Defects Modulation Of Metal Chalcogenides And Their Impacts On Potassium-ion Storage

Potassium（K）resources are abundant and cost-effective.Moreover,the standard electrode potential of K is close to that of Li.These merits make potassium ion batteries（PIBs）a viable option for high-energy-density,low-cost energy storage components in large-scale energy storage systems,which are capable of storing intermittent green energy and providing a stable energy output to the outside world.Metal chalcogenides are composed of various materials with a wide range of structures and sources.They have a high theoretical capacity based on multi-electron conversion reactions and are well-suited to serve as the negative electrode of PIBs.However,the following factors limit the large-scale application of metal chalcogenides in PIBs:（1）the poor conductivity of metal chalcogenides results in slow electron transfer during charging and discharging processes;（2）the large K⁺radius leads to slow K⁺diffusion in metal chalcogenides,resulting in poor reaction kinetics;（3）large volume changes occurring upon the cycling process leads to electrode pulverization,cracking,and ultimately failure.This dissertation focuses on defect modulation in metal chalcogenides.By utilizing various defects,such as vacancies,anion doping,cation doping,and phase boundaries,the electronic structure and bond strength of metal chalcogenides can be modified to optimize their intrinsic electrochemical properties.By combining nanosizing and carbon coating with this approach,the development of high-capacity,high-rate,and long-cycling metal-chalcogenide anode materials can be enabled.The study investigates the effects of different defects on the conversion reaction and their evolution during the cycling process through theoretical simulations and experimental characterizations.Herein,the mechanism of defect-enhanced potassium-ion storage performance of metal chalcogenides was revealed in this dissertation.The main research contents are summarized as follows:（1）P-VSe_2-x was synthesized by bombarding VSe₂ nanosheets with Ar plasma to induce Se vacancies on their surface.The resulting material exhibited exceptional potassium-ion storage capabilities in a half-cell,achieving a capacity of 143 m A h g^-1 at 3.0 A g^-1 after 1000cycles,and a high energy density of 206.8 Wh kg^-1 in a full-cell.The mechanism behind the enhanced potassium-ion storage of P-VSe_2-x through Se-vacancy was investigated using various electrochemical tests.The results revealed that Se vacancies can reduce polarization voltage during potassium uptake/release and facilitate K⁺transport in the electrode.In-situ and ex-situ observations indicated that the vacancies disappear after the（de）potassiation reaction,suggesting that they only play a role during the first discharge process.First-principle calculations demonstrated that Se vacancies promote K⁺insertion by enhancing K⁺adsorption and providing additional diffusion routes,as well as promoting conversion reactions by weakening the V-Se bonding strength during the initial discharge process.（2）To obtain In₂S₃@C nanocomposites,the precursor of the indium-based metal-organic framework（MIL-68-In）was sulfurized,and Se atoms were subsequently doped into them to create In₂S_3-xSe_x@C.In half-cell tests,In₂S_3-xSe_x@C showed a high potassium-ion storage capacity of 709 m A h g^-1 at 0.1 A g^-1,along with excellent cycling performance(118 m A h g^-1at 10.0 A g^-1 after 1000 cycles),and maintained an energy density of 146 Wh kg^-1 after 180cycles in full-cell tests.The study investigated the mechanism behind the Se-doping-enhanced potassium-ion storage of In₂S_3-xSe_x@C.Electrochemical tests showed that Se doping continuously promoted electron transfer and ion diffusion during cycling,thus enhancing potassium-ion storage kinetics.Ex-situ observations revealed that Se is doped into K₂S after potassiation(K₂S_1-x/3Se_x/3)and re-doped into In₂S₃ after depotassiation(In₂S_3-xSe_x).First-principle calculations suggested that Se doping could improve the conductivity and K⁺diffusion by regulating the electronic structure of In₂S_3-xSe_x,thereby promoting K⁺insertion.Additionally,it could weaken the In-S bond strength and promote conversion reactions.（3）The Bi₂S₃@Ti₃C₂ nanocomposite material was synthesized through the in-situ growth of Bi₂S₃ on the surface of Ti₃C₂ nanosheets with high electrical conductivity.To enhance its potassium-ion storage capacity,Cu²⁺was doped into the Bi₂S₃ to form Cu-Bi₂S₃@Ti₃C₂.The resulting material demonstrated excellent performance in half-cell tests,with a high potassium-ion storage capacity of 600 m A h g^-1 at 0.1 A g^-1 and good cycling stability,retaining 91 m A h g^-1 at 5.0 A g^-1 after 1000 cycles.In full-cell tests,an energy density of 163 Wh kg^-1 was achieved after 300 cycles.We investigated the mechanism of Cu²⁺-doping-enhanced potassium-ion storage of Cu-Bi₂S₃@Ti₃C₂ and the influence of Ti₃C₂ on the potassium-ion storage of Bi₂S₃.Electrochemical tests revealed that the presence of Ti₃C₂ and Cu²⁺doping enhances electron and K⁺transport in Bi₂S₃,and maintains this improvement throughout the cycling process.Ex-situ observations indicated that Cu²⁺doping can recover to its initial doping state after cycling.First-principle calculations showed that Cu²⁺doping and Ti₃C₂ have an internal-external coupling effect that synergistically enhances ion diffusion to promote the insertion reaction and weakens the Bi-S bond to promote the conversion reaction.（4）Pyrolysis and selenization were used to prepare Co Se₂/Zn Se@C composite materials.The boundary between Co Se₂ and Zn Se was found to induce P doping,resulting in a high P-doping content of P-Co Se₂/Zn Se@C.The effects of this boundary on doping and the mechanisms behind the enhanced potassium-ion storage of P-Co Se₂/Zn Se@C were investigated.Material analysis and theoretical calculations showed that,compared to single-phase materials,the boundary in P-Co Se₂/Zn Se@C effectively reduced the formation energy of P-doping and promoted P-doping at the boundary.As a result,P-Co Se₂/Zn Se@C exhibited excellent rate performance(166 m A h g^-1 at 10.0 A g^-1)and cycling performance(180 m A h g^-1 at 1400 cycles under 5.0 A g^-1)in half-cell tests,as well as a high energy density of 146 Wh kg^-1 in full-cell tests.Various ex-situ observations revealed that the boundary disappeared after one cycle,and P-doping was distributed uniformly throughout Co Se₂ and Zn Se rather than concentrated at the boundary.