Preparation Of Carbon Nanotube Film-Based Sulfur Cathode For Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries are considered to be the most promising secondary batteries to replace Li-ion batteries due to their extremely high discharge specific capacity.However,the poor electrical conductivity,volume expansion,and shuttle effect in electrode limit its commercialization.In order to solve these problems,sulfur combined with carbon materials has been explored.However,uneven distribution of active materials in supporting,leads to low utilization of the sulfur,Furthermore,the electrode is difficult to maintain the flexibility with the sulfur loading increased,Polysulfides（Li PSs）also have been used as active materials.The most important problems are the reactants of the Li PSs cannot be reacted completely and the concentration cannot be controlled easily.To slove the shuttle effect problem,the modified substrates have been studied to catalyze the electrochemical reaction and inhibit the shuttle in the system.However,the poor electrical conductivity and decomposition property of substrates limit their practical application.In this dissertion,we propose to prepare liquid sulfur cathodes（Polysulfides）by electrochemical discharge approach based on a direct observation of the electrochemical reaction in the battery.The pressed carbon nanotube sponges（CNSs）was used as supports to provide sulfur utilization.And thereby the modified CNTs could as an anchor to catalyze conversion of the Li PSs,and improve electrochemical reaction kinetics of the battery,beacuse more active sites for lithium sulfide（Li₂S）deposition could be provided.In this dissertation,the corresponding relationship between different charge/discharge voltages with colors of the battery was established by studing the through the direct observation of the electrochemical reaction.The optimal potential for preparation of Polysulfides by electrochemical discharge was determined through measuring the substances at different voltages,The polysulfides could be prepared by discharging the cell to the first plateau of 2.3V,and it could be adsorbed effectively by CNSs as a carrier.And thereby the active materials could be distributed uniformly,which increased the utilization of the active materials.The cell can be cycled 1500 times at a rate of 1 C.Due to the S₈ and Li₂S nucleated with fine nanoparticles in polysulfides-CNSs electrode at the electrochemical reaction,the utilization rate of active materials coud be enhanced,and excellent electrochemical performance.However,the inhibitory effect of carbon materials on the shuttle effect is not strong,and the cycle performance of high-capacity and soft-pack batteries cannot meet the commercialization requirements and needs to be further strengthened.In addition,the CNTs also have been modified by WO_2.9-W₂N.W₂N has an excellent electrical conductivity as a metallci nitride,which not only could improve the poor electrical conductivity of transition metal oxides,but also could form Li-O bonds and Li-N with Li PSs.The chemical bond could strengthen the chemical absorption and ancho the matrix on Li PSs.Moreover,WO_2.9 also can provide more active sites for the deposition of Li₂S due to the existence of oxygen vacancies for different atomic structures.Therefore,the assembled cells show excellent electrochemical reaction kinetics and excellent catalytic performance for electrochemical reactions.The capacity retention rate of the cell is 80%after 500 cycles.In addition,the pouch cells also have been prepared for their practical applications.The main reason is the WO_2.9-W₂N/CNTs as shuttling inhibition experiences the three processes of smooth trapping-anchoring-catalytic conversion,and the dissolved Li PSs can be reacted effectively in the system.