Construction Of Functionalized Hierarchical S-loaded Cathodes For Li-S Batteries

With the ever-increasing attention paid to new energy vehicles,developing an efficient battery energy storage system becomes urgent and necessary.The specific capacity of the cathode material for conventional plug-in lithium ion battery has reached its limit,and there is insufficient space for the widespread development.The up to date lithium-sulfur?Li-S?battery is considered to be one of the most promising energy storage systems featured by the remarkable theoretical energy density of 2600 W h kg^-1,coupled with abundant sulfur cathode materials and low prices.However,their commercial application remains depressed because of the tough problems such as poor Coulombic efficient,limited cyclic lifetime and low electrochemical utilization efficiency,mainly induced by the inherent poor conductivity of S cathode,volume expansion during the redox process,dissolution of polysulfide into the electrolyte and inevitable shuttle effect.Generally,the active sulfur is combined with conductive carbon materials,metal compounds?oxides,sulfides,nitrides,etc.?,organic polymers and other host materials to solve the above problems.However,the single electrode materials have been unable to meet the requirements of high-power density and high-energy density.To this end,researchers are committed to researching different electrode materials to recombine,through the synergistic effects between each component to improve the performance of Li-S batteries.In this paper,in view of the dilemma of Li-S batteries,the nanocomposites with unique hierarchical structure were designed for accommodating active sulfur.Physical/structural characterization and electrochemical analysis of the obtained electrodes were performed,and then interactions between the components were thoroughly explored.The main research contents are as follows:1.An efficient battery improvement strategy was designed based on the advantages combination of Se and S.The S/Se@CB precursors were obtained by infusing Se and S into the carbon black matrix step-by-step via a simple ball milling method and melt diffusion.Then,as-resulted nanoparticles were coated with ultrathin nickel nitrate hydroxide?NNH,thickness of per layer:7 nm?to get the S/Se@CB???NNH complex with a hierarchical core-shell structure.Highly conductive elemental Se not only benefits to improve the utilization of active materials,but also can participate in the reversible redox reaction of the electrode and contribute additional specific capacity.At the same time,the outer NNH protective layer plays a significant role in protecting the structural integrity during the charge and discharge process,and alleviates the solution of polysulfide/polyselenide into the electrolyte to some extent.Such S/Se@CB???NNH composite used as cathodes for Li-S batteries delivers an initial discharge capacity of 913mAh g^-11 at a current rate of 0.2 mA g^-1.While the reversible capacity is still retained 488mAh g^-1,even under high current density of 5 mA g^-1.After 500 deep cycles,its specific capacity can still reach 753 mAh g^-1 with excellent cycling stability and superior rate capability.2.Using traditional electrospinning technology,ZIF-67@PAN nanofibers were synthesized from purple ZIF-67 nanoparticles which were previously prepared via an improved precipitation method.In the subsequent step,the ZIF-67@PAN precursors were converted into a functionalized hollow Co₉S₈@CNFs by a liquid-phase sulfidation reaction?thioacetamide,TAA?and a high-temperature heat treatment process.Finally,a melt diffusion method was applied to infuse active S into the highly-puffed Co₉S₈@CNFs.Each Co₉S₈ with void substructures provides large void space to store active materials and endure volume expansion during charge and discharge.In addition,the polar Co₉S₈nanoreactors can effectively alleviate the diffusion/dissolution of polysulfides through useful chemical bonds,and improve the conversion reaction kinetics of polysulfides.The linear electrode constructions are conducive to the long-distance electron transmission and ensures the fast charge/discharge capabilities of electrodes.When used as cathode for Li-S batteries,the S???Co₉S₈@CNFs composites reveal a high specific capacity of 1080mAh g^-1(at 0.1 A g^-1).Even after 500 cycles,it can still maintain a significant capacity of891 mAh g^-1?capacity loss rate per cycle:0.03%?.