| Lithium-sulfur batteries have been paid considerable attention owing to the high theoretical energy density,and also recognized probably the most promising candidates for next-generation power source.Furthermore,the abundant reserve and widespread distribution of sulfur can ensure the premise and foundation for commercial application of lithium-sulfur batteries.Conductive carbonaceous materials are traditionally employed as electron transfer skeleton and are composited with sulfur,in order to solve the issue of insulating for sulfur and sulfides.In general,aluminum foil,as the current collector for lithium-sulfur batteries,is oxidized into Al2O3 layer with a thickness of 4-5 nm on the surface.On the one hand,the Al2O3 layer can prevent further oxidation of the Al foil,whereas on the other hand,it also impedes the electron transfer between Al foil and cathode active materials.Simultaneously,the active materials will crack and peel off from the current collector with high specific sulfur loading.In addition,during the charge and discharge process,the polysulfides can dissolve in electrolyte and form “shuttle”effect,which will lead to low utilization of sulfur,low capacity,and decrease the cyclic life of lithium-sulfur batteries.To cope with the above issues,this work includes three sections:First,the flexible cathode electrodes are employed as 3D current collector by using super-long CNTs synthesis through CVD method,so as to replace Al foil.The composites containing nitrogen doped graphene and sulfur element are sheared with super-long CNTs to form graphene@S-CNT free-standing cathode electrode.The free-standing cathode can be bent and applied to flexible devices.However,the abundant macroporous structure of the flexible electrodes can not limit the shuttle of the polysulfide.The facile calendering method has thus been proposed to tune the mechanical properties,bulk conductivity,and the pore distribution of the flexible graphene@S-CNT electrodes.The calendering process of the flexible electrodes to a large degree hindered the polysulfides shuttle effect and enhanced the capacity of lithium-sulfur batteries.Secondly,because the flexible cathode can only be used for some devices and electrode tabs,graphene/CNTs hybrids(GNHs)modifying layer can be regarded as an effective way to improve corresponding issue on the basis of original Al foil current collector.The GNHs were used for modifying interlayer between Al foil current collector and cathode electrode to form 3D conductive network,with a significant improvement in electrodes conductivity of 2.7 folds of that without GNHs layer.The lithium-sulfur batteries with GNHs layer exhibit an initial specific discharge capacity of 1113 mA h gs-1 and the corresponding sulfur utilization 67 % with sulfurloading 1.1 mg cm-2 and current density of 0.5 C(1 C=1675 m A h gs-1).The GNHs layer reduces the polarization phenomenon,and also enhances sulfur utilization as well as discharge capacity.Furthermore,the adhesion of cathode materials and current collector was enhanced,which was favorable for decreasing or even avoiding the falling off of cathode materials from the Al foil.Thirdly,modification by the carbonic materials on the surface of separator facing to the cathode side,is an efficient method to inhibit polysulfide shuttle.Porous graphene(PG)was used to modify polypropylene(PP)membrane,denoted as PG separator.The coin cells have a sulfur loading of 1.8-2.0 mg cm-2 and PG separators exhibited a very high sulfur utilization of 86.5% at0.05 C,a very low self-discharge rate of 90% retention,and an enhanced rate capability.In addition,the fabrication of PG-modified separators was scaled-up for assembling lithium-sulfur pouch cells(30×50 cm2).The pouch cells exhibited an initial capacity of 1135 mA h g-1 with an areal sulfur loading of 7.8 mg cm-2 at 0.1 C.Compared with the PP separator cells,the PG layer significantly enhanced the capacity and cyclic stability. |
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