Preparation Of Porous Carbon Based Material As Cathode And Separator For Application In Lithium-sulfur Batteries

Environment and energy issues have emerged as a serious threat with the booming development of the economy in recent years.Among the endeavors to tackle the renewable energy and environmental issues,lithium-sulfur(Li-S)battery is one of the most promising candidates for the next generation high-performance secondary batteries.However,the commercial application of Li-S batteries is hindered by several inherent issues,including the poor electrical conductivity of sulfur,the poor cycle stability,the"shuttle" effect,and the safety issues due to the deposition of lithium dendrites.Therefore,a series of novel technologies and methods combined with the special microstructure of solid waste have been used for the preparation of cathode materials and modification of separator.This will provide a theoretical basis for the wide application of high-performance Li-S batteries,contributing to the alleviation of environment issue and energy demand.This dissertation focuses on the following four parts:1.Lamellar mesoporous carbon derived from bagasse as the cathode materials for lithium-sulfur batteryThe lamellar mesoporous carbon was prepared using the solid waste of bagasse as precursor.The influence of different activation routes on the carbon structure and batteries performance was explored.The mesoporous lamellar carbon was produced from direct carbonization without the widely adopted chemical activation process,and the unique mesoporous structure was found to be beneficial to adsorb polysulfides during the charge-discharge process.The first cycle capacity of battery was 1377 mAh g-1,and the capacity remained as high as 670 mAh g-1 after 200 cycles with 48.6%capacity retention,implying a capacity loss of 0.26%per charge-discharge cycle.The electrochemical results show that bagasse can be utilized as promising cathode material due to its unique lamellar microstructure.2.The preparation of carbon material using sodium chloride as a template and its electrochemical performance as the cathode material for lithium-sulfur battery N,S co-doped graphene platelet(NSG)was produced utilizing sodium chloride as a template and L-cysteine as the precursor,and used as the cathode material and for separator modification.When NSG was used as the cathode material,the cycle stability tests were compared at 0.5 C,1 C and 2 C.After 200 cycles,the batteries delivered a reversible capacity retention of 47.0%,44.4%,and 47.7%,implying a capacity loss of 0.24%,0.22%,and 0.24%per cycle,respectively.When NSG was used as the material for separator modification,the initial cycle capacity of battery was 1410.1 mAh g-1 at 0.1 C rate,and the capacity remained as high as 822.5 mAh g-1 after 200 cycles.From the X-ray photoelectron spectroscopy(XPS)analysis,it can be seen that pyridinic-type N,pyrrolic-like N,and thiosulphate,originating from the amine and thiol groups in L-cysteine,are the dominant factors contributing to the superior electrochemical performance of the NSG/S cathode.In addition,DFT computation was used to study the influence of different elemental types of dopant on the performance of batteries.The strongly negative charged pyrollic N doped sites are believed to promote nucleophilic attraction to anchor Li23x(x=3,4,8)by charge attraction with Li+ and the interaction between Li2S8 and pyrrolic N doped sites was stronger than other configurations through DFT simulation.This study sheds light on interaction of polysulfides with the heteroatoms doped NSG and provides some reference for the application of different elements in lithium-sulfur battery.3.The preparation of carbon material using egg shell as a template for application in Li-S batteryThe doping with heteroatoms in the carbon material is privotal to improve the performance of Li-S batteries.N,S co-doped hollow porous carbon(NSPC)was produced using egg shell as a template and glucose as the precursor,which contains certain ingredient of amino acids and protein besides calcium carbonate.The prepared NSPC material was employed for the modification of separator.The discharge capacity of the second cycle at 1 C was 1914 mAh g-1,which is 1.7 times higher than the batteries with a pristine polypropylene(PP)separator.The cathode convertible capacity retained at 1467 mAh g-1 after 100 cycles,2.4 times higher than the batteries with a pristine PP separator,implying excellent cycling stability of the batteries.The specific capacity of first cycle at 0.5 and 1 C rate was greater than 2000 mAh g-1,slightly exceeded the theoretical capacity of a lithium sulfur battery.This could be attributed to the energy storage capacity contributed from super-capacitive effect of cathode and NSPC materials.The study showed that this particular solid waste could effectively suppress the shuttle effect in the process of charge and discharge and promote the performance in lithium-sulfur battery applications.4.The preparation of carbon material using chemical vapour deposition(CVD)method for the modification of separator in Li-S batteryTwo types of carbon materials,egg shell template derived carbon(EC)and CaO template derived carbon(CC)were prepared through CVD method using egg shell or calcium oxide as the substrate respectively,and the prepared materials were used for the modification of separator.The different morphologies of these carbon materials were beneficial for the modification of separator,and lower mass density of EC and CC than that of conventional activated carbon is beneficial to achieve a higher energy density.The cycle stability of the batteries was improved significantly using the modified separator.The batteries using CC for separator modification retained high cycle stability even at high current density.The use of this solid waste product as a functional modifier improved the discharge performance at higher current density for lithium-sulfur batteries.These studies can provide a theoretical guideline and experimental basis for the manufacturing of mesoporous carbon derived from biomass and the synthesis of the heteroatoms doped porous carbon using solid waste as the template.It provides a novel route for the preparation of heteroatoms graphene platelet using templating method for application in Li-S batteries.