Modification Of Yeast-based Carbon Materials And Its Application In Lithium-sulfur Batteries

Increasingly popular portable electronic products and rapidly developing electric vehicles have placed increasing demands on the energy density and performance of secondary batteries.In order to meet the urgent needs of the development of the new energy industry,a research boom in lithium-sulfur batteries has been set up because lithium-sulfur batteries have ultra-high theoretical specific capacity and specific energy.However,before the real application of lithium-sulfur batteries,there are still some problems in the research of cathode materials.For example,the insulating material of the active substance-sulfur and its discharge products Li₂S₂and Li₂S,the volume strain generated by the sulfur positive electrode during charge and discharge process,and the"shuttle effect"caused by the dissolution and diffusion of intermediate products,etc.,lead to the utilization of active substances in lithium-sulfur batteries.Low,poor cycle stability.In view of the above problems,this paper starts with carbon materials in the carbon/sulfur composite materials,mainly using yeast as a carbon material precursor,and modifying it to prepare different yeast-based carbon microsphere materials,aiming at solving the problems faced by lithium-sulfur batteries,and finally to improve the electrochemical performance of the battery.Firstly,hollow porous yeast-based carbon（HYC）microspheres were prepared by hydrothermal carbonization,the elemental analysis showed that the carbon substrate had a nitrogen content of8.52 wt%.Then,the microspheres are loaded with sulfur to obtain a carbon/sulfur composite material,and the surface thereof is coated with a layer of reduced graphene oxide to obtain a modified carbon/sulfur composite material（rGO@HYC/S）.Electrochemical tests show that the rGO coated modified composites have a capacity of up to 921 mA h g^-11 after 200 cycles at 0.2 C rate,and the average decay rate per revolution is as low as 0.094%,and the attenuation rate of300 cycles at 2 C high rate is lower to 0.08%per cycle.Excellent electrochemical performance benefit by nitrogen-doped hollow yeast carbon that can store sulfur and limit the diffusion of polysulfides,graphene can ensure the rapid transmission of electrons and lithium ions,and can also be used as a second barrier to retain more dissolved polysulfide.The experimental results provide a simple and feasible method for the study of lithium-sulfur battery cathode materials.In addition,two kinds of metals,copper and zinc,were doped into yeast-based carbon materials by biosorption method,and two kinds of metal-doped yeast-based carbon materials（Cu-HYC and Zn-HYC）wereobtained after high-temperature carbonization.The characterization results show that both materials are rich in mesoporous structure and nitrogen,besides,the elements of them are evenly distributed.Copper is mainly present in the form of copper element in copper-doped composite microspheres.Copper doping can enhance the strength of the carbon skeleton and improve the conductivity of the sulfur cathode.The zinc element in the zinc-doped composite material encapsulates the outside of the yeast carbon in the form of zinc oxide.On the one hand,the coating layer can retain the lost polysulfide ions.On the other hand,the zinc oxide can form a strong bond with the polysulfide,which weakens the"shuttle effect"impact.After hot-melting,both materials exhibit electrochemical performance superior to that of HYC when used in lithium-sulfur batteries,the Cu-HYC and Zn-HYC electrodes retain a high capacity of up to 896 mAh g^-11 and 993 mAh g^-11 after cycling 100 times at a 0.2 C rate,respectively.The coulombic efficiency of Cu-HYC electrode has been stable at about 99%,and the Zn-HYC electrode around 97%during 1 C high current,.