Preparation And Electrochemical Properties Of Porous Carbon Based Host Materials Of Sulfur For Lithium Sulfur Batteries

With the development of society and progress of science and technology,the demand of high specific energy batteries has become increasingly urgent.As a new secondary battery system,Lithium sulfur(Li-S)batteries have attracted considerable attention because of its high theoretical capacity and energy density,which are 1675 mA h g-1 and 2600 W h kg-1,respectively.In addition,sulfur owns the advantages of nature abundance,cost effective,and environment-friendly that can meet the huge demand for energy storage in the future.While,Li-S batteries also face a series of problems and challenges.For example,the insulation nature of S and Li2S leads to a low utilization of active materials,and large polarization,low specific capacity of batteries under a high sulfur loading or large charge/discharge current density;Polysulfide lithium(LIPSs),an intermediate product from the process of battery charging and discharging,which easily dissolve in the electrolyte,lead to "shuttle effect"and cause seriously degradation of the cycle performance;Besides,in the process of S reduced into Li2S,the volume will increases by nearly 80%,the resulting stress will cause the separation of the conductive matrix and the active material,then the performance of battery will deteriorate rapidly.Aiming at the above problems,this thesis focuses on developing and designing suitable sulfur host material to improve the performance of sulfur battery cathode.Two kinds of host materials with different structures were designed and prepared by using the low density and good conductivity carbon-based materials.Besides,the inhibition to "shuttle effect" was enhanced by carbon activation and modification of host materials.The main research contents and results are as follows:(1)Using 2,6-diaminopydine monomer(DAP)and formaldehyde as raw materials,the polymer micron spheres(PMS)with multi-cavity structure were synthesized under the system of alkaline water solvent in normal temperature.Hollow carbon micron spheres with rich microporous shells(aHCMS)were prepared as host materials for sulfur by using PMS as the precursor.In this work,the influence of pH on the morphology of PMS was studied.The results reveals that with the pH value gets smaller,the cavities of PMS become smaller and their number will increase.The carbon wall of HCMS obtained by direct carbonization of PMS is too thick,resulting in a low battery discharge specific capacity.While the carbon wall of HCMS obtained by carbonization of ethanol treated PMS becomes thinner.The aHCMS obtained after KOH activation has abundant microporous structure,which is a good porous carbon host material of sulfur.The high surface area and pore volume not only can provide sulfur with electrochemical reaction interface but also ensuring sufficient contact between sulfur and the conductive matrix.On the other hand,the physical adsorption of micropores can confine the LIPSs to the host material and effectively alleviate the "shuttle effect".Therefore,with an areal sulfur loading about 0.7 mg cm-2,the specific discharge capacity of 0.2 C after 200 cycles is maintained at 886 mA h g-1 and when the current density increases to 3 C,it can still deliver a discharge specific capacity of 561 mA h g1.(2)The conventional lithium sulfur battery cathode based on aluminum foil current collector generally have a low areal sulfur loading,in order to achieve higher areal sulfur loading cathode to improve the energy density of the battery.A selfsupporting membrane with three-dimensional network structure and constructed by Co modification and N doping porous carbon nanofibers(Co/N-PCNF)was prepared by means of electrospinning.CoN-PCNF not only serves as the host of sulfur,but also as the current collector of composite electrode,its three-dimensional network structure can effectively promote the permeability of electrolyte and the transport of ions and electrons.The results of LIPSs-adsorption experiment and the theoretical calculation prove that the chemisorption of introduced polar CoNx species and the physical adsorption from porous structure greatly enhanced the fixing of LIPSs.In addition,the experimental results show that the cobalt species can improve the reaction kinetics of LIPSs transformation.Owing to the rational design of host materials,with the areal sulfur loading about 2.0 mg cm-2,the Co/N-PCNF@S composite cathode delivers an initial discharge specific capacity of 878 mA h g-1 at 1 C,and maintains 728 mA h g-1 after 200 cycles.Even when the areal sulfur loading increased to 9.3 mg cm-2,the areal specific capacity still maintained 7.16 mA h cm-2 after 100 cycles at 0.2 C,which was about twice of those(4.00 mA h cm-2)of ordinary commercial lithium ion batteries.Through the integrated design of space limitation and chemical adsorption catalysis of LIPSs,a kind of high sulfur loading composite cathode applied to high-specific energy Li-S battery was constructed.This work may provide a new approach for development of Li-S batteries with high sulfur loading.