| Lithium/sulfur(Li/S) battery is a new type of energy storage device, where sulfur is aspositive electrode active material and lithium as the negative electrode. It has theadvantages of having2600Wh/kg theoretical specific energy, low cost, environmentfriendly. In recent, Li/S battery is considered the most promising for next generationbattery system owing to a series of achievements. However, there are some shortcomingsincluding low cathode active material utilization, poor electrochemical cyclingperformance and chemical blunt, based on dissolved/deposition reaction in Li/S batterysystem. These factors have seriously hampered the development of Li/S battery. In thisthesis, carbon materials with different structures(graphene, porous carbon) were preparedby chemical and physical methods. Then, sulfur/carbon composites were prepared, wherecarbon was regarded as conductive carrier. The electrochemical performances impact ofcarbon materials with different structures were measured and analyzed by CV, EIS andcharge/discharge tests in a given experimental conditions.Graphene oxide was prepared by Hummer method and varying the oxidation time,characterized by XRD, SEM, TEM and IR. The results showed graphene oxide wasprepared under the conditions, where oxidation time is3h at35℃, high temperatureoxidation time is0.5h at95℃and ultrasonic time of0.5h. Meanwhile, graphene oxidewas reduced more completely at high temperature reduction (1000℃) and chemicalreduction. The morphology of graphene appeared a significant piece layered, multi-stripping and larger spacing layer than graphite.To further study battery performance, sulfur/graphene composites cathode materialswere prepared by using different methods including mechanical mixing, chemicaltwo-step method, melt method. Electrochemical tests demonstrated that sulfur/graphenecomposites exhibite maximum initial discharge capacity corresponding to chemicaltwo-step synthesis. Further, sulfur/graphene composite with a sulfur content54.94wt%cathode exhibited better capacity retention compared with the pure cathode, where aninitial discharge capacity was up to1500mAh/g and it remained about602mAh/g after30cycles. Graphene would play a role of loading sulfur and conductor, which contributedto improve electrochemical properties for Li/S battery.Mesoporous carbons had been synthesized directly through the evapo ration-inducedself-assembly method, by using triblock copolymers F127as an organic structure directing agent (template) adding a certain amount of dodecane and phenolic resin as carbonprecursor. Through the structure characterization of XRD, SEM and BET, the result werethat porous carbon samples are block and full of mcroporous on the surface. When n(dodecane)/n (F127)=30, the highest pore size of12.5nm was obtained. So the poresize was expanded by adding a certain amount of dodecane, which would contribute toadsorb and fix sulfur particles fully. Further, S/porous carbon composites cathode wereprepared via melting method, with the initial discharge capacity of715mAh/g, decayingto350mAh/g after30cycles.Silica was synthesized respectively by various amounts of tetraethylorthosilicate(TEOS). Next, porous carbon samples were synthesized via Silica as hard template,sucrose as precursor and dissolving away template finally. Through the structurecharacterization of XRD, SEM and BET, the results showed the porous carbons wereaccumulated curly and full of deep holes and pores clearly. When n(F127)/n(TEOS)=1:400, the larger specific surface area, pore volume and uniform pore sizedistribution of about2-4nm were obtained. To further study Li/S batteries performanceimpact of pore structure, S/porous carbon composites cathode were obtained by meltingmethod. The electrochemical tests showed the initial discharge capacity was600mAh/gand declined to about285mAh/g after30cycles. So porous carbon with pore size of2-4nm would not contribute to fix sulfur, which cause poor cycle performance for Li/Sbattery. |
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