Design, Synthesis And Electrochemical Performance Of Sulfur Cathode Materials For Lithium Sulfur Batteries

Lithium sulfur (Li-S) batteries are considered to be one of the most promising high energy density battery technologies due to their high theoretical energy densities (2600Wh kg-1), environmental friendliness and low cost. However, some inherent problems remain to be further solved, such as low sulfur utilization, poor cycle life, limited rate capability and serious safety problem. In response to these challenges, current state-of-the-art strategy is coupling sulfur or lithium sulfide with the carbonous materials. Although improvements are obtained, the systematic studies of carbon/sulfur composite cathodes for Li-S batteries, including the effect of surface chemistry and metal oxide decoration of carbon hosts on the electrochemical performance, the optimization of sulfur dispersion condition, the simple systhesis of Li2S cathode for high safe Li-S batteries, the design of sulfur cathode with hierarchical mesocrystal structure and the further improvement of electrochemical performance of Li-S batteries, are still limited. Based on the application of carbon/sulfur composites in Li-S batteries, this thesis mainly focused on the control of structures of carbon hosts and their compositing method with sulfur, to develop the efficient and scalable approach for the advanced sulfur cathodes with long cycle life, high power density and high safety. The main results of this thesis are summarized as follows:(1) Nitrogen-enriched mesoporous carbon/sulfur with long cycle life:Nitrogen-enriched mesoporous carbons with developed mesoporous structure and tunable nitrogen content have been prepared by a facile colloid silica nanocasting to house sulfur for lithium-sulfur batteries. Nitrogen doping could assist mesoporous carbon to suppress the shuttling phenomenon, possibly via an enhanced surface interaction between the basic nitrogen functionalities and polysulfide species. Nitrogen doping only within an appropriate level (4-8wt.%) can improve the electronic conductivity of the carbon matrix. Among three nitrogen species, pyridinic nitrogen atoms are responsible more for the enhanced surface interaction. At an optimal nitrogen content (8.1wt.%) and nitrogen species, the carbon/sulfur composites deliver a high sulfur utilization and good cycle stability. A initial discharge capacity of1140mAh g-1and reversible discharge capacity of828mAh g-1after100cycles were obtained at0.2C. A PPy/PEG hybrid polymer coating was further employed to suppress the polysulfide shuttle and form a stable interface between the liquid electrolyte and the sulfur cathode, allowing fast ion and charge transfer, thus improve the cycle stability (a reversible capacity of891mAh g-1 was obtained after100cycles at0.2C).(2) Nitrogen-enriched mesoporous carbon/La2O3/sulfur with high rate:Nitrogen-enriched mesoporous carbons decorated with ultrafine La2O3nanoparticles via a simple wet impregnation method were served as scaffolds to house sulfur for high rate lithium-sulfur batteries. The nitrogen species improve the dispersion of the La2O3nano-particles in the carbon framework. Apart from their on-site trapping of polysulfi des, the La2O3nano-particles decorated on the mesoporous carbon framework have a strong catalytic effect on sulfur reduction, offering fast electrochemical reaction kinetics. Combining the multiple effects of the well developed mesopores, nitrogen doping and La2O3nanoparticles, the resulting ternary NMC/La2O3/S nanocomposites delivere high discharge voltages and ultrahigh rate capacity. They maintain rate capacities of579and475mAh g-1at3C and5C, respectively, after100cycles.(3) H2S catalytic oxidation approach for carbon/sulfur cathode:H2S catalytic oxidation approach was developed to produce high performance carbon/sulfur cathodes. Nitrogen doping makes nitrogen enriched mesoporous carbons have a catalytic activity to directly oxidize H2S to elemental sulfur via the reaction H2S+1/2O2â†'S+H2O. Low temperature is favorable for highly efficient oxidation of HoS and improved oxiation selectivity. The sulfur content in the carbon/sulfur composite can be controlled by adjusting the reaction time. The catalytic-growth sulfur in situ deposite on the carbon framework via the layer-by-layer way and bind strongly with carbon framework. This intimate interaction provide efficient immobilization of sulfur and improve the electronic and ionic conductivity of insulated sulfur. Compared with melt-impregntion sulfur/carbon composites, the catalytic-growth sulfur/carbon cathodes exhibite higher utilization, better cycle stablity and rate performance. This composites delieve a initial capacity of1172mAh g-1, a reversible capacity of874mAh g-1after100cycles at0.2C and a rate capability of420mAh g-1at5C after80cycles.(4) Self-discharge method for Li2S/carbon microsphere with high safety:A self-discharge method was developed to facilely prelithiate sulfur/carbon microsphere using the cheap lithium metal foil as the lithium source for Li2S/carbon microsphere cathode with high performance. Furthermore, this Li2S/carbon microsphere cathode can be coupled with the commercialized C-Si alloy anode to form the lithium metal-free Li-S batteries with high safety. In the presence of electrolyte, the lithium metal can spontaneously react with sulfur encapsulated in the microporous carbon to form the Li2S which is completely encapsulated in the micropore of microsphere carbon. The assembled Li2S//C-Si full cells show excellent cycle stability. The discharge capacity is701mAh g-1at1st cycle and maintain583mAh g-1after90cycles. Compared with Li2S/irregular carbon composites, the Li2S/microsphere carbon cathode exhibits better electrode structural stability and electrochemical performance due to the advantages of microsphere morphology on buffering volume change during lithiation process.(5) Carbon@MoS2core-shell microspheres with hierarchical mesocrystal structures: Hierarchical carbon@MoS2core-shell microspheres were prepared via a facile hydrothermal-carbonization method for the first time to obtain high performance cathode for Li-S battery. Ultrathin MoS2nanosheets self-assemble on the nitrogen-enriched carbon microspheres, forming the hierarchical mesocrystal structure. The addition of basic melamine favors the formation of the microspheres and the coverage of high MoS2content. The nitrogen-enriched carbon cores and the cavities among self-assembled MoS2nanosheets can more effectively buffer the volume change during the electrochemical process and maintain the hierarchical mesocrystal structured Li2S-Mo cathode. In addition, the carbon cores also improve the electrical conduction. Ultrathin MoS2nanosheets and shell can provide fast Li+diffusion. Thus, carbon@MoS2core-shell microspheres show excellent electrochemical performance. At the current rate of100mA g-1, the composites are deeply discharged to0.01V and delieve a initial capacity of1054mAh g-1; Cycling the composites between1.00-3.00V after the above deep discharge (0.01V) afford a capacity of395mAh g-1and a reversible capacity of296mAh g-1after100cycles. A in-situ carbon coating was further employed to inhibit the Li2S loss and the side reaction of electrolyte with Li2S, improving the surface stability between the electrolyte and Li2S. In addition, carbon coating further enhance the electronic conductivity.