The Application And Performance Of Multi-branched Multi-Functional Phosphate-based Gel Polymer Electrolytes For Sodium-ion Batteries

Due to the abundant sodium resources,low price and similar charge-discharge mechanism with lithium-ion batteries,sodium-ion batteries are considered as the most promising energy storage devices for large-scale energy storage systems and low-speed electric cars and electric bicycles.Organic liquid electrolytes which are commonly used in sodium ion batteries have the disadvantages of being volatile,flammable and easy to leak,posing certain safety risks.Gel polymer electrolytes combine the high conductivity of liquid electrolytes with the safety of solid electrolytes,which are considered as an effective solution to the performance defects and potential safety hazards of liquid electrolytes such as electrolyte decomposition and sodium dendrite growth.In the previous work of our group,the monomers used in phosphonate/phosphate ester-based gel polymer electrolytes are all phosphonate/phosphate esters containing two or one polymerizable double bonds,and copolymerized with comonomers can form a cross-linked polymer backbone that can encapsulate 90 wt.%of liquid electrolyte.In this thesis,we designed and synthesized multi-branched functional phosphate ester-based monomers with three polymerizable double bonds,which can form a polymer backbone with a larger spatial network by cross-linking and copolymerizing with the comonomers.On the basis of increased mechanical properties,this polymer backbone can wrap more liquid electrolyte to form a gel electrolyte to reduce polarization and further improve the overall performance of the battery.In this thesis,two new types of multi-branched cross-linked gel copolymer electrolytes for sodium ion batteries were designed and synthesized.(1)Two gel polymer electrolytes,MTP90 and MTP92,containing different electrolyte mass ratios,were obtained by in situ polymerization of multi-branched phosphate monomer with methyl methacrylate and trifluoroethyl methacrylate in the presence of liquid electrolyte,and their electrochemical properties were comparatively investigated.The experimental results showed that MTP92 which containing 92 wt.%liquid electrolytes could form a stable interface within 48 h,which could better inhibit electrolyte decomposition and maintain good interfacial stability compared with liquid electrolytes,thus having smaller polarization.The assembled Na3V2(PO4)3‖MTP92‖Na cells have good cycling stability over a wide temperature range from-10℃ to 60℃,showing significantly better low and high temperature cycling stability than the liquid cell.Meanwhile,it maintains 93%of the initial capacity after up to 3000 cycles under 2 C at room temperature,showing excellent long cycle stability.In addition,the MTP92-based battery also exhibits good rate performance from 0.1 C to 5 C.In a word,the gel polymer electrolyte has good compatibility with a variety of electrode materials and can match different electrode materials to assemble gel polymer electrolyte cells and exhibit good electrochemical performance.(2)In order to further improve the flame retardant performance of the electrolyte,two phosphorus-containing monomers previously designed and synthesized by the group as comonomers were used to prepare the gel polymer electrolytes 321P90 and 321P92 with all phosphorus-containing monomers including three,two and one double bonds respectively.Combustion tests show that the increase of phosphorus content effectively improved the flame retardant performance of the electrolyte,which can improve the safety performance of the battery.After replacing the fully phosphoruscontaining monomer,321P90 exhibits superior electrochemical performance at room temperature,with a capacity retention rate of 96%after 3000 cycles,and is slightly better than the performance of MTP92.Although the performance of 321P92 is slightly worse,it still exhibits 88%cycling capacity retention.This indicates that the 321P series gel electrolytes obtained after replacement with all-phosphorus-containing monomers have improved flame retardancy without affecting the electrochemical performance,especially the 321P90 system,which has superior overall performance.