Preparation Of PEO-based/PCL-based Copolymer Electrolytes And Composite Polymer Electrolyte And Corresponding Applications In Lithium Batteries

Since the first commercial Li-ion battery introduced into the market,high-performance Li-ion batteries have been widely used in portable electronic devices and electric vehicles,making a huge contribution to the sustainable development from traditional energy to new energy.The electrolyte,as an intermediate part for Li⁺transport in a Li-ion battery,plays a vital role in the performances of the battery.In most of the commercial batteries,liquid electrolytes are composed of organic solvents and lithium salts.However,flammable and volatile organic solvents incur severe safety issues,impeding the progress of development.Furthermore,the demands for sustainable and high-energy density Li-ion batteries are constantly increasing with the development of society.Li metal has an extremely high theoretical specific capacity(3860 mA h g^-1),low mass and the lowest negative electrochemical potential?-3.04 V vs.SHE?,which is considered as an ideal anode material for improving the energy density of Li-ion batteries.However,Li metal is incompatible with liquid electrolyte because of the safety issue caused by the formation of dendrites during repeated charge-discharge.The application of solid-state electrolyte makes it possible to solve safety issues while using Li metal anode to improve energy density.Since the theory that using polymer containing alkali metal salts as electrolyte proposed,solid polymer electrolytes?SPEs?have been extensively studied.Considered as the next generation of secondary batteries,solid-state batteries with SPEs have not reached the stage of large-scale commercial production.The main challenges and critical issues with SPE are the inherently poor ion conductivity,large resistance at the interface between the SPE and active material,and narrow electrochemical stability window.Therefore,this thesis briefly introduces the development process and working mechanism of Li-ion batteries,introduces liquid electrolytes and solid electrolytes separately,summarizes the current research progress of inorganic and polymer solid electrolytes,and proposes the current challenges and perspectives for solid polymer electrolytes.The main topics of this thesis are the modification research of polyether-based electrolyte;the design and synthesis of novel polyester-based copolymer electrolyte;the preparation and research of composite polymer electrolyte,listed as follows:1.The poly?ethylene glycol??PEG?was chosen as Li⁺-conductor which is analogous to PEO.The PEG with low molecular weights exhibits better molecular mobility but unfavorable mechanical strength compared with PEO.Thus,polystyrene?PS?was introduced onto PEG chain to synthesize PS-PEG-PS.The research demonstrated that PS effectively improved the mechanical strength and decreased the crystalline phase.The ionic conductivity of PS-PEG-PS was 1.1×10^-3S cm^-1at 70°C.Meanwhile,the HOMO-LUMO energy levels of the electrolyte were analyzed by the theoretical calculations.The LUMO level decreased more rapidly than the HOMO level with increasing degree of polymerization.Furthermore,the electrochemical window estimated by the theoretical calculations was consistent with the experimental value.After a series of analyses on interface performances,PS-PEG-PS exhibited better compatibility toward Li metal anode than PEO.The Li-metal batteries with either LFP or NCM cathode materials exhibited excellent charge-discharge performances.For NCM//Li batteries,the increased interfacial resistance will affect the battery kinetics,resulting in poor cyclability and rate performance.2.PCL-PPC-PCL block copolymer electrolyte was prepared by using poly??-caprolactone??PCL?as host material and introducing poly?propylene carbonate??PPC?as a block.The research demonstrated that the crystallinity of PCL-PPC-PCL was tuned by the content of PPC block,leading to a high ionic conductivity of 3×10^-5S cm^-1at 30°C.Meantime,the electrochemical window is up to 5 V with a high lithium transference number(t_Li⁺=0.4).The DFT calculations proved that weaker coordination environment and reduced binding energy of Li⁺-PCL-PPC-PCL lead to a higher lithium ion transference number than that of Li⁺-PEO.Furthermore,this polyester-based copolymer electrolyte exhibited excellent compatibility toward Li metal anode.A stable SEI layer with high content of LiF and Li₃N formed after cycling,which is beneficial to suppress Li dendritic.The LFP//Li battery delivered a high discharge capacity of 161mA h g^-11 at 0.1 C,70°C with a capacity retention of 90%after 200 cycles,and 141 mA h g^-1at 0.05 C,room temperature.The NCM//Li battery also exhibited a high reversibility with Coulombic efficiency of 98%after ten cycles.3.Composite polymer electrolyte was prepared by using PCL as polymer matrix and the fast ion conductor Li_1.5Al_0.5Ge_1.5?PO₄?₃?LAGP?as filler.The research demonstrated that the crystallinity of PCL decreased with LAGP increasing.When the concentration of LAGP was up to 75 wt.%,Li⁺favored the pathway through the LAGP,leading to the highest conductivity of 1.7×10^-4S cm^-1at 30°C in this“polymer-in-ceramic”electrolyte.This composite electrolyte exhibited a large electrochemical stability window up to 5 V vs.Li⁺/Li and better compatibility against Li anode.After long-term Li//Li cycling,the overpotential was still lower than 0.2 V without any Li dendrites formation.Meanwhile,the LFP//Li cells with this electrolyte delivered a capacity of 157 mA h g^-1,and high capacity retention of 95.8%after130 cycles at 30°C,which were obviously better than the other PCL-based electrolytes.