Fabrication Of Polyimide And Its Composites With Graphene For Lithium-ion Batteries

Rechargeable lithium-ion batteries have dominated the market of portable electronics,even applied on vehicles owing to their high working voltage,high energy density,long cycling life and low self-discharge rate.Traditional cathode materials are lithium-contained transition-metal oxides,which possess the low theoretical capacity(less than 200 mAh/g),and inorganic compounds are nonrenewable resources.So,it’s urgent to explore new materials with low cost,abundant resources and high capacity as alternatives to inorganic materials.Organic polymers have been considered as promising replacers to traditional inorganic electrode materials because of their high capacities,designable structure,low cost and abundant resources.The charge transfer reaction-based organic electroactive material is more advantageous for achieving fast charge and discharge capability than transition metal oxide electrodes based on the insertion/extraction mechanism with sluggish kinetics.In spite of the capacity decay brought by the dissolution of organic small molecule compounds in the organic electrolyte,it is expected to suppress the dissolution of active materials and promote the cycling stability by the polymerization of small molecules to increase the molecular weight.Among various electroactive polymers,carbonyl-containing polyimide is the most comprehensive engineering plastic with high theoretical capacity,high electrochemical activity,excellent mechanical and thermal stability,and is very suitable for new lithium-ion battery cathode with high energy density,excellent cycle stability and high safety.Based on the structural design of organic polymers,the capacity,potential,and cycle life of polyimide can be improved by changing the linking groups between imide structural unit,the number of reactive carbonyl groups,the substituent groups and the topology.However,the main problem of polyimide positive electrode is unsatisfied practical capacity resulted from the low carbonyl group utilization.Besides,the electrical insulation of polyimide also leads to the poor rate capability.It is expected to promote ion conductivity and carbonyl utilization through proper molecular designing.And incorporation of conductive carbon materials is the main strategy to improve its electronic conductivity of polyimides.Based on these,the effects of different molecular structures on the electrochemical performances of polyimides were studied by the regulation of diamine linkages in this paper.At the same time,conductive graphene was introduced as a substrate for the growth of polyimide nanosheet arrays,and the effects of graphene content and nanostructures on the electronic conductivity and carbonyl utilization were investigated.The specific research contents are as follows:(1)Four PI samples were fabricated by using 1,4,5,8-naphthalenetetracarboxylic dianhydride and different diamine as monomers through in-situ hydrothermal polymerization.The products are named PI-1,PI-2,PI-3 and PI-4,corresponding to ethylene diamine,para-phenylenediamine,1,4-butanediamine and 1,10-diaminodecane,respectively.Morphology images show that PI-1 and PI-3 exhibit a sheet-like honeycomb structure,and the sheet structure of PI-4 is severely stacked,while PI-2 does not exhibit a regular morphology due to the existence of rigid structure.When used as LIBs cathodes,PI-1 shows the highest reversible capacity(132 mAh/g at 50 mA/g)and excellent cycle stability with a capacity retention as high as 94% after 3000 cycles at 1000 mA/g.Although PI-4 shows a higher retention rate(96%),its practical specific capacity is relatively lower due to the presence of large molecular weight structural units.(2)Graphene/polyimide composites(GPI)with different graphene contents were synthesized by in-situ hydrothermal polymerization.The morphology images show that the polyimide nanosheets grow perpendicular on the surface of graphene with an average thickness of about 10 nm.It is worth mentioning that the graphene is reduced in the hydrothermal polymerization process,which further increases the electronic conductivity of the composite.Meanwhile,the increase of graphene content provides more growth sites for polyimide nanosheet array.Thus more active carbonyl groups are exposed to electrolyte,which can significantly improve the carbonyl utilization rate.Besides,the graphene skeleton also provides a robust structural stability.The polyimide composite with 20 wt.% rGO content(GPI-c)exhibits the largest capacity and carbonyl utilization(232 mAh/g at 50 mA/g,64%compared to the theoretical capacity)with the best cycling stability(85% capacity retention after 200 cycles at 100 mA/g).At the same time,the vertical nanosheet array channel provides more paths for lithium-ion transfer,so GPI-c possesses the largest lithium-ion diffusion coefficient.