| Rechargeable batteries with high energy density and long cycle characteristics are playing an increasingly important role in current society.Compared with other batteries systems,sulfur as the cathode has the advantages of environmentally friendly,abundant raw materials and low price,lithium-sulfur batteries become one of the most promising rechargeable battery systems.However,many problems such as very low sulfur conductivity,slow polysulfides conversion kinetics and difficult deposition of lithium sulfide(Li2S)restrict the further development of lithium-sulfur batteries.It is found that the coulomb efficiency and cycle life of batteries can be improved by modification of sulfur conductive skeleton material,separator modification and lithium metal electrode protection strategy.In recent reports,the strategy of modifying sulfur electrodes with highly conductive catalyzed functional materials is considered as the effective methods for inhibiting polysulfide shuttle effect and increasing battery cycle life.Therefore,based on the design of an efficient conductive catalyst,the following studies were carried out in this paper to regulate the kinetics of sulfur electrode redox reaction,study the mechanism of sulfur conversion reaction process,and improve the electrochemical performance of lithium-sulfur batteries under high sulfur loading and lean electrolyte.1.Ultrafine zirconium boride(ZrB2)nanoparticles have been synthesized at relatively low temperatures,benefiting from its high conductivity,strong chemical adsorption with polysulfides,and bi-functional catalytic conversion polysulfides features,then composited with nitrogen doped graphene(NG)as the sulfur host improved batteries performance.Compared with zirconium-based oxides,the zirconium boride has more obvious advantages,which not only can be operated at a high current density of 5 C within 4000 overlength cycles,but also can be tested under a high rate of 20 C and with sulfur loading of 7.77 mg cm-2 at a high current density of 1 C.And then pouch cells have been assembled to further demonstrate its excellent electrochemical performance in potential applications.2.Niobium boride(NbB2)nanoparticles with higher compaction density alleviates have been prepared and applied to alleviate the "shuttle effect" of polysulfide and promote the transformation kinetics of polysulfide.Compared with other niobium-based compounds,NbB2 nanoparticles have higher electrical conductivity and more abundant catalytic sites.The Li2S deposition mechanism of NbB2 was revealed by timing amperometry(four classical electrochemical deposition mechanisms)and scanning electron microscopy(SEM)characterization,demonstrating that the high deposition capacity of NbB2 is due to its three-dimensional "flower" Li2S deposition.The Arrhenius equation proved that NbB2 can reduce the activation energy of polysulfide conversion.In-situ XRD and first-principle calculations are used to elucidated the mechanism of the enhanced reaction kinetics due to the high catalytic performance of NbB2.In addition,the batteries achieve a high areal capacity of 17 mAh cm-2 with a sulfur loading of 16.5 mg cm-2.3.The ordered self-assembly of zero-dimensional nanoparticles or two-dimensional nanosheets into three-dimensional spherical superstructure is one of the challenges of materials synthesis.A spherical superstructure hafnium diboride derived from metal-organic framework(MOF)was synthesized by one-step borification.Benefiting from its unique superstructure,the obtained waxberry-like HfB2 exhibits excellent active sites and catalytic activity for the conversion of polysulfides.The 5d metal Hf modulates the p orbitals of nonmetallic B atoms and moves them close to the Fermi energy level,which causes the empty hybrid orbital of B generated by sp3 hybridization to build Lewis acid-base interaction with sulfur,and the mechanism for accelerating polysulfide conversion is verified in combination with in-situ XRD,ex-situ EIS,and theoretical calculation.The electrode delivers a high initial discharge capacity of 1433 mAh g-1 at 0.2 C and 580 mAh g-1 at 5 C.Achieving high sulfur loading and lean electrolyte,the electrode exhibits a high areal capacity of 15.5 mAh cm-2 with sulfur loading of 12.8 mg cm-2 and electrolyte of 4 μL mg-1.4.Zn-doped cobalt-ditelluride polyhedra(Co0.9Zn0.1Te2@NC)catalyst has been successfully prepared,which reduced the amount of electrocatalyst used and added only 4 wt%added in the cathode to reduce the activation energy and accelerate the conversion rate during sulfur conversion.It is proved by XRD refinement technique that the addition of zinc atoms distorted the lattice,resulting in a change in the coordination environment of cobalt atoms,which enables its antibonding orbitals occupy more electrons,thus balances the constitutive catalytic effect and the adsorption of polysulfides,accelerates the transformation kinetics of polysulfides,and promotes Li2S nucleation.The electrode exhibits the excellent cycling stability for more than 1500/2500 cycles at high current density of 1 C/2 C,the initial discharge capacity at 1 C is 1030 mAh g-1.Achieving high sulfur loading and lean electrolyte,the electrode exhibits a high areal capacity of 12.8 mAh cm-2 with sulfur loading of 7.7 mg cm-2 and electrolyte dosage of 4 μL mg-1. |
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