| As a kind of lithium battery using sulfur as the positive electrode,Li-S batteries have ultra-high theoretical energy density(2600 Wh·kg-1)and theoretical specific capacity(1675 mAh·g-1),which are higher than the traditional Li-ion batteries.In addition,sulfur is a common element in nature with abundant reserves,and the low prices as well as environmentally friendly properties indicate the broad development of Li-S batteries.However,there are still many problems in the practical application of Li-S batteries.For example,the poor electrical conductivity of sulfur and polysulfides formed during discharging process;the large volume difference of products during charging and discharging;the shuttle effect of polysulfide intermediates;the dendrite problem of lithium anodes,et al.The shuttle effect and dendrite problems of Li-S batteries can be greatly alleviated by modifying the separator or adding functional coatings.At the same time,the utilization of sulfur cathode can be effectively improved.Carbon materials receive wide attentions in various separator coating materials because of their structural diversities,high electrical conductivity,light weight and other advantages.In the first chapter of this thesis,research progress in different carbon materials and their composite material as separator coatings is summarized based on the dimensions of materials.From the perspective of commercial applications of Li-S batteries,the cost of coating materials,the difficulty of preparation,the loading amount,and performance improvement towards to the battery are important factors that require comprehensive considerations.Based on the same battery preparation conditions in the laboratory,the third chapter of this thesis systematically studied the electrochemical performance of three different dimensions of commercial carbon materials(zero-dimensional carbon nanoparticles(0D CNPs),one-dimensional carbon nanotubes(1D CNTs)and two-dimensional graphene nanosheets(2D GNSs))as the separator coating materials of Li-S batteries,providing a guidance for the design of the structure of the separator coatings.To further decrease the loading amount of coatings,the hollow carbon nanomaterials were prepared and systematically as the separator coating for Li-S batteries in the fourth chapter.The conclusions are as follows:(1)Commercial CNPs,CNTs and GNSs were loaded on the commercial separators by vacuum filtration.By controlling different loading amount,the effects of different carbon structure and coating mass density on the electrochemical performance of the battery were studied.At a mass loading of 0.1 mg·cm-2,CNPs,CNTs and GNSs separators significantly improved the performance of the battery.With the increasing of the loading amount,the performances of the batteries were greatly improved.The improvement effect of the three structural materials at 0.2 C is similar,however,0D CNPs shows good high rate performance.At the rate of 3 C,the reversible capacity of CNPs could reach 631 m Ah·g-1.Combined with cost,bulk density and battery performances,0D CNPs exhibit more advantages in application.(2)To further reduce the loading amount of coatings,hollow carbon spheres were prepared by chemical vapor deposition using nanometer MgO as template.The hollow carbon spheres have lighter weight and higher specific surface area as a result of the hollow structure.Under the same mass loading amount of 0.1 mg·cm-2,the specific capacity,cycle stability and rate performance of hollow carbon spheres were much higher than those of CNPs.When the loading amount was further reduced to 0.025mg·cm-2,the electrochemical performance was still much higher than the commercial separator.These results proved the superiority of the hollow zero-dimensional structure,which is instructive to the development of high-performance lithium-sulfur batteries. |
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