Preparation And Properties Of Poly(Ethylene Oxide)-Based Electrolytes Via Lithium Salts-Induced Polymerizations

Lithium-metal batteries（LMBs）have been regarded as one of the most promising next-generation energy storage system due to their high-energy densities.However,the commercial applications of LMBs are limited by the traditional liquid electrolytes（LEs）which face safety issues such as leakage,combustion and even explosion.Polymer electrolytes（PEs）are the most important part of polymer-based LMBs because of their advantages such as non-volatilization and simple-processing.Poly（ethylene oxide）（PEO）-based PEs has been the most popular and researched polymer electrolytes for LMBs owing to the advantages of light weight,easy to be prepared to film,good chemical stability and so on.But the high crystallinity of PEO leads to the low ionic conductivity of PEO-based PEs.And the strong complexation effect between ethoxy（EO）chain segment and lithium-ion（Li⁺）leads to a low lithium transference number(t_Li⁺)of the electrolytes.In addition,the poor wettability between PEs and electrodes also leads to slow interfacial transference of Li⁺.And the overgrowth lithium dendrites caused by the uneven lithium plating would pierce the separator,then damage the electrochemical properties,life-span and safety of LMBs.Based on the above background,in this thesis,a variety of high-performance PEs for LMBs were constructed by lithium salts-induced radical polymerization,cationic polymerization and ring-open polymerization.At the systems of in-situ preparation of PEs,the lithium-induced polymerization can reduce the use of non-electrochemically active materials and removal of metal ion catalysts,and then simplify the preparation process of PEs.The ionic conductivity and t_Li⁺of PEO-based PEs were improved by adjusting the bulk structure of polymer matrix,introducing polymer which has a weakly complexation effect with Li⁺and alternative composition of PEs.On the other hand,the in-situ constructed robust solid electrolyte interphase（SEI）layer on lithium-metal surface inhibited the growth of lithium dendrites and improved coulombic efficiency and cycling stability of batteries,which provided a novel idea for the high-performance LMBs.Firstly,the reaction kinetics of lithium salts-induced polymerization of polyethylene glycol methyl ether acrylate（PEGA）initiated by ethyl 2-bromoisobutyrate was studied.Lithium salts commonly used in electrolytes can effectively catalyze the polymerization of PEGA,and the monomer conversion ratio reaches more than 90%.Based on this reaction mechanism,the cross-linked polymer membranes with carbon-bromo bond were constructed through reversible addition-fragmentation chain transfer polymerization,the PEs（BNPPEs）with a dense polyethylene glycol（PEG）chain segment were prepared by lithium salts-induced radical polymerization.The electrochemical properties of BNPPEs were further enhanced through adding deep eutectic electrolyte（DEE）.The assembled Li|BNPPEs|LiFe PO4 battery exhibits good rate performances and cycling stabilities.Secondly,in order to develop the application of lithium salts-induced polymerization in electrolytes,（3-Aminopropyl）trimethoxysilane（APS）and 2-bromoisobutyryl bromide were adopted to modify cellulose.Then,solid polymer electrolytes（SIPEs）with PEG brush grafted on the modified cellulose surface were obtained via lithium salts-induced surface-initiated polymerization and"grafting from"strategy.The APS on cellulose surface accelerates the Li⁺migration and surface-initiated polymerization method enhances the interaction between the dense PEG brush and the cellulose which also improves the Li⁺transfer and increases the ionic conductivity of SIPEs.Thirdly,since the goals of the enhanced rate performance and improved electrode-electrolyte interfacial contact,the polyacrylate acid and its neutralized derivatives（PAAX,X=Li,Na,K）were employed as binder to prepare lithium iron phosphate cathode（LFP@PAAX,LFP=LiFe PO4）and formed an artificial cathode electrolyte interphase（CEI）layer on cathode surface to improve rate performance and cycling stability of LFP@PAAX.Notably,the LFP@PAALihas the best electrochemical properties.Meanwhile,the gel polymer electrolytes（GPE-PAAX）were constructed through in-situ radical copolymerization of poly（ethylene glycol）methyl ether methacrylate（PEGMA）and poly（ethylene glycol）dimethacrylate（PEGDMA）catalyzed by PAAX.The GPE-PAALidisplays the highest ionic conductivity at ambient temperature(5.0×10^-5 S·cm^-1).And PAALion cellulose surface is beneficial for Li⁺migration so that it shows a high t_Li⁺（0.61）.And the assembled Li|GPE-PAALi|LFP@PAALibattery exhibits an excellent cycle performance.Fourthly,so as to further enhance the interfacial stability between electrolytes and lithium metal anode,DEE-based gel polymer electrolytes（GPEs）were constructed by the in-situ cationic polymerization of vinyl ether（DEGVE-i BBr）monomers initiated by lithium hexafluorophosphate（LiPF6）and radical polymerization of PEGMA induced by lithium perchlorate（LiClO4）.It is shown that the Li||LFP@PAALibattery based on the DEE-based electrolyte formed by tetramethylene sulfone has the best cycling stability.And the battery has a high-capacity retention of 84%after 1200 cycles at 60 ℃ with a current density of 1C.In addition,the LiBr-rich SEI layer formed from the reaction between lithium metal and bromine atoms has obviously suppressed the growth of lithium dendrites.Finally,as the targets of improving electrochemical stability and lithium-ion transference number of PEO-based electrolytes,the GPEs（GPE-PEG-PTMC）with polyether and polycarbonate were in-situ prepared through the LiBr/LiTFSI co-induced radical polymerization of PEGMA and ring-opening polymerization of trimethyl carbonate（TMC）.And the effects of different components and monomer ratio on electrochemical performance was compared.The GPE-PEG-PTMC displays enhanced t_Li⁺and electrochemical stability owing to weakly complexation effect with Li⁺of PTMC.And it also shows a good interfacial compatibility with lithium metal.The assembled Li|GPE-PEG-PTMC|LFP@PAALibatteries have excellent cycling stability（the capacity retentions are more than 80%after 1000 cycles at 1C）.