Cathode Design And Performance Of Graphene-Based Lithium Sulfur Battery

With the development of electric vehicles,portable mobile tools and scale energy storage,the demand for new secondary batteries with high energy density,low cost and long cycle life is urgent.Lithium-sulfur(Li-S)battery is one of the most competitive systems for next generation high-energy secondary batteries due to its high theoretical capacity(1675mAhg^-1)and energy density(2600Whkg^-1).However,several problems seriously affect the actual specific capacity and cycle performance of Li-S battery.For instance,the "shuttle effect" caused by the dissolution and migration of lithium polysulfides(LiPSs).Graphene is one of the most promising electrode materials for Li-S battery due to its large theoretical specific surface area,excellent conductivity,high electrochemical stability and scalable synthesis method.In this thesis,the high-quality graphene materials have been firstly prepared and investigated.Meanwhile,a series of cathodes of graphene-based Li-S battery were designed by introducing different mediator materials.The electrochemical properties of the battery were investigated by various analytical approaches,and the effect of mediator materials on the dynamic process of the battery was also discussed.The main results are as follows.Graphene oxide(GO)sheets were synthesized via a modified Hummers method.GO dispersion with a high concentration was chosen to form GO hydrogel,followed by chemical reduction to derive a free-standing reduced GO(rGO)film.The rGO film has a densely stacked laminated structure and highly anisotropic characteristic of electrical conductivities.The light-weight rGO film also demonstrates its excellent flexible and fire-retardant characteristics.Electrical transport measurement indicates that the rGO film exhibits semiconducting behavior.A temperature dependence of the conductivity from 20 to 297K reveals that the carrier transport mechanism is thermally activated band conduction above 200K and three-dimensional Mott's variable range hopping below 100K.The as-prepared graphene material has variously excellent properties,which can be applied in lithium sulfur battery.Freestanding cathode with sandwich-structured characteristic was synthesized for high-performance Li-S battery.N-doped graphene(NG)/sulfur composites were prepared as primary active material.Furthermore,the active layer with reinforced concrete structure was constructed by braiding NG/sulfur composites with carbon nanotube(CNT)and nanofibrillated cellulose(NFC).CNT contributes to the construction of long range conductive skeleton and ion transport channel.NFC rich in hydroxyl functional groups can effectively bind LiPSs produced during charge and discharge process.Interconnected CNT/NFC layers on both sides of the active layer can improve the electrical conductivity and mechanical properties of the electrode,and effectively inhibit the diffusion of LiPSs.The non-use of current collector and binders can effectively increase areal capacity of the freestanding electrode.Specifically,the electrode with high areal sulfur loading of 8.1mgcm^-2 exhibits an areal capacity of-8mAhcm^-2 and an ultralow capacity fading of 0.067%per cycle over 1000 discharge/charge cycles at 0.5C rate,while the average coulombic efficiency is around 97.3%,meaning a good electrochemical reversibility.This novel and low-cost fabrication procedure is readily scalable and provides a promising avenue for potential industrial applications.A heterostructured sulfur host composed of a porous two-dimensional g-C₃N₄(PCN)and NG hybrid was fabricated by a template-free strategy.The proposed superposition design not only affords a large electrode surface area with evenly distributed mesopores for smooth electrolyte accessibility and rapid Li ion diffusion,but also renders interconnected building blocks for efficient electronic transport.The heterostructured PCN/NG hybrid can stabilize the redox cycling due to the enhanced LiPSs anchoring-conversion capability.The obtained hybrid sulfur cathodes demonstrate excellent cyclability and high coulombic efficiency.Specifically,the Li-S battery based on the optimized PCN/NG hybrid delivered a high reversible capacity of 1236mAhg^-1 and a good long-term cycling stability(i.e.,70.4%capacity retention after 500 cycles at 0.2C).Due to the facile and scalable in situ fabrication strategy,the PCN/NG hybrid can be envisioned as a promising heterostructured sulfur host for Li-S batteries with superior performance.Iron phthalocyanine(FePc)was introduced as an effective mediator material for high-performance graphene-based Li-S battery to achieve the LiPSs anchoring-conversion.The rGO acts as conductive infrastructure to ensure adequate electron transport and smooth electrolyte accessibility,while the additional conjugate molecule by coupling FePc with rGO through ?-? stacking interaction functions as active sites to accelerate the reactions kinetic,enhance the anchoring-conversion process,and thus effectively inhibit the shuttling of LiPSs.The fabricated electrode with 10wt.% of FePc exhibits a high initial discharge capacity(1307mAhg^-1 at 0.1C)and a superior cyclic stability(a low capacity fading rate of 0.049% per cycle over 500 cycles at 0.5C).The proposed hybridization strategy will offer new ways for achieving high-stable and long-lifetime Li-S batteries.