Facile Fabrication Of Polymer Electrolytes Via Lithium Salt-accelerated Thiol-michael Addition For Application In Lithium-ion Batteries

Lithium-ion batteries(LIBs)are widely used in areas of electric vehicles and portable digital devices due to the high energy density,long cycle life,and environmental friendliness.However,nowadays most commercial LIBs on the market use liquid electrolytes,and the battery capacity decays very quickly.Besides,when liquid batteries have the situation of internal short circuit or overheating,it's extremely possible to ignite the liquid electrolytes,which will cause batteries to explode,leading to potential danger.Solid electrolytes are developed to solve the problems in liquid batteries during practical applications.And the safety performance of LIBs can be effectively improved by good mechanical performance and stable electrochemical properties.Currently,polymer electrolytes based on poly(ethylene oxide)(PEO)is the most widely used solid electrolyte.However,the crystallinity of PEO at room temperature is very high,which severely restricts its application in polymer electrolytes because lithium ion is hard to transfer in the crystallization region.Therefore,by fabricating comb polymers,block copolymers,crosslinking network,and composite polymer electrolytes,the crystallinity of PEO can be decreased,which will increase the ionic conductivity of polymer electrolytes based on PEO.Nevertheless,severe reaction conditions and complex production processes in the methods above resulting in serious limitations on practical applications.To develop a facile and simple method for the preparation of polymer electrolytes,we use the lithium salts accelerated thiol-Michael addition to prepare polymer electrolytes(PEs)with well-defined cross-linking network and applied the PEs to LIBs.The main contents of the research are as follows:(1)Real-time FT-IR is used to study the kinetics of lithium salts accelerated thiol-Michael addition with TEA as a catalyst.The reaction conversion within 30 min increased from 3%to almost 100%after adding LiClO₄ compared to the conversion with TEA as a catalyst only.Besides,several other alkali metal salts also showed an acceleration effect,includinglithiumbis(trifluoromethanesulfonyl)imide(Li TFSI),lithium trifluoromethanesulfonate(Li CF₃SO₃),and sodium perchlorate(Na Cl O₄).At the same time,using ¹H-NMR to study the mechanism of acceleration effect of thiol-Michael addition.The experimental results showed that,electron cloud density of double bond which is from acrylate group of poly(ethylene glycol)diacrylate(PEGDA)decrease because lithium ion can coordinate with the oxygen of carbonyl group,which is conducive for thiolate anion to attack electron-deficient double bond,leading to a significant increase of thiol-Michael addition reaction rate.Finally,the mechanism above was testified by density-functional theory(DFT)calculation.The results show that the decrease of electron cloud density of vinyl moiety may be related to the complexation,and the acceleration effect caused by alkali metal salts with smaller cation radius is more obvious.(2)Using thiol-Michael addition with PEGDA and multifunctional thiol monomer as reactants,TEA as a catalyst,in the present of LiClO₄,with a stoichiometry ratio of thiol:vinyl=1:1 to prepare cross-linking polymer electrolytes.In this process LiClO₄ not only provides ion conduction property,but also accelerates the thiol-Michael addition as a co-catalyst.The cross-linking PEs with a uniform network structure improve the semi-crystalline of PEO and improve the motion of lithium ion.PEs prepared using polyhedral oligomeric silsesquioxane(POSS-SH)have an organic-inorganic hybrid structure.On one hand,the addition of POSS can further suppress the crystallization of PEO,on the other hand,ion migration channels near the surface of POSS contribute to the improvement of ion conductivity.Finally,assembling LIBs with polymer electrolytes prepared using POSS.The LIBs showed excellent rate capability and stable cycling performance.(3)Based on the experiments above,combining lithium salt-accelerated thiol-Michael addition with lithium salt-induced free radical polymerization to prepare double network polymer electrolytes(DN-PEs)using thiol-Michael addition and radical polymerization in sequence.The double network structure improves the degree of crystallization in PEO,increasing the ionic conductivity of DN-PEs.And by increasing the ratio of PEGDA,the cross-linking density of polymer electrolytes can be reduced,which can also solve the fragility of polymer electrolytes.The elongation can reach 140%at the maximum,which can effectively prevent electrolyte membranes from cracking during the battery assembling process to improve the safety performance of LIBs.