The Application Of Ethylene Vinyl Alcohol Copolymer And Its Derivatives In Lithium Metal Battery

Advanced energy storage technique plays a more and more important role in modern society.Unfortunately,safety issues with lithium secondary batteries is strangling the evolution of electronic products and electric vehicles.Solid polymer electrolytes effectively avoid the damage of organic electrolytes（combustion,explosion,etc.）without organic solvent.Furthermore,polymer electrolytes have good flexibility and excellent electrochemical stability,which makes it easy to be used in industrial scale.The first part of this thesis is to obtain poly（ethylene vinyl alcohol-β-acrylonitrile ether）（EVOH-β-OCH₂CH₂CN,abbreviated as EBC）via addition reaction of ethylene vinyl alcohol copolymer（EVOH）with acrylonitrile,and the properties of EBC-based polymer electrolytes are studied.Lithium（Li）metal anode is a promising candidate for the next-generation high capacity energy storage system.Nevertheless,the rechargeable Li metal batteries have not been released at a large-scale due to poor interface stability,uncontrolled growth of dendrites and infinite volume expansion.Among these problems,unstable interface is regarded as one of the most critical issues.Therefore,building an artificial protective layer is a promising approach to address the issues of Li anodes.（1）In this thesis,the polymer precursor solution was prepared,and the film was formed in situ on Teflon（PTFE）plate by solution casting.The basic properties and performances of EBC as the polymer matrix were characterized.When the ratio of EBC to lithium bis-trifluoromethanesulfonimide（LiTFSI）is 5:1,the ionic conductivity can reach 3.75×10^-4 S/cm at 60°C,the lithium ion transference number is 0.39,and the electrochemical window is greater than 4.7 V.The first charge specific capacity in full cell of LiFePO₄ can reach 154 mAh/g,and the coulombic efficiency can maintain at99%within 400 weeks,indicating that EBC can be used as a polymer electrolyte matrix in Li batteries.（2）A robust and deformable polymer electrolyte film（EBC-LiTFSI）as the advanced protective layer on Li metal is developed by a simple tape-casting method,in which the polymer endows a comfortable interfacial contact as well as membrane flexibility to adapt the volume change,meanwhile the coordination between the polymer and Li salt provides fast Li⁺ion diffusion channels.The modified Li metal anode can maintain a stable cycle of 1000 times at a current density of 1 mA/cm² and deliver a stable cycling over 600 cycles under a high current density of 3 mA/cm² in the ether electrolyte.In addition,the Li₄Ti₅O₁₂|Li@（EBC-LiTFSI）battery at a rate of 2 C can be stable for more than 700 cycles with a capacity retention of 94.6%and a coulombic efficiency of around 99.5%.The enhancement of the cycling performance is mainly attributed to Li metal deposited under the EBC-LiTFSI layer rather than on the EBC-LiTFSI layer,which is not only beneficial to the uniform distribution of Li⁺ions,but also can further inhibit the growth of Li dendrites.（3）The EBC-LiTFSI film was coated on Cu foil by spin coating method,and the coating thickness was controlled by adjusting the spin coating time,in which the Cu electrode with 90 s of spin coating showed the best performance.The（EBC-LiTFSI）-coated electrode maintained an average coulombic efficiency of 98.6%over 120 cycles at a current density of 1.0 mA/cm² and a capacity load of 3 mAh/cm²,indicating that the modified Cu electrode showed a more stable interfacial performance and higher coulombic efficiency.（4）In order to further enhance the stability of the Li-Cu dual-phase alloy anode,the EBC-LiTFSI film was coated on the alloy surface by a simple coating method.The CuLi60 alloy electrode was studied before and after modification,and the performance of CuLi60 alloy was further improved by polymer coating.The CuLi60 alloy electrode with EBC-LiTFSI coated has stable cycle of 800 h at the current density of 1.0 mA/cm²and the capacity load of 1 mAh/cm².From the evolution of AC impedance plots,it can also be seen that after surface modification,the interfacial stability of CuLi60 alloy anode is improved and the cycle life is prolonged.Accordingly,the polymer electrolyte coating of Li metal anode to enhance Li metal battery performance is an simple and effective methodology.