroscopy.XRD and TEM results showed the uniform dispersion of poly-EDOT（PEDOT）chains between the Ti₃C₂T_x layers in the d-Ti₃C₂T_x/PEDOT hybrid.First-principle calculations indicated transference of 0.34 electrons from each EDOT monomer to MXene flake upon adsorption.This electron transfer initiated the in-situ polymerization of EDOT on MXenes,evidenced by the fact that its energy cost decreased with the charge transfer.The effective combination of Ti₃C₂T_x and PEDOT renders enhanced lithium-ion-storage capacity compared to the pristine Ti₃C₂T_x and PEDOT,as well as the increased cycle stability,due to the increased interlayer spacing for better ions transport.Fivethly,the interface constructed by two different 2D materials could combine superiority from each component,leading to better electrons and ions diffusion transport.We prepared Mo₂ TiC₂T_x/MoS₂ binary heterostructure by in situ sulfidation.The 2D Mo₂ TiC₂T_x/MoS₂ heterostructure features intimate interfacial interactions,which maximize the potential of conductive MXene as support for MoS₂.First-principle calculations indicated that MoS₂ is semiconductor,while Mo₂ TiC₂T_x/MoS₂ heterostructure is conductive.Therefore,the Mo₂ TiC₂T_x/MoS₂ heterostructures exhibited a stable cycling performance and promising rate capability.Meanwhile,the mechanism was analyzed from theoretical calculations,namely the enhanced Li and LiS₂ adsorption during intercalation and conversion reactions.