Investigation Of Sulfide Transition Metal Electrodes And Multi-electron Reaction Mechanism

The rapid development of energy storage market has raised higher requirements for the energy density of lithium-ion batteries.Transition metal sulfides(TMSs)electrodes have received great attention,due to the high theoretical capacity,nature abundance,low price,and potential applications.At the same time,due to the complexity of the energy storage mechanisms in the multi-electron reaction process,the TMSs electrode materials have also attracted extensive attention from researchers.There are still several problems hindering its practical implementation.First,their poor ionic and electronic conductivities result in poor rate performance.Second,huge volume changes during cycling process,which causes active particle pulverization,leading to poor cycling performance.Third,intercalation-type TMSs electrode materials suffer from the limit of crystal structure,which leads to structural damage upon multi-electron reaction process and poor electrochemical properties.In contrast to intercalation electrodes,conversion materials typically suffer from agglomeration,dissolution of the active materials or their components,and shuttle of soluble species,all of which may lead to poor cycling performance.In this paper,three representative TMSs materials such as FeS2,TiS2,and CoS2 are taken as the research objects,considering the combination of structural design,synthesis and theoretical analysis:(1)Microstructure design.The carbon matrix anchors TMSs via chemical binding,which acts as a strong buffer and a polysulfide trapper to maintain their electrochemical activity during ultra-long cycling.(2)Sulfide vacancies.It is found that the cluster vacancy can serve as an effective control factor for the improvement of the crystal structure stability,and enhances the electrochemical stability during multi-electron reaction process.(3)Metal-Organic Frameworks(MOFs)precursor.The porous carbonaceous frameworks obtain abundant ion/electron-diffusion paths and promote rate performance.The structure characteristic,electrochemical properties and the reaction mechanism of the prepared various multi-electron reaction TMSs materials were systematically investigated.Micron FeS2 particles with bifunctional carbon nanotubes(FeS2@B-CNTs)including CNTs' internal conductive network and external protecting cage have been prepared by a one step solvothermal method.The internal CNTs anchor FeS2 via the chemical binding,which analysed by TEM and XPS test.The growth of FeS2 on CNTs via chemical binding,thus can not only inhibit the FeS2 from peeling off the CNTs substrate during cycling,but also provide facile conductive pathways.While,the continuous CNTs network from the surface to the interior of the micron particles shortens ion and electron transport paths,enhancing lithium storage reaction kinetics.Additionally,the reticular CNTs cages generate on the surface of the composite FeS2 micron particles,buffering the volumetric change during cycling and further restraining the shuttle of polysulfides.Therefore,the FeS2@B-CNTs are expected to show a long cycle life and a good rate performance.After 500 cycles,the electrode shows a reversible capacity of 698 mAh/g at 1000mA/g,and FeS2@B-CNTs still has a high reversible capacity of 575mAh/g at 5000mA/g.Based on phase equilibrium analysis,we introduce a facile solid-state method to prepare TiS2 microparticles with abundant sulfur vacancies(CV-TiS2-x).In addition,the multiple comparison for XRD,Raman,EPR,XPS,TEM results of this material suggests that more sulfur vacancies exist in CV-TiS2-x.According to DFT calculations,it is found that the cluster-vacancy can serve as an effective control factor for the improvement of Fermi level,enhances the electrochemical stability of CV-TiS2-x,and retains the structural integrity during the more lithium ions insertion process.In situ XRD measurements are performed to monitor the structure evolution of CV-TiS2-x,accompanying initial and 50th cycling,respectively.The CV-TiS2-x samples exhibit the steady extended interplanar spacing in accompany with cycling and promote the rapid diffusion of lithium ions.Sulfur vacancy clusters not only increase the stability of the structure,but also stimulate anion redox reaction and pseudocapacitive contribution,and improve the multi-electron reaction performance and cycling performance.The CV-TiS2-x exhibits a high reversible capacity(650 mAh/g)after 300 cycles at 220mA/g(IC),and at the rate current of 25 C,the electrode can still deliver a high reversible capacity of 200 mAh/g.CoS2 electrode undergoes complex phase reactions during the multi-electron storage process,suffering more difficult conductivity during cycling and requiring a more effective conductive microstructure design.Considering the structural advantages of the large specific surface area and abundant pores of MOFs,CoS2 with carbon-based porous framework(CoS2@CNT@C)is prepared by a two-step solid phase method using ZIF-67 composited with CNTs as a precursor.The obtained CoS2 nanoparticles are uniformly embedded in porous carbonaceous micro-polyhedrons,interwoven with CNTs to ensure high electronic conductivity and inhibit volume expansion and side reactions.CoS2@CNTs@C performs a large specific surface area of 459.3 m2/g and 8nm pore size,and inherits the functional groups of MOFs and CNTs.Combined with analysis results such as TEM,EIS and DRT,it is found that the diffusion performance of lithium ions in the multi-electron reaction process is significantly improved.The surface Faraday process is enhanced,and the contribution of pseudocapacitance is increased.At the voltage range of 1-3V and the current density of 500mA/g,the initial discharge specific capacity is 763.7mAh/g.After 160 cycles,the reversible capacity is 556.2mAh/g.At the high current density of 2000mA/g,the CoS2@CNTs@C electrode still has a reversible capacity of 454.8mAh/g.