Preparation And Application Of Comb-like Poly(D,L-lactide)-based Solid Electrolytes For Lithium Ion Batteries

The development of lithium ion batteries is now mainly restricted by safety risks and poor cycling stability caused by the conventional organic liquid electrolytes.To solve this,solid polymer electrolytes?SPEs?have been a promising alternativedue to their safety,lighter weight,and easier processability.However,there are still many issues that hinder the application of solid polymer electrolytes such as low ionic conductivities and electrochemical window,poor mechanical and cycling stability.Aiming at the above problems,this paper aims to prepare free-standing SPEs with high ionic conductivity,wide electrochemical window and good cycling stability.In this thesis,a comb-like SPE and a star-comb SPE based on poly?D,L-lactide??PDLLA?,poly?ethylene glycol?methyl ether methacrylate?PEGMA?and LiTFSI were prepared by solvent-free free radical polymerization.And the physical properties,electrochemical properties and cell performance of these SPEs were systematically investigated.The main research methods and conclusions are as follows:?1?The vinyl functionalized linear macromonomer PLA1 and six-arm star macromonomer PLA6 were synthesized by a ring-opening polymerization?ROP?of D,L-lactide and subsequently an acylation of the hydroxy end-groups.The chemical structure of the obtained polymers was confirmed by ¹H NMR and FTIR.?2?A novel comb copolymer based on linear PDLLA macromonomer PLA1 and PEGMA was prepared,and the electrochemical properties were studied,with an aim of using as solid polymer electrolyte in lithium ion batteries.A series of free-standing solid polymer electrolyte membranes from different ratios of PLA1,PEGMA and LiTFSI were prepared through solvent-free free radical polymerization under UV radiation.The as-preparedcomb solid polymer electrolytes?PLA1-SPEs?exhibit good thermal stability with T_d^5%s of²73^oC and low T_gs of-54^-29^oC.The electrochemical characterization shows that the PLA1-SPEs possess a wide electrochemical window up to 4.8 V with optimal ionic conductivity of4.3?10^-4 S·cm^-1at 60^oC and 6.9?10^-5 S·cm^-1at 25^oC with an EO/Li⁺ratio of 32:1.Furthermore,the all-solid-state LiFePO₄/Li cells display extraordinary cycling and rate performances at60^oC by curing the PLA1-SPEs directly on the cathode.These superior properties of the comb-like PLA1-SPE make it a promising candidate of solid electrolyte for lithium batteries.?3?A star-comb copolymer based on six-arm PDLLAmacromonomer PLA6 and PEGMA was prepared,and the electrochemical properties were studied,with an aim of using as solid polymer electrolyte in lithium ion batteries.A series of free-standing solid polymer electrolyte membranes from different ratios of PLA6,PEGMA and LiTFSI were prepared through solvent-free free radical polymerization under UV radiation.The as-prepared six-arm star-comb solid polymer electrolytes?PLA6-SPEs?exhibit good thermal stability with T_d^5%s of²70^oC and low T_gs of-48^-34^oC.The electrochemical characterization shows that the PLA6-SPEs possess a wide electrochemical window up to 5.1 V with an optimal ionic conductivity of 9.7×10^-5 S·cm^-1 at 60^oC at an EO/Li⁺ratio of 16:1.Furthermore,the all-solid-state LiFePO₄/Li cells display extraordinary cycling and rate performances at 60^oC by curing the PLA6-SPEs directly on the cathode.These superior properties of the six-arm star-comb PLA6-SPE make it a promising candidate of solid electrolyte for lithium batteries.