Stable Cross-linked Gel Copolymer Electrolyte Containing Methyl Phosphonate For Sodium Ion Batteries

With the increasing demand for large-scale stationary energy storage devices and portable electronic devices,sodium-ion batteries have attracted wide attention in the research field due to their relatively rich sodium resources and low cost.The electrolyte is the blood of the battery,which affects the electrochemical performance and safety performance of the sodium ion battery.The electrolyte not only determines the electrochemical stability window of the battery but also affects the interface between the electrode and the electrolyte.It is also the key factor affecting the energy density of the battery.However,the commercially available electrolyte is mostly flammable,the packaging process is troublesome,and it is easy to leak,which has great safety hazards.A series of methyl phosphonate cross-linkers with different chain lengths were synthesized.These methyl phosphonate cross-linkers were copolymerized with acrylate comonomers to prepare gel polymer electrolytes,and are applied to sodium ion battery systems.The electrochemical performance was studied in detail.The main content of this thesis is divided into three parts.The specific research contents are as follows:The first part of this thesis is mainly about the synthesis of materials and the characterization of the gel polymer electrolyte.The bifunctional methyl phosphonate which combines the good polymerization ability of acrylate,the adjustable flexibility of ethylenedioxy group and the flame retardant properties of methyl phosphonate is designed and 8 phosphonate crosslinkers were synthesized and characterized by ¹H NMR,¹³C NMR,¹³P NMR,IR and HR-MS spectrometry.Relevant structural characterization data show that a series of pure methyl phosphonate crosslinkers were synthesized and the purity meets the requirements of electrolyte configuration.The gel copolymer electrolyte containing methyl phosphonate and acrylate comonomers was prepared by in-situ cross-linking copolymerization.The gel polymer electrolyte has good thermal stability with the onset decomposition temperature of 250°C,excellent electrochemical stability of up to 5.0 V?vs Na⁺/Na?and high ionic conductivity of 5.13 mS cm^-1.SEM images show the uniform and homogeneous gel polymer electrolyte was formed with the glass fiber separator filled with gel polymer electrolyte.Furthermore the gel polymer electrolyte can promote strong interfacial adhesion between the electrodes and the membrane,which shows the good interfacial stability of gel polymer electrolyte with electrodes.The second part of this thesis is mainly to prepare the gel polymer electrolyte by in-situ cross-linking copolymerization and assemble the sodium ion batteries.The Na₃V₂?PO₄?₃/Na cell using gel polymer electrolyte and Na₃V₂?PO₄?₃ cathode shows ultra-long cycling performance and better cycle stability and rate performance than the corresponding conventional liquid electrolyte based cell.Meanwhile,the glass fiber separator filled with gel polymer electrolyte maintains better integrity than liquid electrolyte after 1000 cycles.That means the gel polymer electrolyte has good mechanical property.When running at high temperature of 60?,the gel polymer electrolyte based sodium-ion Na₃V₂?PO₄?₃ cell also exhibits superior cycling stability compared to a battery with liquid electrolyte.The gel terpolymer electrolyte based sodium ion cell all shows better cycling performance than the gel bipolymer electrolyte system based cell and other commercial crosslinker based cells.At the same time,it also has superior cycle stability than the commercialized phosphorus-free crosslinking agent.The strong interaction between methyl phosphonate and sodium ion is studied by FT-IR,³¹P NMR and HR-MS,and demonstrated by density functional theoretical calculation.The third part of this paper is to characterize the electrochemical performance of gel polymer electrolyte with anode of SnS₂.The gel polymer electrolyte full-cell with Na₃V₂?PO₄?₃ as cathode and SnS₂ as anode is assembled.The assembled full-cell has relatively good electrochemical performance and has important guidance for the commercialization of the gel polymer electrolytes.