Preparation And Electrochemical Performance Of Carbon And Metal Oxide Coaxial Composite Material For Lithium Sulfur Battery

Nowadays,the two main challenges of ecological and environmental issues for human beings demand a steady development of high energy density,environmental friendly,and renewable energy-storage technologies,which are the incentives for the exploitation of lithium sulfur（Li-S）batteries.Li-S batteries have attracted great attention because of their high specific capacity(1675 mAh g^-1)and energy density(2600 Wh kg^-1).Moreover,Li?S batteries are of low cost due to the natural abundance,intrinsically low cost and easy recycling of sulfur.However,several inherent issues of lithium sulfur batteries should be solved before achieving satisfied performance:electric/ionic insulating of elemental sulfur and the discharge products leading to poor electrical conductivity of electrode and low sulfur utilization;dissolution of polysulfides into the organic electrolyte,causing the loss of active material and capacity fading;large volumetric expansion（about 80%）upon the charge/discharge process,resulting in the destruction of the electrode;lithium dendrites produced on the anode leading to safety concern.To address the above-mentioned problems and improve the performance of Li?S batteries,this work mainly focuses on preparing porous carbon,special structure of carbon materials,composite of metal oxides and carbon as the substrates for sulfur.A series of characterization methods and electrochemical measurements were conducted to evaluate the performance and reaction mechanisms of Li?S batteries.The experimental details and major conclusions in this work are summarized as follows:1.Porous carbon was prepared by calcination and chemical activation using corn flour as raw materials and KOH as activator,followed by the incorporation of sulfur.The porous carbon could improve the conductivity of the electrode and also provide adsorption sites to immobilize polysulfides because of the physical confinement property of pore structure.The results demonstrated that the carbon material under the optimal activated temperature of800°C was beneficial for obtaining the porous structure with high specific surface area,and total hole volume thus leading to excellent performance with discharge capacity of 451 mAh g^-1 after 170 cycles at 0.1 C,388 mAh g^-1 after 150 cycles at 0.5 C,and 608 mAh g^-11 at0.1?2 C.2.The coaxial nanostructure of MWCNTs@MPC was synthesized by in-situ hydrothermal method,using D-glucose,pretreated MWCNTs,sodium dodecyl sulfate（SDS）as raw materials,then calcinated under argon atmosphere.The inner“axis”MWCNTs not only supported the integrity of stable structure,but also improved the conductivity of sulfur composite.The outer“shell”MPC provided adsorption sites to host the sulfur species for hindering the dissolution of polysulfides.The composite exhibited a higher reversible capacity and excellent high-rate capability,which could maintain 505 mAh g^-1 after 200cycles at 0.1 C,450 mAh g^-1 after 100 cycles at 0.5 C,270 mAh g^-1 after 100 cycles at 2 C.3.The novel composite MWCNTs@TiO₂ was prepared by grafting TiO₂ onto the surface of MWCNTs followed by incorporating sulfur into the composite.The inner MWCNTs improved the mechanical strength and conductivity of the electrode and the outer TiO₂ provided the adsorption sites to immobilize polysulfides due to the bonding interaction between TiO₂ and polysulfides.The MWCNTs@TiO₂-S composite with a mass ratio of 50%（MWCNTs in MWCNTs@TiO₂）exhibited the highest electrochemistry performance among all compositing ratios of MWCNTs/TiO₂ with discharge capacity of 679 mAh g^-1 after 50cycles at 0.1 C,385 mAh g^-1 after 170 cycles at 1 C,and 262 mAh g^-1 after 170 cycles at 2C.The performance improvement might be attributed to the downward shift of the apparent Fermi level to a more positive potential and electron rich space region at the interface of MWCNTs-TiO₂ that facilitated the reduction of lithium polysulfide at a higher potential.This study provided a theoretical basis for the preparation of mesoporous carbon derived from biomass,coaxial nanostructure of MWCNTs@MPC and MWCNTs@TiO2composite.The work also proposed a new route to solve the environmental issue from the origin in the field of energy storage.