Fabrication Of Polymer Electrolytes Based On Lithium Salt-induced Controlled Radical Polymerizations

Lithium-ion batteries(LIBs)have been widely used in our daily life,such as electric vehicles or portable electronic products.Polymer electrolytes(PEs)have attracted much attention for their good safety,flexibility,thermal stability and chemical stability.Designing and synthesizing polymers with well-defined structure could provide an effective method for preparing PEs with high ionic conductivity and excellent cycling performance of LIBs.However,there are problems including the removal of the catalyst,cumbersome process,environmental pollution and high cost in synthesizing polymers with well-defined structures based on controlled radical polymerization mechanism.Therefore,the design of low-toxic,low-cost,high-activity catalytic systems is the development direction of living radical polymerization and provides a method for the design of functionalized and high-performance polymer electrolytes.In this thesis,based on the mechanism of inorganic salt-induced controlled radical polymerization,the lithium salt-autocatalytic strategy was applied to construct polymer electrolytes.The electrochemical performance of PEs based on the cross-linking copolymerization system of poly(ethylene glycol)methacrylate(PEGMA)and polyethylene glycol dimethacrylate(PEGDMA),and the in-situ copolymerization system of PEGMA and methyl methacrylate(MMA)were systematically studied.Moreover,the effects of lithium fluoride(Li F),lithium chloride(Li Cl),lithium bromide(Li Br)or lithium nitrate(Li NO₃)additives on the interface between in-situ formed PEs and electrodes were discussed.The main research results are as follows:(1)The activation efficiency of various lithium salts on halogenated alkanes,the types of initiators and solvents on the polymerization activity were investigated with MMA as the monomer and halogenated alkanes as the initiator.2-iodo-2-methylpropionitrile(CP-I)/lithium iodide(Li I)catalytic system has the highest initiation efficiency.Based on the mechanism of controlled radical polymerization induced by lithium salts,the cross-linked polymer electrolytes based on the copolymerization of PEGMA and PEGDMA were successfully prepared.The lithium salt can be used as the activator of halogenated alkanes,as well as the lithium source for transporting lithium ions in PEs.The cross-linked PEs have good thermal,mechanical and electrochemical properties,and the Li//Li Fe PO₄ battery assembled by the PEs exhibits stable cycling performance.The method for preparing PEs based on lithium salt-induced controlled radical polymerization mechanism is simple and easy to operate,which provides a new idea for the preparation of novel PEs.(2)The Li/P(PEGMA-co-MMA)/Li Fe PO₄ battery was prepared by in-situ method.Compared with the battery prepared by ex-situ method,PEs in in-situ batteries have better compatibility with the positive and negative electrodes,which can effectively inhibit the growth of lithium dendrites.The battery can cycle stably at 0.2C for 260cycles.More importantly,the addition of Li I can form a Li I-rich SEI layer on the surface of the lithium anode to further prevent the growth of lithium dendrites.18CE6can dissolve the lithium salt and also be used as a plasticizer in PEs.Moreover,it has an auxiliary effect on the formation of the SEI layer.The method of constructing PEs based on the in-situ polymerization induced by lithium salt is simple,green and efficient.The PEs prepared by in-situ method are in close contact with the electrode,and the LIBs exhibit excellent cycle stability.(3)Based on the in-situ polymerization induced by lithium salt,the PEs based on cross-linked polymers or random copolymers are prepared by in-situ method using Li F,Li Cl,Li Br or Li NO₃ as additive.The additives not only can activate the halogenated alkanes to induce the polymerization reaction of monomers,but also can form the SEI layer between the lithium anode and PEs.The SEI layer plays an important role in regulating the interface.The in-situ battery based on the above additives has good cycle stability.The generated SEI layer can protect the lithium anode,avoid the side reactions between Li metal with PEs,and inhibit the growth of lithium dendrites.The effect of lithium salt is further explored and utilized,which is expected to provide an effective method for the preparation of high-performance PEs.