Preparation And Electrochemical Study Of Cathodes For Room Temperature Sodium-Sulfur Batteries

In order to reduce severe climate change and environmental pollution caused by burning fossil fuels,there is an urgent need to innovate battery technology and explore advanced electrode materials.Room temperature sodium-sulfur batteries have received wide attention because of its high energy density(1274 W h kg^-1)and high theoretical specific capacity(1675 mA h g^-1),which is considered to be a promising energy storage system.Furthermore,due to the abundance of sodium and sulfur in the crust,it has brought broad prospects for practical application.Unfortunately,some problems inherent in room temperature sodium-sulfur batteries still seriously hinder their further development.First,the cathode sulfur(5×10^-30 S cm^-1)and its discharge product?Na₂S₂/Na₂S?have poor conductivity,which result in slow reaction kinetics of sodium-sulfur batteries.Second,sulfur cathodes undergo severe volume expansion during?dis?charge,which makes the structure of host easy to smash,and the cycle performance of battery is poor.Finally,sodium polysulfides generated during the reaction of sodium-sulfur batteries is easily dissolved in the electrolyte and reduced on the surface of Na,leading to the dissolution of the active substance and rapid capacity decay.In the paper,I focus on sulfur cathodes and construct three hosts with different structures and characteristics to solve above problems in room temperature sodium-sulfur batteries.These different hosts have targeted different problems of poor conductivity,volume expansion,and shuttle effect in sodium-sulfur batteries.In addition,through the physical characterization and electrochemical tests of the cathodes and batteries,the feasibility of those sulfur carriers for room temperature sodium-sulfur batteries is fully explored,the results are as follows:1.Synthesis of carbon nanotubes in series with polyhedral carbon?S@CNT/NPC?sulfur hosts:The carbon nanotubes are connected in series with ZIF-8?CNT/ZIF-8?as the precursor.Then,carbon nanotubes connected polyhedral carbon?CNT/NPC?with rich pore structure are obtained after carbonization and acid treatment.The particle diameter of polyhedrons is about 1.2?m,and then sulfur and CNT/NPC are melt-mixed to prepare cathodes for sodium-sulfur batteries.Finally,the S@CNT/NPC electrode exhibits a specific discharge capacity of?601 mA h g^-1 at a current density of 0.5C.The good electrochemical performance comes from the combination of metal-organic framework-derived carbon and carbon nanotubes.The rich pore structure of NPC can contain a certain amount of sulfur,while alleviating volume expansion,and carbon nanotubes play a vital role in stabilizing the structure of electrodes and improving cathode conductivity.2.Synthesis of S@HCS/MoS₂ sulfur carriers:firstly,dopamine carbon hollow spheres?HCS?have been prepared using SiO₂ spheres as templates.After removing templates,polar MoS₂ nanosheets are grown on the surface of the hollow spheres by hydrothermal method.The HCS/MoS₂ composite can combine active substance sulfur and effectively adsorb the intermediates?NaPSs?.In addition,considering that the sulfur attached to the surface of the carrier will move in the electrolyte during discharge and charge process,I further prepared HCS/MoS₂ modified separator to prevent NaPSs from moving into the anode.The cathode-separator design exhibits a high specific capacity of?1309 mA h g^-1 at a current density of 0.1C and continuously circulates 1000 times at1C in the sodium-sulfur battery.Besides,in-situ Raman and ex-situ XPS have been appiled to further analyze the products during discharging and charging.3.Synthesis of core-shell Co₉S₈@SiO₂/C selenium disulfide?SeS₂?hosts:according to the difference in the rate of nucleation of metal-organic frameworks?MOFs?and hydrolysis of tetraethyl silicate,a layer of SiO₂ shell is grown in situ on the surface of ZIF-67.The Co₉S₈@SiO₂/C composite is finally prepared after vulcanization and carbonization as the host for SeS₂.Here,using selenium disulfide instead of sulfur can enhance the conductivity of the active substance and improve the utilization of SeS₂.The core-shell MOF-derived sulfides can accommodate active materials and effectively adsorb sodium polysulfides/polyselenides?NaPSs/NaPSes?.In addition,the in-situ Raman and ex-situ XRD are used to analyze the conversion reaction and intermediate products during discharging and charging in detail.And the reaction mechanism of Na-SeS₂ battery in ether electrolyte is briefly described.In summary,we have designed and synthesized three composites with different characteristics,and used them as sulfur/selenium disulfide hosts to show excellent electrochemical performance.These composites can not only contain a certain amount of sulfur/selenium disulfide,effectively relieve the volume expansion during?dis?charging,but also have the functions of improving conductivity and reducing polysulfides/polyselenides leakage.Overall,the work can provide research references for the development of room temperature sodium-sulfur batteries.