Preparation And Electrochemical Performance Of Carbon-based Cathode Materials For Lithium-sulfur Batteries

Lithium-sulfur batteries have high theoretical specific capacity and theoretical energy density,which are considered to be one of the most promising energy storage systems.Besides,sulfur has the advantages of low cost,natural abundance and environmentally benignity.However,the practical application of lithium-sulfur batteries is restrict by many factors.The most important problem is that the soluble polysulfide lithium produced during the charge-discharge cycle is easily dissolved into the organic electrolyte,resulting in loss of active materials,shuttle effect and irreversible capacity fading.In addition,the insulativity of the sulfur and the discharge products?Li₂S₂/Li₂S?increases the internal resistance of the cell,and the volume expansion during cycling decreases the stability of the cathode.To address the mentioned issues,we have designed different the positive electrode structure and material to improve the electrochemical performance of the batteries in this work.The main research contents include:1.carbon nitride?g-C₃N₄?/carbon nanotube?CNTs?composite has been prepared with the help of ultrasonication and coated on the surface of carbon paper in which manganese dioxide?MnO₂?is grown in situ to obtain the g-C₃N₄@CNTs-MnO₂ sulfur cathode.The g-C₃N₄ with high N-doping can capture lithium polysulfide through forming a chemical bond with lithium,thereby alleviating the shuttle effect of lithium polysulfide.Besides,MnO₂ nanosheet also can bond with lithium polysulfide and have strong adsorption to lithium polysulfide,thereby further inhibiting the dissolution and diffusion of lithium polysulfide.Benefiting from the synergistic adsorption of g-C₃N₄ and MnO₂ and the design of the layered structure,the shuttle effect of lithium polysulfide has been effectively alleviated.In addition,the conductive frame of the CNTs improves the conductivity of the electrode and promotes the transport of electrons during charging and discharging.Based on the above advantages,the electrochemical performance of the g-C₃N₄@CNTs-MnO₂based sulfur cathode has been significantly improved,including a high discharge specific capacity?1182.7 mAh·g^-11 at 0.2 C?,a good rate performance(738.2 mAh·g^-1at 2.0 C)and a superior cycling stability?792 mAh·g^-11 after 200 cycles?.2.The ruthenium oxide?RuO₂?/ultrathin porous carbon shell?UPCS?composite has been synthesized by thermal reduction combined with solvothermal method.The carbon shell provides a large number of channels and sufficient space for the penetration of sulfur.This not only inhibits the dissolution and diffusion of lithium polysulfides,but also alleviates the volume expansion of the sulfur cathode during the lithiation/delithiation process.The use of polar RuO₂ nanoparticles as the cathode material of the lithium sulfur battery can effectively inhibit the dissolution of polysulfides by the chemisorption to the polysulfides,and significantly enhance redox reactions.Therefore,the RuO₂-UPCS based sulfur host exhibits excellent electrochemical performance,high specific capacity?1301 mAh·g^-11 at 0.2 C?,superior rate performance?985 mAh·g^-11 at 2.0 C?,and stable cycle performance.