Structural Regulation Of Layered Transition Metal Dichalcogenides And Their Electrochemical Behaviors

Electrochemical energy storage and conversion is one of the important pathways to solve the energy crisis and environmental pollution in today’s society,and the development of energy storage and electrocatalytic materials with high performance is the center of the advancement of energy storage and conversion technology.The transition metal dichalcogenides（TMDs）is of stable layered structure,rich element compositions and diverse electronic structures,which brings new promise for the development of novel energy storage materials and electrocatalysts.Based on the crystal structures and electronic properties of TMDs,the goal of this dissertation is to regulate their structures through specific methods such as composite hybridization,interlayer spacing expansion,surface defects engineering and heteroatom doping,in order to achieve the optimization of their electrochemical behaviors and the improvement of their electrochemical performances.The details of this dissertation are summarized briefly as follows:1.The author developled a facile one-step solid-phase general synthesis strategy for synthesizing MoS₂/carbon compounds,and a binder-free MoS₂/carbon fibers paper（MoS₂/CFP）anode was successfully synthesized for lithium-ion batteries.The mixture of Mo and S sources was firstly loaded on cotton filter paper by solution soaking and subsequent freeze drying,and then the MoS₂/CFP material was obtained by sintering the cotton filter paper with Mo and S sources.The MoS₂/CFP is composed of carbon fibers covered with MoS₂ layers interlaced into a three-dimensional network structure,in which the load percentage of MoS₂ is 68.8 wt.%.The MoS₂/CFP anode exhibits a reversible capacity of 261 mAh/g after 100 cycles at 100 mA/g.After subtracting the capacity of CFP,the MoS₂ in the composite anode delivers a reversible capacity of 539 mAh/g and shows better cycle stability than commercial MoS₂.The remarkable electrochemical performances could be attributed to the buffer space among carbon fibers for the volume changes of MoS₂as well as the highly conductive carbon fibers network.The one-step solid-phase synthesis strategy can realize the synchronous synthesis and combination of carbon fiber and MoS₂,avoiding the complicated multi-step synthesis processes of preparing carbon matrix and loading MoS₂ in the previous literatures.This strategy has the advantages of simple flowsheet and low cost,etc.2.A sandwich-like VS₂/C composite was successfully synthesized via a solvothermal method followed by a heat treatment in Ar-5%H₂ environment.Firstly,the octylamine molecules were successfully intercalated into the VS₂ layers in the solvothermal process.During the followed heat treatment in Ar-5%H₂ environment,the octylamine molecules were carbonized and decomposed into amorphous carbon,which was left between the VS₂ layers to form the sandwich-like VS₂/C composite.The sandwich-like VS₂/C anode exhibits superior cycling performance of 711 mAh/g after 100 cycles at 100 mA/g and excellent rate performance with 548 mAh/g even at 1 A/g,markedly better than those of VS₂ counterpart synthesized by hydrothermal.The excellent lithium-storage performance is attributed to the novel VS₂/C“sandwiched structure”synergistically combining the advantages of ideal atomic interface contact for improving electrical conductivity and the interlayer amorphous carbon layer for buffering the volume change.The“sandwich structure”of VS₂/C composite avoids the disadvantage of the limited interface contact between VS₂ and carbon in the VS₂/C composites reported in the previous literatures,and the sandwich-like VS₂/C composite is expected to be a prospective anode material for lithium-ion batteries.3.The sandwich-like VS₂/C composite was used as the active material of electrode for supercapacitor,and the electrochemical performances were tested.The electrochemical measurements showed that the sandwich-like VS₂/C composite exhibits better electrochemical performance than the pure VS₂ synthesized by hydrothermal.The specific capacity of the sandwich-like VS₂/C composite could reach 168.2 F/g at the current density of 0.5 A/g and 103.6 F/g even at 5 A/g,indicating its ideal rate capability.In addition,the VS₂/C composite electrode showed excellent cycling stability with 92.5%capacity retention over 1000 galvanostatic charge-discharge cycles at a current density of1 A/g.The excellent performance of the VS₂/C composite is attributed to the ideal interface contact between the VS₂ and carbon to improve the conductivity and the interlayer amorphous carbon layer to suppress the volume change during the charge-discharge process.This VS₂/C composite is expected to be a promising candidate material for supercapacitor electrode.4.For the first time,we find that the basal-plane activity of VS₂for hydrogen evolution reaction（HER）can be significantly strengthened by Mo doping based on the combined experimental and computational approach.Experimentally,the Mo-doped VS₂ with different doping concentrations were successfully synthesized by one-pot hydrothermal method,and the structural characterization of the Mo-doped VS₂ confirmed that Mo atoms were successfully doped into VS₂ matrix by replacing V atoms.Electrochemical tests showed that the HER performance of VS₂ microflowers after Mo doping is significantly improved in 0.5 MH₂SO₄ solution.The Mo-doped VS₂ with optimal Mo doping concentration（Nominal composition:Mo/（V+Mo）at.%=10）exhibits a small Tafel slope of 52.6 mV/dec and a low overpotential of 243 mV at-10 mA/cm²,and the turnover frequency at 300 mV and electrochemical surface area reveal 19.2-and 27.2-fold enhancements compared with the pure VS₂,together with significantly improved long-term cycle stability.The first-principles calculations suggest that Mo dopant reduces the hydrogen adsorption free energy on S sites,and consequently strengthens the basal-plane activity.We uncover a charge transfer mechanism from Mo to the outer surface of S atoms,leading to the enhancement of p-orbital density of states of S at Fermi level and thus far super HER performance.Our findings provide a promising route and theoretical framework to improve the electrocatalytic HER performance of VS₂,and may have general implications to other TMDs.