Performance Of Benzonorbornadiene Block Copolymer-based Solid Electrolyte Membrane And Lithium-ion Battery

Lithium-ion battery（LIB）have received extensive attention due to their high specific energy,good cycling performance,and green environmental protection,and are widely used in mobile communication equipment and electric new energy vehicles and other fields.However,the liquid electrolyte in LIB has the safety risks of electrolyte leakage and flammable and explosive,so a new generation of electrolyte——solid electrolyte came into being.Solid polymer electrolyte（SPE）in solid electrolyte has attracted widespread attention in the LIB field due to its good safety performance,flexible structural design and inhibition of lithium dendrite growth.Norbornene polymers have excellent properties such as good membrane-forming,low moisture absorption,easy processing,and less-flammability.Polyionic liquids（PILs）have a wide application prospect in LIBs as a new ionic liquid polymer electrolyte because of their high ionic conductivity,excellent electrochemical stability,high thermal stability and low flammability.Synthesis of epoxy-functionalized norbornene（NB-MGE）monomer,imidazole cations are grafted as functional groups on the side chain structure of benzonorbornadiene,synthesis of bis-imidazole functionalized BenzoNBD-BisIm⁺Br^-monomer.NB-MGE,BenzoNBD-BisIm⁺Br^-and norbornene（NB）were used to prepare block copolymers by ring-opening translocation polymerization（ROMP）,and after anion substitution,which were blended with different contents of bis（trifluoromethane）lithium sulfonimide（LiTFSI）to obtain a solid polymer electrolyte membrane（SPEM）.The block copolymer rP（NB-MGE-b-BenzoNBD-BisIm⁺Br^-）-40%was anionically displaced,which was blended with 30 wt%LiTFSI and different SiO₂ content to prepare a composite solid polymer electrolyte（CSPE）.It can further improve the ionic conductivity,thermal stability and mechanical properties of SPEM.The specific research content is as follows:（1）NB-MGE,BenzoNBD-BisIm⁺Br^-and NB under the action of Grubbs’^2ndcatalyst to prepare block copolymer rP（NB-MGE-b-BenzoNBD-BisIm⁺Br^--b-NB）-x%,（x=20,30,40）.After that,the block copolymer was anionically replaced and blended with different contents of LiTFSI to prepare into a series of SPEMs.The effects of different contents of PILs and different contents of LiTFSI on the thermal stability,crystallinization performance,electrochemical properties and assembled battery performance of SPEM were studied.The initial thermal decomposition temperature of SPEM is about 240℃,the crystallinity is low,and most of them are amorphous regions,which is conducive to the transport of Li⁺,and the ionic conductivity can reach up to3.1×10^-5 S/cm at room temperature 30℃,and 1.7×10^-3 S/cm at 100℃.The initial reversible discharge specific capacities of NCM811/SPEM rP（NB-MGE-b-BenzoNBD-BisIm⁺TFSI^--b-NB）-40%/30 wt%LiTFSI/Li battery at 0.1 C,0.2 C,0.5 C and 1 C are 149.1,145.7,135.5 and 99.5 mAh/g.The battery can discharge power stably after 50 cycles at 0.2 C magnification,and the coulomb efficiency is above 95%.（2）CSPEM was prepared by adding nano-SiO₂ blending to SPE,which could effectively improve the mechanical properties,ionic conductivity,electrochemical stability window of the electrolyte membrane.A series of CSPEMs with different SiO₂content were prepared by anion substitution of block copolymer rP（NB-MGE-b-BenzoNBD-BisIm⁺Br^-）-40%with 30 wt%LiTFSI and nano-SiO₂ content.The test results show that the addition of nano-SiO₂ improve the thermal stability of the electrolyte（T_d>350℃）,the mechanical properties are also improved,the maximum tensile strength reaches 4.72 MPa.Compared with SPEM,CSPEM increased ionic conductivity to 9.3×10^-5 S/cm at 30℃ and 1.5×10^-3 S/cm at 80℃,and it was found that the ionic conductivity was highest when SiO₂ was added at 5 wt%.CSPEM’s electrochemical stabilization window has also been further improved,with a stable voltage above 4.5 V,which can meet the needs of high-voltage NCM811 cathode materials.The initial reversible discharge specific capacities of NCM811/CSPEM rP（NB-MGE-b-BenzoNBD-BisIm⁺TFSI^-）-40%/30 wt%LiTFSI/5 wt%SiO₂/Li battery at0.1 C,0.2 C,0.5 C and 1 C were 162.6,149.7,133.4 and 116.7 mAh/g.The battery underwent 50 cycles test at a rate of 0.2 C,and its discharge capacity remained at148.9±8 mAh/g,and the coulomb efficiency was above 96%.