Multi-imidazolium Ionic Liquid Norbornene Block Copolymer Solid Polyelectrolyte Membrane And Lithium Ion Battereries Performance

Lithium-ion batteries have received widespread attention due to their high energy density,high working voltage,good cycle performance,and light weight,and are widely used in portable electronic products and electric vehicles.However,the liquid electrolyte used in conventional lithium batteries has the safety risks of leakage and flammability and explosion,and solid polymer electrolytes have become the first choice for a new generation of electrolytes.Polymer electrolyte has the advantages of good film formation,easy processing and high safety,which can avoid liquid electrolyte leakage,fire,explosion and other safety problems,and has become the research focus of a new generation of lithium-ion batteries.As a new type of electrolyte,polymer ionic liquid electrolyte not only maintains the advantages of ionic liquid high conductivity,non-flammability,high thermal stability,etc.,but also has the convenience of polymer processing.In this paper,aiming at the lower room temperature ionic conductivity of solid polymer electrolytes,a norbornene-based ionic liquid with multiple imidazole cations is designed.Block copolymers are prepared by ring-opening metathesis polymerization（ROMP）.The block copolymers are combined with lithium bistrifluoromethanesulfonimide（LiTFSI）to prepare a solid polymer electrolyte membrane.In order to further improve the conductivity and thermal stability of the solid polymer electrolyte membrane,it is designed to increase the content of imidazole ionic liquid and the rigidity of the main chain.The specific research content is as follows:（1）Bis-imidazole functionalized norbornene ionic liquid monomers（NB-O-Im⁺Br^--Im⁺Br^-）and 5-norbornene-2-methylhexane monomers（NB-O-Hex）were synthesized,and ROMP block copolymers rP（NB-O-Hex-b-NB-O-Im⁺TFSI^--Im⁺TFSI^-）-x%（x=10,20,30,40）were synthesized using Grubbs’2^ndcatalyst.A series of solid polymer electrolyte membranes rP（NB-O-Hex-b-NB-O-Im⁺TFSI^--Im⁺TFSI^-）-x%（x=10,20,30 and 40）/ywt%LiTFSI（y=5,10,15 and 20）were prepared by blending imidazole functionalized copolymers with LiTFSI.The effects of polyimidazole ionic liquid content on the thermal properties,mechanical properties,morphology and ionic conductivity of solid polymer electrolytes and the effects of different lithium salt doping levels on polymer conductivity were studied.According to the characterization results,the prepared solid polymer electrolyte membrane has good thermal stability,the thermal decomposition temperature is not lower than 260℃,the crystallinity is small,and the amorphous region is large.The mechanical properties are good,the tensile strength is 1.43MPa,and the elongation at break is 120%.Among them,the conductivity of the polyelectrolyte membrane of rP（NB-O-Hex-b-NB-O-Im⁺TFSI^--Im⁺TFSI^-）-40%/20 wt%LiTFSI is the highest,the ionic conductivity at room temperature is 8.43×10^-6 S/cm,and the conductivity is5.25×10^-4S/cm at 100°C.（2）In order to improve the conductivity and thermal stability of the polymer,halogenated benzonorbornene monnomers were synthesized,and more imidazole ion groups were introduced into polymer side chain.Multi-imidazole functionalized long side-chain benzobornadiene monomers（BenzoNBD-Bis（Im⁺Br^–Im⁺I^–））and epoxy-functionalized norbornene monomers（NB-MGE）were synthesized,respectively.A series of ROMP block copolymers with different imidazole functional group contents were polymerized using Grubbs’3nd catalyst to obtain rP（NB-MGE-b-BenzoNBD-Bis（Im⁺TFSI^–-Im⁺TFSI^-））-x%（x=4.76,6.25,9.09 and16.7）.The characterization results proved that with the increase of the content of imidazole ionic liquid,the crystallization performance of the copolymer became weaker,which proved that the ionic liquid inhibited the formation of crystal nuclear,increased the amorphous region,improved the mobility of the chain segment,and decreased with the introduction of benzene ring.With the main chain structure of benzonorbornene,the thermal stability of the polymer electrolyte was improved（T_d>280°C）.The mechanical properties become better,the tensile strength reaches4.65MPa.The solid polyelectrolyte membranes rP（NB-MGE-b-BenzoNBD-Bis（Im⁺TFSI^–-Im⁺TFSI^-））-16.7%/20wt%LiTFSI ion conductivity is highest.The ion conductivity is 1.39×10^-5S/cm at 30℃,1.53×10^-4 S/cm at 100℃,respectively.