Synthesis And Electrochemical Performance Of Self-Supporting Cathodes For Lithium-Sulfur Batteries

In order to meet the increasing energy demand of batteries,there is an urgent need to develop new energy storage devices with high energy density,environmental friendliness and low cost.Lithium-sulfur batteries have a very high theoretical specific capacity and specific energy,which is equivalent to several times that of commercial lithium-ion batteries.In addition,sulfur reserves are abundant,low-price,and environmental friendly.Therefore,lithium-sulfur batteries are considered as the most promising next-generation high-performance energy storage systems.Sulfur-containing cathode materials are the key to determining the performance of lithium-sulfur batteries.They mainly face the problems of poor conductivity of sulfur,“shuttle effect”and volume expansion of electrodes.Therefore,designing and constructing high-performance sulfur cathode materials is research hotspot of lithium-sulfur battery research.Based on the investigation and analysis of the research trends of lithium-sulfur battery cathode materials,this paper selects commercial carbon fiber cloth/paper as a flexible conductive support framework to design and build a high-performance self-supporting flexible lithium-sulfur battery cathode material.The metal-assisted chemical etching technology is used to perform in-situ etching on the surface of the carbon fiber?cloth?to obtain a hierarchical porous carbon layer.The porous carbon structure can contain sulfur,inhibit the shuttle effect.We can adjust its microstructure to obtain high sulfur loading.Then the carbon/sulfur electrodes?PC@CFC/S?for lithium-sulfur batteries are contructed.A catalytic carbonization method is used to grow a hierarchical porous carbon layer containing polar nickel-phosphorus compound nanocrystals on the surface of carbon fiber?paper?,and obtain a self-supporting flexible lithium-sulfur battery cathode material with high specific capacity and long cycle life?NP-3DGC@CFP?.The main research contents and conclusions are as follows:?1?Based on carbon fiber cloth as matrix,the core-shell porous carbon fiber?PC@CFC?with adjustable microstructure was prepared by depositing Ni?OH?₂layer on the surface of carbon fiber,and then high-temperature calcination and acid-washing.Carbon fiber?core?acts as both an electrode support structure and a highly conductive skeleton.Porous carbon layer?shell?not only provides a three-dimensional conductive interconnection network,but also has the dual functions of hosting sulfur and confining sulfur.By adjusting the process parameters to regulate the morphology of porous carbon,and thus the electrochemical performance of PC@CFC/S electrode was optimized.The results showed that 6 m A cm^-2 is the best deposition current density,and 800?C is the optimal calcination temperature.Based on optimal parameters,the core-shell carbon fibers exhibit porous structure and still maintained excellent mechanical strength and flexibility.PC@CFC?6/800?/S electrode show evry promising electrochemical performance and good cycling stability.PC@CFC?6/800?/S cathode with sulfur loading of 3.0 mg/cm² delivers a high initial capacity of 903.2m Ah g^-1 at 0.5 C with retaining capacity of 461.9 m Ah g^-1?51.1%retention?after300 cycles,and the rate performance is excellent as well.?2?Choose carbon fiber paper?CFP?with high conductivity and favorable flexibility as a free-standing skeleton/scaffold,glucose as carbon source,potassium hydroxide as activator,red phosphorus and nickel acetate as additives,nickel phosphide nanodot-studded 3D graphene-like carbon?NP-3DGC?were directly grown on the carbon fiber paper using a simple one-step template method.Then,such well-integrated NP-3DGC@CFP architecture was used as a robust sulfur host to construct NP-3DGC@CFP/S cathode for lithium-sulfur battery.The comprehensive characterization analysis and theoretical calculation results show that the highly conductive carbon fiber skeleton and the porous carbon layer not only provide a fast channel for electron transport,but also the hierarchical po rous carbon layer can accommodate active sulfur and physically limit the dissolution of polysulfide ions.Ni₂P nanocrystals dispersed on the surface of porous carbon not only can chemically adsorb the dissolved polysulfides,but also can accelerate the reversible conversion of lithium polysulfides,thereby effectively inhibiting the“shuttle effect”.Therefore,the battery based on the NP-3DGC?CFP?/S electrode exhibits very excellent electrochemical pe rformance,including high specific capacitance,ultra-long cycle life,excellent rate performance,and effective relieved self-discharge behavier.NP-3DGC@CFP/S cathode delivers an initial discharge capacity of 1081.6 m Ah g^-1,and retains a capacity of 786.3m Ah g^-1?72.7%?after 300 cycles at 0.5 C when the sulfur loading is 4.0 mg cm^-2.NP-3DGC@CFP/S cathode also show superior rate performance with 535.6m Ah g^-1 at 5.0 C.