Design And Performance Of Carbon-based Sulfur Host Materials For Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries have been brought into focus as the development direction of the next-generation power battery system due to their high energy density,eco-friendliness,and low cost,which has a broad application prospect in the field of energy storage such as electric vehicles,unmanned aerial vehicles,and silent-submarines.However,some problems are still unresolved in the sulfur cathode,e.g.,poor electric conductivity,serious volume expansion of sulfur,shuttle effect caused by easy dissolution of polysulfides（Li PS）in the electrolyte,and slow redox reaction kinetics of sulfur species.These issues lead to poor cycle stability and rate performance,making it hard to meet the requirement for Li-S batteries in practical applications.Since the inherent nature of sulfur is the root cause of the above problems,the rational design of functional sulfur host materials will be an effective way to break through the current bottlenecks of Li-S batteries.Aiming at preparing multi-functional sulfur host materials by integrating physical confinement,chemical adsorption,and catalytic effect towards Li PS,this thesis mainly focuses on designing and optimizing the structure of carbon materials,preparing carbon-based polar sulfur host materials,and constructing a three-dimensional electrode with a high sulfur areal loading.The main contents are as follows:（1）Using benzoxazine polymer spheres（SPS）as precursors,gridded hollow carbon spheres（HCS）as sulfur hosts were synthesized by regulating the pyrolysis microenvironment to limit gas escape by means of a silica"fence".By adjusting the aging time and pyrolysis conditions of SPS,the density of the internal grid structure of HCS can be controlled.After CO₂ activation,the physical confinement on sulfur of the HCS with a dense grid structure was enhanced,and electron transport was accelerated.Therefore,the HCS with dense grid structure/sulfur cathode shows good cycle stability and high rate performance.The specific capacity is 654 m A h g^-1after 200 cycles at 0.5 C with a low capacity decay of 0.034%,and the capacity at 3 C is 593 m A h g^-1,which is better than that of activated solid carbon spheres/sulfur cathodes.（2）Sulfur hosts with high conductivity,high porosity,and strong polarity were prepared by one-step co-pyrolysis,which were nickel phosphide/carbon nanotubes（P/Ni-x@CNTs）and borocarbonitride/conductive carbon hybrids（KB@BCN-x）,respectively.By changing the P/Ni molar and urea/KB mass ratio,respectively,the composition and pore structure of the hybrids can be adjusted.When P/Ni is 2,P/Ni-2@CNTs consists of Ni/Ni₁₂P₅/CNTs,possessing a high specific surface area and electrical conductivity.Experimental results show that CNTs are interconnected to form a conductive network,accelerating the electron transport.Ni and Ni₁₂P₅nanoparticles have strong adsorption and catalytic activity for Li PS,enhancing the redox reaction kinetics of sulfur species.Accordingly,the P/Ni-2@CNTs-S cathode shows good cycle stability and high rate performance.The P/Ni-2@CNTs-S cathode delivers a low capacity decay of 0.057%at 0.5 C after 1000 cycles.The specific capacity at 4 C is 784 m A h g^-1.Polar BCN and conductive KB nanoparticles were assembled into micron-sized particles denoted as KB@BCN-x.When urea/KB is 3,KB@BCN-2 shows high electrical conductivity and enhanced tap density.The binding energy calculated by the DFT method,and the kinetic characteristics of the reaction studied by Tafel curves show that KB@BCN-2 has strong chemical adsorption and catalytic effect of Li PS.Therefore,KB@BCN-2-S cathode shows good cycle stability under lean electrolyte.Specifically,KB@BCN-2-S cathode delivers a capacity of 677 m A h g^-1 after 100 cycles at 0.2 C,and the capacity retention is 77%under a condition of low E/S=5μL mg^-1.（3）A three-dimensional cobalt-nitrogen（Co-N_x）doped carbon nanotubes/multi-cavity carbon nanofiber film（3D Co-NCNTs@CNF）was prepared by a template and chemical vapor deposition method as a sulfur host and an interlayer.The carbon nanofiber with a multi-cavity structure can provide enough space for high sulfur loading and prevent the dissolution of Li PS by physical confinement.Co-Al LDH as the substrate promotes the growth of micron CNTs,thus forming the conductive network composed of CNF and long CNTs,which enhances the conductivity of the electrode.The quality and pore structure of the binder-free film can be controlled by adjusting the volume ratio of Co-Al LDH/Mn O₂ and the amount of carbon source.With the increase of Co-Al LDH/Mn O₂ from 0 to 0.84,the mesoporous ratio in the film increases gradually,and the low-nanoporosity structure is conducive to electrolyte penetration,thus reducing electrolyte usage.Co-N_x in situ formed can provide chemisorption and catalytic sites to enhance the adsorption effect and catalyze the conversion of Li PS.Through the triple effect of physical confinement,chemisorption,and catalytic conversion,the Co-NCNTs@CNF-0.42-S cathode combined with the Co-NCNTs@CNF-0.21 interlayer shows high sulfur utilization and good cyclic stability under high sulfur loading and low E/S.When the sulfur loading was 6.7 mg cm^-2 and E/S=10μL mg^-1,the initial areal specific capacity of the binder-free sulfur cathode was 6.37 m A h cm^-2 at 0.2 C and maintained at 4.77 m A h cm^-2after 100 cycles with a capacity retention of 75%.