Preparation Of Crosslinked Porous Polyimide Material And Its Application In Lithium-sulfur Batteries

Lithium-sulfur batteries（LSBs）have captured tremendous attention because of their high energy densities,low raw material cost,excellent environmental compatibility.However,LSBs remain plagued by some challenges such as large volume change upon lithiation/de-lithiation processes,shuttle effect of soluble lithium polysulfides,and sluggish redox kinetics during discharge and charge processes.Porous polyimides（PPIs）with excellent pore structure parameters,abundant nitrogen and oxygen heteroatoms and good chemical stability have potential application as cathode hosts in LSBs.Porous polyimides are prepared by polycondensation reaction between multifunctional monomers such as polyamines and polyanhydrides.However,low production,complex synthesis process,expensive monomer and polar solvent with high boiling point limit the application of PPIs.To solve above issues,we used N,N’-1,4-Phenylenedimaleimide as building unit,optimized synthesis of PPI by regulating conditions of polymerization reaction,and investigated their electrochemical performance as cathode hosts for LSBs.Then,we prepared a new type of composites with better pore structures and conductivity through in situ polymerization of multi-walled carbon nanotubes and N,N’-1,4-Phenylenedimaleimide.The strategy can further improve electrochemical performance of as-prepared LSBs.The exploration is as follows:Firstly,controllable preparation of PPI.The bismaleimide-based porous polyimides were prepared from N,N’-1,4-Phenylenedimaleimide by the thermal initiated radical polymerization under reaction condition without sealing tubes.Then,we investigate the relationship between polymerization reaction time or monomer content and pore structure parameters of prepared PPI.The results demonstrate that at the initial stage of polymerization,BET surface areas and pore volumes of PPI increase with the time and reach the maximum value at 10 h,then the pore structure parameters of PPI stay stable.The polymerization time is controlled unchanged,the BET surface areas and pore volumes of PPI ascend firstly and then decrease with the increase of monomer content.When the monomer content is 1%,PPI show the highest BET surface areas and pore volumes.Secondly,performance studies of PPI used as sulfur host for LSBs.X-ray photoelectron spectroscopy and visualized adsorption test of samples for Li₂S₆ show PPI-1-10 have strong chemical adsorption interaction with polysulfides.PPI-1-10 with highest BET surface areas(857 m² g^-1)and total pore volumes(0.533 cm³ g^-1)among PPI at different synthetic conditions was selected as cathode host.The S/PPI-1-10 and S/AB composites were prepared by melt diffusion method.Then the composites were used to assemble cell.Compared with S/AB,S/PPI-1-10 shows higher rate capacity and discharge specific capacity.S/PPI-1-10 electrode delivers a highly reversible discharge specific capacity of 400 m A h g^-1 at 1 C after 500 cycles,with a capacity fading rate of0.034%.Finally,preparation of PPI@MWCNT composite and its performance as cathode host for LSBs.N,N’-1,4-Phenylenedimaleimide and multi-walled carbon nanotubes（MWCNTs）with different content were used to prepare PPI@MWCNT-23,PPI@MWCNT-38,PPI@MWCNT-47 through in situ polymerization.The results of nitrogen adsorption test show that compared with PPI-1-10,PPI@MWCNT composites show more mesoporous structures and higher pore volumes.With the increase of content of MWCNTs in the process of polymerization,the specific surface areas of PPI@MWCNT gradually decreased and the pore volumes gradually increased.The S/PPI-1-10,S/PPI@MWCNT-38 and S/MWCNTs composites was prepared by melt diffusion method.Then these composites were used to assemble cells.Among these cathodes,S/PPIs@MWCNTs-38 cathodes deliver highest discharge specific capacity of 1338 m A h g^-1 at 1 C.After 100 cycles,S/PPI@MWCNT-38 cathodes still stay discharge specific capacity of 865 m A h g^-1.Increasing of performance of cathodes is related to unique composition and structure of PPI@MWCNT-38:Polar PPI can provide polar chemical adsorption sites and micropore structures,which suppress shuttle effect of polysulfides and promote the nucleation and growth of Li₂S.MWCNTs can improve conductivity of composites and promote electrons transfer at the process of electrochemical oxidation of Li₂S.Compared with PPI,PPI@MWCNT-38 shows more mesoporous structure,which is beneficial for diffusion of lithium ions.The synergy of these effects makes the battery show higher discharge specific capacity,rate performance and cycling performance.