Preparation And Electrochemical Performance Of Flexible Carbon-based Lithium-sulfur Battery Cathode Materials

Lithium-sulfur battery,a new type of secondary battery energy system,has re-entered the research laboratory after decades.Because it not only has a high energy density of 2600 Wh kg^-1,it can also meet people's growing energy needs.Moreover,the element sulfur is non-toxic and rich in content,which is friendly to the environment.However,most of the secondary batteries are prepared by traditional mixing and coating procedures,likely to cause some human errors,and the materials easily fall off from the current collector during the charging and discharging process.Therefore,in this thesis,through using carbon cloth as a flexible substrate,where the Ni Co₂O₄ in-situ grew,and then the electrode is prepared by using molten sulfur-bearing sulfur.The details as follows:?1?A honeycomb S/Ni Co₂O₄/CC composite was prepared by a solvothermal method and a molten sulfur-loaded method.Using methanol as a solvent,nickel ions,cobalt ions,carbonate ions and hydroxide ions were self-assembled respectively,after calcined at 350?for 3 hours and smelted at high temperature,the target composite material was obtained.SEM image showed that the honeycomb Ni Co₂O₄ can not only provide a rich electron transport path,but also the holes in it can be used as a load container for sulfur,which can slowly release the stress caused by the volume expansion and contraction of sulfur during the charge and discharge process.Electrochemical tests showed that the S/Ni Co₂O₄/CC electrode with an sulfur loading of 1.12 mg cm^-2 can achieve a specific discharge capacity of 1480 m Ah g^-1 at0.1 C,and a specific discharge capacity from 1090 m Ah g^-1 decays to 826 m Ah g^-1,from calculation,the capacity decay rate is 0.06%per turn.When the sulfur-loaded areal densities reach to 2.10 mg cm^-2 and 3.08 mg cm^-2,the specific discharge capacities at 0.1 C still showed1139 m Ah g^-1 and 689 m Ah g^-1 respectively.?2?Needle S/Ni Co₂O₄/CC composites were prepared by solvothermal method and molten sulfur-loaded method.By changing the solvent of the reaction system and controlling the morphology,three micro-morphologies of pine needles,nanoflakes and shallow pits were synthesized.Through analysis and comparison,we finally decided to use pine needles as sulfur-loaded structures.First,the pine needle-like nickel cobaltate can cover the carbon fiber surface well.Second,the pine needles that grow vertically and are very densely packed can be combined into a stronger three-dimensional nano needle cluster.it not only provides more abundant pores,and can also increase contact with the electrolyte,which is conducive to the diffusion of ions.BET test shows that the specific surface area of the material is 57.428 m² g^-1,which is a considerable value for monolithic materials,which also creates a good environment for sulfur loading.Electrochemical tests showed that the sulfur-loaded areal density was 1.09mg cm^-2,the specific discharge capacity was 1467 m Ah g^-1 at 0.1 C,and the specific discharge capacity decreased from 633 m Ah g^-1 to 363 m Ah g^-1after 300 cycles at a larger rate of 1 C,and the attenuation rate per cycle is 0.3%,showing excellent electrochemical performance.?3?Light-weight nano-flower S/Ni Co₂O₄/CC composites were prepared by high-temperature roasting,solvothermal and molten sulfur-loaded methods.The carbon cloth was pretreated with ethanol,hydrochloric acid and deionized water,and then calcined at 600°C for 5 hours.The mass loss rate of the carbon cloth was between 48.6 and 52.4%.Then,different solvent thermal times were studied in the ethanol system.A nano flower structure supported by a dense nano sheet was identified as a sulfur-loaded container.The prepared composite electrode was tested for battery loading.The results showed that the discharge capacity at 0.1 C was 808 m Ah g^-1,and after 250 cycles at a larger rate of 1 C,the capacity decayed from 278 m Ah g^-1 to 174 m Ah g^-1.The attenuation rate of each cycle is 0.1%.