| With the rapid development of portable electronic devices and wearable devices,the development of flexible secondary batteries is a hot topic in the field of energy storage.At present,lithium-ion batteries have been widely used in various electronic devices and smart fabrics.The functions of devices are becoming more and more complex and intelligent,and the energy density of lithium-ion batteries has been difficult to meet people’s needs.Therefore,it is urgent to develop flexible energy storage devices with higher energy density.Sulfur is one of the most abundant elements on earth,with an ultra-high theoretical capacity of 1675 m Ah g-1,taking into account the advantages of low price and environmental friendliness.The lithium-sulfur battery assembled with the lithium metal anode has a theoretical energy density of 2500 Wh kg-1,which is very promising as a new generation of high-energy-density energy storage devices.Its high capacity is due to the reversible combination of two electrons per sulfur atom to form lithium sulfide（Li2S）,however,the conversion reaction is accompanied by a volume change of about 80%.At the same time,the intermediate product of the reaction,lithium polysulfide,can be dissolved in the currently used ether electrolytes,which brings great challenges to the development of stable and efficient sulfur cathodes.Numerous efforts have been made to address these issues,developing novel cathode materials,such as mesoporous/microporous carbons,protective conductive polymer coatings,polysulfide-mediated inorganic materials,metal oxide organometallic frameworks,to promote Physical and chemical adsorption of active materials.However,with the increasing demand for flexible electronic products,flexible lithium-sulfur batteries have begun to attract the attention of researchers.Similar to flexible lithium-ion batteries,there have been many reports on flexible planar lithium-sulfur batteries,but they are basically around flexible planes.Lithium-sulfur batteries,there are few reports on fibrous lithium-sulfur batteries.In this paper,aiming at the problem of sulfur volume expansion and polysulfide shuttle during the charging and discharging process of lithium-sulfur batteries,starting from the structural design of the cathode,an integrated asymmetric porous cathode is designed through a simple phase separation method.The possibility of using it as an electrochemical energy storage material was verified under the two energy storage systems of supercapacitor and lithium-sulfur battery,and then it was evaluated in the field of lithium-sulfur battery.The structure can effectively alleviate the shuttle effect of polysulfides,and the internal porous structure can buffer the volume expansion of sulfur during charging and discharging,and a flexible fiber positive electrode is directly obtained by wet spinning.A high-sulfur-content fibrous lithium-sulfur battery with a current collector was obtained with a coaxial spinning needle.The main research contents of this paper are as follows:（1）.First,we explored the possibility of using a hierarchical porous asymmetric membrane obtained by non-solvent-induced phase separation as a battery material.The N,N-dimethylformamide（DMF）solution of polyacrylonitrile（PAN）was used as a casting solution.Membrane precursor solution,blade-coated on a glass plate,immersed in deionized water,then thoroughly dried,carbonized to obtain a graded porous membrane material（HPM）,and then polyaniline（PANI）was deposited in situ on the HPM surface to obtain HPM@PANI The composite film material realizes the effective combination of electric double layer material and pseudocapacitive material,which greatly improves the charge storage capacity of the material,and we find that when the dense film layer is close to the side of the separator,the long-term cycle performance of lithium-sulfur batteries has obvious.promotion,confirming the possibility of asymmetric hierarchical porous membranes as excellent cathode materials for lithium-sulfur batteries.（2）.On the basis of work 1,we abandoned the carbonization step and significantly improved the flexibility of the membrane on the premise of retaining the hierarchical porous asymmetric structure.Methylformamide（DMF）solution is used as the dispersion liquid,conductive carbon black（C）and carbon nanotubes（CNT）are used as the carrier of active material sulfur（S）,S and C and CNT are fully ground,melted and added to the above dispersion The positive electrode slurry is obtained after stirring evenly in the liquid,and then the aluminum foil coated with the positive electrode is directly immersed in deionized water to form a flexible positive electrode with an asymmetric structure,the surface of which is a dense porous layer,and the interior is a three-dimensional interconnected porous structure,This special structure can limit the shuttle effect of polysulfides while buffering the volume change of the positive electrode during charge and discharge,and realize the integrated design of the physical confinement separator and the three-dimensional interconnected porous structure.The prepared positive electrode shows a good performance.The electrochemical performance of 0.5C:after 500 charge-discharge cycles at a current density of 0.5C,there is still a specific discharge capacity of 706.8 m Ah g-1,and the capacity retention rate is as high as 74.5%.When the surface loading of sulfur was 3.7 mg cm-2,the reversible capacity of the battery could still reach 708.4 m Ah g-1 after 200 cycles.The electrodes were assembled into a commercial pouch battery,which remained stable even when bent.The LED strip can be stably lit.Further wet spinning method was used to directly inject the positive electrode slurry into the deionized water coagulation bath through a syringe,and a flexible fibrous positive electrode was obtained.The mixed fibers still retained the hierarchical porous asymmetric structure,which was assembled with lithium sheets to form a button battery.,a discharge capacity of 680 m Ah g-1 can be achieved at a current density of0.5 C,demonstrating the feasibility of this wet-spun fiber as a cathode for lithium-sulfur batteries.Then we used aluminum wire as the current collector,immersed it in the positive electrode slurry,took it out and quickly immersed it in water,the positive electrode slurry formed a uniform whole and tightly wrapped on the aluminum wire,while maintaining the outer surface dense layer and the internal three-dimensional interconnected porous structure,and abandoning the commercial separator,using the same phase separation principle to form a porous PAN film on the outside of the positive electrode layer in situ,with lithium-plated copper foil and heat-shrinkable polyethylene tube,successfully assembled a fibrous lithium-sulfur battery,at 0.1C At a high current density,a discharge capacity of 821.9 m Ah g-1 is achieved,and it is worth noting that a single fiber battery can continuously supply power for more than 6 hours for a light strip with more than 80 light-emitting diodes.Due to the rough packaging process,leakage occurs,which makes the battery electrochemical The performance cannot be compared with the button battery,but it still provides ideas for the application of lithium-sulfur batteries in wearable devices.In order to further improve the energy density of fibrous lithium-sulfur batteries and enable them to power wearable devices for a long time,we used a coaxial spinning strategy to simultaneously extrude the cathode slurry and PAN solution to obtain ultra-long current collector-free fibers The lithium-sulfur battery cathode,the integrated design of the cathode and the separator can not only reduce the relative displacement of the cathode and the separator during the bending process of the battery,but also the all-round wrapping of the separator also protects the cathode,ensuring that the active material and the close contact between current collectors.This ingenious design can greatly increase the content of active materials,improve the problem of low energy density of fiber batteries,and provide an idea for high energy density fibrous lithium-sulfur batteries. |
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