Design Of Sandwiched Cathodes For High-performance Lithium-sulfur Battery

Lithium-sulfur batteries(LSBs)have received more and more attention due to their high theoretical specific capacity(1675 mA h g-1),high energy density(2600 Wh kg-1),environmental friendliness,and low cost.However,a series of problems such as volume expansion of sulfur electrode during the cycle,low conductivity of the final charge and discharge products(S and Li2S2/Li2S),and shuttle of polysulfides have restricted the practical application of lithium-sulfur batteries.In order to address these problems,in this thesis,the self-supported film of carbon fiber cloth modified with CoS2 nanoparticles(CC-CoS2)is synthezied and employed for high-performance LSBs.On one hand,the 3D porous CC-CoS2 film serves as a multifunctional interlayer that exhibits efficient physical blocking,strong chemisorption and fast catalytic redox reaction kinetics towards soluble poly sulfides.On the other hand,the CC-CoS2 film works as a robust 3D current collector and host with high conductivity,high sulfur loading and high capability of capturing polysulfides.The effects of CC-CoS2 as an interlayer and cathode current collector on the performance of LSBs were studied respectively.In addition,CC-CoS2 was used as a current collector and an interlayer at the same time to fabricate a sandwich structure cathode,which helps to improve the performance of high sulfur loading LSBs.The main research contents of this dissertation are as follows:(1)A simple one-step hydrothermal route was developed to prepare CC-CoS2 film,where the CoS2 nanoparticles are uniformly grown on the surface of carbon fibers.And the ploysulfide adsorption test and H tank experiment reveal that CoS2 has a strong adsorption capacity for polysulfides,which can effectively prevent the migration of polysulfides.(2)The CC-CoS2 film is employed as a multifunctional interlayer of LSBs.The Al@S/AB@CC-CoS2 cell with a sulfur loading density of 1.2 mg cm-2 has a high initial discharge specific capacity of 1488 mAh g-1 at 0.5 C;the cell remains a capacity of 610 mA h g-1 after 1000 cycles at 4C,and the average capacity decay rate per cycle is only 0.021%,showing excellent cycle stability.(3)A sandwiched structure cathode of CC-CoS2@S/AB@CC-CoS2 is designed for high sulfur loading LSBs,where CC-CoS2 is used as both the current collector and the interlayer at the same time.The CC-CoS2@S/AB@CC-CoS2 cell with a sulfur loading of 3 mg cm-2 yields the the discharge specific capacity of 1395 mA h g-1 at 0.1 C.When the sulfur loading is as high as 4.2 and 6.1 mg cm-2,the cell with the sandwiched structure cathode exhibits outstanding cycling stability,and has no capacity degradation after 100 cycles at 0.2 C.In conclusion,CC-CoS2 composite material was successfully prepared and used as a current collector and multifunctional interlayer to improve the electrochemical performance of lithium-sulfur batteries.The experimental results show that this strategy provides a new idea for the development of high-performance lithium-sulfur batteries.