Preparation And Surface Structure Of Perfluorinated Side Chain Lithium Single-ion Conductive Polymer Electrolytes Membrane

As a representative of green energy in batteries,lithium-ion batteries have been widely used in various industries in the past twenty years.At present,commercial liquid electrolytes has poor thermal stability and is easy to leak,which makes lithium-ion batteries have potential safety hazards.Solid polymer electrolytes have good mechanical properties and machinability of shape and size,which avoid the shortcomings of liquid electrolytes.In recent years,the appearance of lithium single-ion conductive polymer electrolytes have eliminated the phenomenon of concentration polarization caused by anion transference in dual-ion conductors.However,there are still many problems to be solved before commercialization.Firstly,the low conductivity of polymer electrolyte at room temperature limits the improvement of rate performance of battery.Moreover,when lithium metal is used as the negative electrode,the stability of Li/SPEs interface seriously affects the charge-discharge capacity and cycle life of the battery.In view of the above problems,this paper has carried out research work,and the conclusions as follows:（1）At first,lithium（4-styrenesulfonyl（trifluoromethylsulfonyl）imide）（Li STFSI）was synthesized.Then three kinds of lithium-ion conductive polymer electrolytes,P（Li TFMA）,P（Li OFMA）and P（Li DFMA）were synthesized by copolymerization with Li STFSI,polyethylene glycol methyl methacrylate（PEGM）,tetrafluoropropyl methacrylate（TFMA）,octafluopentyl methacrylate（OFMA）,dodecafluoroheptyl methacrylate（DFMA）as monomers.The structures of the polymer were determined by FT-IR and NMR.Thermal analysis shows that the polymer electrolyte only has a glass transition temperature（T_g）between-100℃～150℃,and all of them are amorphous,and the thermogravimetric temperature can reach 300℃.（2）The electrochemical performance test showed that when EO/Li⁺=29,the conductivity of P（Li TFMA）electrolyte exhibits better conductivity of 2.0×10^-5S/cm at 25℃.The shorter the fluorine-containing side chain is,the shorter the transmission path of extended ions in polymer electrolyte is,which enhances the interaction between EO segments and ions.The growth of fluorine-containing side chain is beneficial to improve the stability of polymer electrolyte.P（Li DFMA）electrolyte has a wide electrochemical stability window of4.5 V at 60℃.At the same time,all the three polymer electrolytes have high lithium-ion transference number（≈1）at 60℃,and the movement of anions is greatly restricted.（3）The polymer electrolytes were heat-treated at 170℃,and the interfacial impedance of Li/SPEs/Li symmetrical battery at 60℃before and after heat-treatment were tested.Electrochemical impedance spectroscopy（EIS）shows that the interfacial impedance of Li/P（Li DFMA）electrolyte is greatly reduced after heat treatment.With the extension of time,especially the increase of the interfacial impedance of Li/P（Li FDMA）electrolyte is the smallest.The surface elements of the electrolyte were characterized by XPS.After heat treatment,inert fluorine-containing groups were more enriched on the surface of electrolyte membranes,and the P（Li DFMA）electrolyte was more abundant.