Plasma-Assisted Material Synthesis And Electrochemical Performance Of Lithium-Sulfur Batteries

High specific energy battery technology is necessary to realize grid intelligence and promote the transformation of transportation electrification.Based on the multi-electron conversion reaction between sulfur and lithium,Lithium-sulfur（Li-S）batteries achieve the high specific capacity and energy density.Moreover,the natural abundance and non-toxic property of sulfur endow Li-S batteries considered to be a promising next-generation battery system.However,the electronic insulation properties of sulfur and Li₂S₂/Li₂S,as well as the dissolution and shuttle effect of lithium polysulfide,seriously restrict the development of large-scale commercial applications of Li-S batteries.Always,sulfur is compounded with porous carbon materials or metal compounds for effectively improving the conductivity of the positive electrode and suppressing the shuttle effect.However,the sluggish reaction kinetics of sulfur species limit the rate capability and lifespan of Li-S batteries.Given this,plasma technology is utilized to design and prepare mediators with abundant catalytic active sites and excellent adsorption activity as sulfur hosts or interlayers to improve the adsorption and catalytic capabilities for lithium polysulfide to enhance the kinetics of multiphase and multielectron reactions for high energy density and long lifespan Li-S batteries.Sulfur-vacant MoS₂@biomass-derived hollow carbon fiber(MoS_2-x@HCF)composite is designed and prepared as the sulfur host using room temperature H₂plasma technology.By introducing sulfur vacancies into MoS₂ to create unsaturated coordination atoms,the effective activation of its basal plane catalytic sites is achieved,and the adsorption capacity and catalytic activity for lithium polysulfide are improved.The ultrathin MoS₂ nanosheets is supported by hollow biomass carbon fibers,and thus the agglomeration of nanosheets is suppressed to expose more active sites.Meanwhile,the hollow carbon fiber accelerates the electron/ion directional transmission and effectively reduces the interface impedance.Therefore,the obtained S/MoS_2-x@HCF cathode delivers an initial discharge specific capacity of 1169.1 m A h g^-1 at 0.1 C,and can be cycled stably for 1000 cycles at 1 C with a capacity decay rate of only 0.036%per cycle.Aiming at the unfavorable phase transition during the preparation of MoS_2-x and the instability of sulfur vacancies during the cell cycling,room temperature NH₃plasma is used to introduce heteroatom nitrogen into MoS₂ to fill the sulfur vacancies.The catalytic active site is stabilized by means of compensatory coordination of heteroatoms,and a sulfur host with high catalytic activity and high stability is obtained.In addition,MoS₂ is gradually transformed from the semiconductor phase 2H phase to the metal phase 1T phase,and its charge transport property is effectively improved after NH₃ plasma treatment.Theoretical analysis indicates that N-MoS₂ has a stronger adsorption for lithium polysulfide for suppressing shuttle effect;meanwhile,N doping reduces the nucleation/decomposition barrier of Li₂S for promoting the uniform deposition of Li₂S and realize bidirectional catalysis.As a result,the obtained electrode exhibits good cycling stability and rate capability with an initial capacity of 708 m A h g^-1 at 1C and a retention capacity of 491 m A h g^-1 after 1000 cycles.In order to enhance the electrochemical performance under high sulfur loading,the in-situ derived ultrafine Co_5.47N nanocrystals(Co_5.74N@Co-NC/PCNF)composite film is used as interlayers for promoted Li-S batteries.The ultra-fine Co_5.47N nanocrystals（5-10 nm）increase the effective surface area for interaction with lithium polysulfide,and the unique porous carbon nanofibers provide fast ion/electron transport channels for redox reactions,reducing the dead sulfur and increasing the actual energy density of Li-S batteries.At a sulfur loading of 1.5 mg cm^-2,the interlayer exhibits an initial discharge specific capacity of 1222.5 m A h g^-1 at 0.2 C and a capacity retention rate of 72.5%for 1000 cycles at 1 C with the Coulomb efficiency over 98%.To further accelerate the reaction kinetics,a carbon cloth-supported N-Ti O₂@Co Mott-Schottky heterojunction is designed and fabricated as a multifunctional interlayer.Benefiting by the strong adsorption of N-Ti O₂ for lithium polysulfide and the spontaneous rearrangement of electrons,lithium polysulfide is directionally transferred and converted under the action of the built-in electric field,accelerating the kinetics of the conversion reaction.Thereby the adsorption-catalysis-conversion integration of lithium polysulfide is developed.At a sulfur loading of 1.5 mg cm^-2,the battery exhibits an initial discharge specific capacity of 1431 m A h g^-1 at 0.1 C and a lower capacity decay rate of 0.0013%per cycle for 1000 cycles at 1 C.Even at high sulfur loading of 10.5 mg cm^-2,it can still cycle stably over 80 cycles.This thesis elucidated that various new composite materials for Li-S batteries are designed and prepared by plasma technology,and obtained the precise control of their microstructures.Through in-depth analysis of the intrinsic relationship between such materials with defect,doping and heterostructure on lithium polysulfide catalysis/conversion,the improvement of electrode reaction kinetics and performance have been achieved,which provides the theoretical basis and technical support for the design and development of novel high-performance batteries.