| Lithium-sulfur batteries(LSBs)have been widely studied because of their high theoretical energy density,low cost and non-toxicity of sulfur.However,practical applications of LSBs are hindered by several obstacles,such as low utilization of sulfur,poor cycle performance for sulfur cathode,serious safety problem associated with Li metal anode and so on.The primarily promising approach toward better performance of sulfur cathode is to utilize the physical/chemical interactions between hosting materials(such as carbon,polar oxide)and the lithium polysulfide species to minimize shuttle effect.However,it is difficult to achieve high utilization of sulfur and stable cycle performances of the cathode simultaneously by this strategy.Recently,Li selective deposition method opened up a new avenue for promoting high performance Li anode,However,high cost of maerials(Au)utilized in this method may restrict its practical application.In this thesis,a solid electrolyte interface(SEI)layer was generated on cathode,separator and anode via electrochemical control and interface design to address the aforementioned problems;this SEI layer can realize encapsulation of lithium polysulfide species and selective deposition of Li.and thus enable cycle stability,high rate performance,large energy density and safety of LSBs simultaneously.The main contents and results of this thesis are summarized as follows:(1)SEI encapsulation method for "shuttle effect" suppression.An SEI layer was generated on porous carbon spheres/sulfur composite cathode,and then the morphology,utilization of sulfur and cycle performances of SEI wrapped cathode were studied.The results show that an SEI layer(8?10 nm)can be synthesized on porous carbon spheres by charging-discharging carbon spheres/sulfur composite cathode a few cycles at 0.3?1.5 V;it can imprison lithium polysulfide and the pre-existing electrolyte within the porous carbon spheres.The SEI layer can not be constructed by charging-discharging the cathode at 1.5?3 V.The SEI wrapped cathode exhibits high utilization of sulfur(1205 mAh/g at 0.2 C)and stable cycle performance(75%capacity retention after 400 cycles at 0.5 C),suggesting that the SEI based encapsulation method can effectively improve the cycle performance of the carbon/sulfur cathode.(2)Improvement of SEI encapsulation strategy.In SEI wrapped porous carbon spheres/sulfur composite cathode,electrons inevitably travel through the high-resistance SEI layer,and then slightly affect the rate performance and Coulombic efficiency of the cathode.To address this problem,an SEI wrapped CNT/sulfur composite array cathode was prepared,allowing electron transport from the current collector to the CNTs without the need to pass the SEI layer.The SEI wrapped CNT/sulfur array cathode demonstrates a high capacity of 1275 mAh/g at 0.2 C,with 81.4%capacity retention after 200 cycles;its capacity reaches 1205,824,631 and 454 mAh/g at 0.2,0.5,1 and 2 C respectively,and the Coulombic efficiency maintains at-99%after 4 activation cycles.These results indicate that the SEI encapsulated CNT array strategy can enable utilization of sulfur.Coulombic efficiency,cycle stability and high rate performance of sulfur cathode simultaneously.(3)SEI based strategy for improving sulfur loading of cathode.An SEI layer was constructed on the surface of carbon-coated polypropylene separator,which was used together with a nano-array structured cathode for cell assembly.Morphology,electrochemical performance and permeability of the SEI modified separator were studied systematically.It is shown that the SEI modified separator has a dense blocking layer compared with that modified by carbon black,hollow carbon sphere and Li4Ti5O12 layer.The SEI layer on the separator can confine the dissolved polysulfide between the cathode and the separator more effectively,insuring the cycling stability of cathode with large areal sulfur loading.At the same time,the carbon nano-tube(CNT)array in the cathode can facilitate electron transport,insuring high sulfur utilization and rate performance.With the synergistic effect of SEI modified separator and CNT cathode structure,the LSBs with an areal sulfur loading as high as 10 mg/cm2 can give a high rate performance(1221 mAh/g or 12.2 mAh/cm2,1096 mAh/g or 10.9 mAh/cm2,929 mAh/g or 9.3 mAh/cm2,752 mAh/g or 7.5 mAh/cm2,400 mAh/g or 4 mAh/cm2 at 0.1 C,0.2 C,0.5 C,1 C and 2 C)and cycle performance(88%capacity retention after 100 cycles at 0.5 C).This novel strategy may open up a new avenue for the facile construction of high-performance sulfur cathodes with large areal loading.(4)SEI enabled high performance Li metal anode.An SEI layer was generated on the surface of CNT array,resulting in different conductivities at inner-and outer surface of CNT.The SEI wrapped CNT structure results in selective electron transport along the CNT to/from the current collector,rather than through the SEI layer so that Lithium selectively deposit on inner CNTs and thus dendrite formation is suppressed.The lithiated,SEI wrapped CNT anode shows good safety(stable cycles>150 h)and high utilization of Li(Coulombic efficiency?98%).The LSBs using SEI wrapped CNT array andoe(3 mAh/cm2 Li)and porous carbon sphere/sulfur composite cathode exhibits a capacity of 837 mAh/g after 100 cycles(59%retention)and a high Coulombic efficiency(-97%),which is similar to that of LSB using commercial Li foil anode,indicating the good application of SEI wrapped CNT array anode in LSBs.Differing from the reported selective deposition strategy,this method is simple,economical,effective and may open up a new avenue for high performance Li metal anode. |
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