Synthesis And Lithium-Sulfur Performance Of Polymer/Carbon Nanotube Composites As Battery Cathodes

Higher requirements for a new generation of energy storage devices had been put forward by technological progress and the rapid development of electric vehicles.Due to the capacity limitation of cathode materials,conventional Li-ion batteries cannot supply applications with high energy density.Lithium-sulfur（Li-S）batteries are high specific energy storage devices that are based on high-capacity sulfur cathodes(1675 m Ah g^-1)and are considered to be a promising alternative to traditional Li-ion batteries due to their ultra-high energy density(2600 Wh kg^-1).While the abundant resources of elemental sulfur and low costs are also expected to promote the large-scale application of Li-S batteries.Nevertheless,the insulating property of sulfur leads to the improvement of the rate performance of Li-S batteries.The lithium polysulfides generated during the charging and discharging process are easily dissolved in the organic electrolyte,which will result in a severe shuttle effect and lead to poor cycle stability.The large volume change of sulfur cathodes during charge and discharge also leads to rapid capacity fading.In order to improve the conductivity of sulfur cathodes and suppress the shuttle effect,the usual strategy is to introduce support materials to design sulfur-based composite cathodes.However,the non-polar surface of the carbon material supports is difficult to restrict the shuttle of polar lithium polysulfides,and the polar metal compound supports have low electronic conductivity.Therefore,exploring sulfur support materials with both strong polysulfide capture ability and fast electron transferability is still the key to developing high-performance sulfur cathodes.Polymers have attracted extensive attention in lithium-sulfur batteries due to their abundant sources,light-weight,and strong structural design ability.Designing and synthesizing polymers with polar functional groups can significantly inhibit the dissolution and shuttling of lithium polysulfides,and the flexible frame of the polymer carrier can also effectively buffer the volume expansion of sulfur during discharge/charge.In addition,polymers can not only serve as sulfur support materials but also some sulfur-containing polymers can directly serve as active materials.However,most polymers have electrical insulating properties,and the composite of functional polymers and conductive carbons is an ideal strategy to improve the electronic conductivity of composite cathodes.In order to develop polymer-based sulfur cathodes with a high specific capacity,excellent rate performance and cycling stability,the following work is carried out from the molecular design of polymer supports and conductive carbon composites:（1）Carbon nanotube-supported polyimide（PI）nanoarrays（PI@CNT）were synthesized by in situ hydrothermal polymerizations with ethylenediamine and pyromellitic dianhydride and used as sulfur carriers while introducing an additional conductive network of carbon nanotubes（CNTs）to obtain PI@CNT/CNT/S composites.PI@CNT exposes sufficient sulfur loading sites due to its large specific surface area,which is beneficial to achieve high sulfur loading.Polyimide enhances the capture of lithium polysulfide and inhibits its dissolution and shuttling through polar Li-N bond interaction.In addition,the PI@CNT/CNT/S three-dimensional network structure can also inhibit the shuttle effect through the physical confinement of lithium polysulfide and buffer the volume expansion of sulfur.The CNTs network forms a continuous charge conduction path,improving its rate capability.The PI@CNT/CNT/S cathode shows a high sulfur loading of 70 wt.%and exhibits capacity retention of 67%after 300 cycles at 1 C(1 C=1675 m A g^-1),which exhibits stable cycling performance.It also shows a reversible capacity of 652 m Ah g^-1 at 0.1 C and capacity retention of 54%at 2 C.（2）Polyacrylonitrile arrays（PAN@CNT）were grown on the surface of carbon nanotubes by in-situ radical polymerization with acrylonitrile as monomer and azobisisobutyronitrile as initiator,and then mixed with sulfur powder and co-heated to obtain sulfurized polyacrylonitrile/carbon nanotube（SPAN@CNT）.The microscopic morphology and sulfur loading mechanism of SPAN@CNT were explored by adjusting the polymerization time and sulfurization time.The results show that the polyacrylonitrile array morphology on the surface of CNTs is more conducive to the exposure of adsorption sites and achieves higher sulfur loading.The short-chain sulfur is fixed in the polyacrylonitrile chain through the C-S bond,which fundamentally avoids the generation of lithium polysulfide and greatly improves the cycle stability.The introduction of CNTs enhanced the electrical conductivity of the composites.The SPAN@CNT with a polymerization time of 4 h and a sulfurization time of 7h exhibits the best performance,with a reversible capacity as high as 1329 m Ah g^-1 at 0.1 A g^-1,and specific capacity retention rate was 70.1%at 5 A g^-1.It also exhibits a reversible capacity of 929 m Ah g^-1 at 10 A g^-1 with a specific capacity retention of 51.7%after 200cycles.