| The high theoretical specific capacity(1675 mAh g-1),high theoretical energy density(2600 Wh kg-1),abundant reserves,low price and non-toxicity,elemental sulfur has become a new type of energy storage material.In this paper,a planar double-layer structure cathode is designed to improve the electrochemical performance of Li-S battery.Three kinds planar double-layer structure cathode were prepared by activated carbon?AC?,graphene?G?,spiral carbon nanotubes?SCNT?:AC/S-X?X=1,2,3,4,5?,G/S-G and S/AC/SCNT-AC.The crystal structure of the material was analyzed by X-ray diffractometer,and migration process of active substances in three kinds of cathode materials before and after the cycle was analyzed by Scanning Electron Microscopy and Energy Dispersive Spectroscopy.The surface area,pore volume and pore size distribution of AC were measured by nitrogen physical adsorption instrument.A series of performance tests of these three kinds of batteries,such as cycle performance,rate performance and impedance performance,were carried out by using Xinwei battery test system and electrochemical workstation.1.The AC/S-X planar double-layer structure cathode were prepared by AC and S in different proportions.It was found that polysulfide could be effectively adsorbed by activated carbon in layered structure.Performance tests showed that AC/S-1proportional batteries with a first discharge specific capacity of 1166 mAh g-1,after200 cycles at 0.1 C were retained at 793 mah g-1,with a capacity retention rate of 68%.This is because the cathode of AC/S-1 ratio provides a suitable pore content and specific surface area,which is beneficial to the storage of electrolyte and the inhibition of the dissolution of intermediate product in electrolyte.2.The G/S-G composite structure cathode was prepared by G and S.It is found that the polysulfides formed in the cycle attach to G,formed an effective interception mechanism.The tests show that the battery has a discharge capacity up to 501 mAh g-1 and a capacity retention rate of 65%,after 200 cycles of 0.5 C cycles.The layered properties and high conductivity of Graphene can physically intercept for soluble polysulfides and insoluble sulphides,improve the chemical reaction efficiency and reduce the loss of active substances.As a result,the battery exhibits good rate performance and cycle performance.3.The composite structural materials of S/AC/SCNT-AC were prepared by using S,AC and SCNT.The performance tests showed that the S/AC/SCNT and S/AC/SCNT-AC batteries showed good rate performance and Coulomb efficiency,with the initial discharge capacity of 1016 and 946 mAh g-1 at 0.2C,respectively,indicating that SCNT played a major role,its excellent conductivity and insertion throughout the cathode,provide an efficient electric charge transfer path for active materials.On the other hand,S/AC/SCNT-AC structured batteries show long cycle stability?0.11%capacity attenuation rate per cycle for 500 cycles at 0.5C?.It can be seen that the planar double-layer cathode can increase the electric charge conductivity,bound polysulfide,and improve the electrochemical performance.Through the comparison of the above three materials,it is found that:?1?the S/AC/SCNT ternary composites combine the adsorptive property of AC and the conductivity of SCNT,so they show better ratio performance and cycle performance;?2?as a cathode,the planar double-layer structure can effectively improve the cycle performance of the battery.It provides a reference solution to the problems faced for lithium-sulfur batteries.?3?the introduction of ternary composites into the planar double-layer structure,can further improve the electrochemical performance of the batteries by taking advantages of the two materials at the same time. |
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