Preparation Of PEG-based Comb Copolymer Electrolyte For Lithium Ion Battery And Its Electrochemical Properties

Polymers with ether-based groups,like polyethylene oxide(PEO),were still selected for their promising performances in polymer electrolyte.It is a known fact that lithium salt can be dissolved into PEO matrix because of the formation of complexes between Li+ and ether oxygen atoms in PEO chains.The transport of Li+ ion in PEO matrix is via the movement of flexible PEO molecule chains.So,it is believed that only the amorphous phase gives rise to Li+ ion's transportation.However,PEO is a semicrystalline polymer.Most PEO chains are in crystalline form at room temperature.So,preventing the flexible PEO molecule chains to crystallize is the only way to enhance the ion conductivity of SPE at room temperature.Besides,the low crystalline and flexible PEO molecule chains result in the low mechanical properties of the electrolyte.In order to obtain a strong self-standing film,polyethylene glycol often blended with the rigid polymer or copolymer to obtain random,block,star or comb polymers which will meet the strength required for the assembly process.After detailed analyzing the ion transfer mechanism and summarizing the previous works,we found that keeping PEO molecules as the side chain and cross-linking the main chain might be a possible way to solve the problem.In our work,a comb like copolymer with short PEO molecule chains,which will not crystallize at room temperature,and polyolefin main chains containing unsaturated C=C double bonds will be prepared through Ring Opening Metathesis Polymerization(ROMP)method.This character makes the electrolyte be easy processed at room temperature and cross-linked at elevated temperature to get good mechanical properties.The polymer was mixed with lithium perchlorate.The SPE films are obtained via casting the mixed solution into a PTFE mold.After evaporation of solvent at room temperature,the electrolyte will be heated to over 100 ? to form a cross-linking membrane.We synthesized a compound according to the literature.The compound is based on TDI as the crosslinking agent which attached 5-hydroxy-1-cyclooctene to polyethylene glycol.We named this compound macromolecules.The second generation Grubbs catalyst was taken to catalysis the polymerization of 5-hydroxy-1-cyclooctene and macromonomer to form the comb polymer.A comb like copolymer with short PEO molecule chains(PEG Mn<1000g/mol),which will not crystallize at room temperature,and polyolefin main chains containing unsaturated C=C double bonds will be prepared through Ring Opening Metathesis Polymerization(ROMP)method.Existed unsaturated C=C double bonds which are stable at room temperature can be cross-linked at elevated temperature(80 ?).This character makes the electrolyte be easy processed at room temperature and cross-linked at elevated temperature to get excellent mechanical properties.The content of PEG in the electrolyte can be easily controlled via changing either the length or the density of PEG molecules along the main chain.1H NMR was taken to characterize the structure of the macromonomer.FTIR was taken to characterize the successful synthetic of polymer.Raman spectroscopy was taken to calculate the amount of the actual amount of 5-hydroxy-1-cyclooctene in the process of polymerization reaction.1H NMR was taken to characterize the polymer which did not participate in the film forming process in electrolyte film-forming.Discuss the change of the polymerization time,the amount of catalyst,the chain length,the density of the chain and the content of the lithium salt which will control the film-forming of the polymer during the experiment.Then polarizing microscope and DSC data were taken to characterize the situation of the crystalline polymer,the results were the comb polymers do not crystallize.The ionic conductivities of different SPE films are measured using an AC impedance analyzer with stainless steel electrode over a frequency range from 0.1 to 100 MHz.Among all obtained polymer electrolyte,this paper discusses the influence of various factors on the ionic conductivity.Such as the amount of lithium salt(LiClO4 or LiPF6),short PEO molecule chains(Mn =350 g/mol,550 g/mol,750 g/mol).The effect of Li salt concentration,PEG chain length and density,Lewis acid content and ultasonicating conditions on the ion conductivity and cyclic life of the polymer electrolyte and Li ion battery will be fully investigate.The result shows that with the increase of the lithium salt content,the ionic conductivity increased first and then decreased,when lithium salt content was 20 wt%,the maximum conductivity was 10-4 S/cm.In same molecular weight PEG,when the PEG content was 35?55 wt%,the ionic conductivity increased after the content got larger.The content of PEG cannot be increased further for rewarding a self-standing film.In the similar content of PEG,the bigger density of PEG is,the higher ionic conductivity it will get.In all short PEO molecule chains,Mn =550 g/mol gets the highest conductivity.In discussing the ionic conductivity of the polymer electrolyte film,we considered the relationship between the mechanical strength of the electrolyte membrane and the ionic conductivity.The results showed that,when the higher the tensile strength of the film was,the smaller the ionic conductivity would be.The conventional SPE is the relatively low Li+ transport number.As the Li+/polymer complex will be formed when lithium salt dissociate into polymer electrolyte,the free anion mobility along the polymer chains is normally higher than the cation's complex,which will reduces the Li+ transport number and cause electrode polarization and deterioration.Although Li+transport numbers may be improved by choosing salts with larger organic anions and huge electron delocalization.Lewis acids are believed to interact with the anion of the lithium salt and form anion complexes,which leads to the enhancement of the Li+transport number in the polymer electrolyte.To enhance the ionic conductivity of the solid electrolyte membrane further,this paper aims to add nano silica,and small molecular weight polyethylene glycol dimethyl ether.Spreading the nanoparticles in electrodes(both cathode and anode)into cross-linked SPE film surface via ultrasonication method which will improve the solid-solid interface stable between SPE and electrode.The results have shown that the ionic conductivity has greatly improved.