Polyimide/Multi-layered Graphene Nanostructure Composites:Preparation And Electrochemical Lithium Storage Performance

Since the 21st century,in order to prevent the further expansion of the energy crisis and environmental pollution,scientists have been committed to the development of a new type energy storage system with environmental benignity.Among the various clean energy systems,lithium-ion battery?LIB?is gradually playing the dominant role in the energy family and becoming an inseparable part in people's normal life due to it's high energy density,high operating voltage,rare memory effect,and long cycle life.However,the anode graphite in conventional lithium ion batteries has only theoretical capacity of 372 m Ah g^-1,which cannot meet the requirements of high energy density and long-term battery life.Meanwhile,with the scale of battery industries increased,people are beginnig to worry about environmental pollution caused by the lack of inorganic materials.Different from conventional inorganic materials,organic polymer electrode materials will be applied to the next-generation"green high-energy power source"due to their green sustainability.Polyimide?PI?is an important type of polymer materials,and its main chain usually contains imide rings,which is synthesized from equimolar organic diamine and organic dianhydride by melt condensation or solution condensation.As a typical carbonyl polymer,polyimide material has made great progress in the application of electrode materials,but its low specific capacity(<300 mAh g^-1)limits its further application.Hence,in this article,we have designed and developed two kinds of polyimide-multi-layered graphene?PI-MG?composites and explored the superlithiation phenomenon of polyimides under specific conditions.The details of this dissertation are as follows:?1?High performance PI-MG composite electrode material was designed and synthesized by co-precipitation and thermal imidization.The PI-MG composite material has a layer-by-layer structure,where the surface of the multilayer graphene is evenly coated with a layer of PI?the thickness is about 5.12 nm?.This composite material exhibited excellent specific capacity(612 mAh g^-1;0.1 A g^-1)when used as anode for LIBs,as well as superior cycle stability(89.3%retention capacity after 500cycles of 0.5 A g^-1).Moreover,the battery can operate stably from-15 to 55°C with a capacity of 884 mAh g^-11 at 55°C,which is greater than major values reported in previous literatures.Subsequently,we proved that BPDA-BPA-based polyimide can electrochemically react with 22 lithium ions and obtain theoretical capacity of 1340mAh g^-1through comparison of experimental data and theoretical calculations.?2?Although the composite prepared by the coprecipitation method has excellent specific capacity,its rate performance is far from satisfactory.The discharge capacity is only 143 m Ah g^-11 at current density of 5.0 A g^-1.Therefore,a porous foam-like material was successfully prepared through a gentle ethanol gradual precipitation method,which will contribute to electrolyte penetration and lithium ion transport.It is shown that the material has a porous structure and a high specific surface area(48.3m² g^-1).When the current density is 0.1 A g^-1,this porous PI-MG composite has higher reversible specific capacity(647 mAh g^-1),and the reversible specific capacity was411 mAh g^-11 when current density reaches 5.0 A g^-1,which was much higher than that of the coprecipitation method.The reason why the porous PI-MG composite have such excellent rate performance is mainly because the porous structure is more conducive to the rapid diffusion of lithium ions and reaction with the active material.