Preparation Of Multi-arm Polymeric Ionic Liquid Gel Electrolyte And Its Application In Lithium And Sodium Batteries

With the development of energy revolution,the storage battery with high safety and high energy density is one of the key research directions in the field of energy storage.Gel polymer electrolyte（GPE）can replace liquid organic electrolytes and enhance battery safety and energy density.The preparation method of gel electrolyte membrane based on quaternary ammonium branched polybranched polymerized ionic liquid and its application in lithium metal battery and sodium metal battery were studied.First of all,six-arm branched polymerized ionic liquids with single cation center and double cationic centers were prepared.The gel electrolyte was assembled by adding lithium salt and ionic liquid,and lithium metal batteries were assembled.At the same time,the polymer ionic liquids were mixed with sodium salt and ionic liquids to prepare gel electrolytes and assembled sodium metal batteries.The structure and electrochemical properties of the single and double ionic center polymerized ionic liquid gel electrolytes were tested in detail,and the performance of the battery was tested.The details are as follow.（1）Using dimethylaminoethyl methacrylate as monomer,the single center six-arm branched polymeric ionic liquid（PIL-6）and double centers six-arm branched polymeric ionic liquid（DPIL-6）were prepared by atom transfer radical polymerization technology,and then the lithium bis（trifluoromethanesulphonyl）imide and 1-ethyl-3-methylimidazolium bis（trifluoromethanesulfonimide）were added to prepare the gel electrolyte PIL-GPE-6（Li TFSI）and DPIL-GPE-6（Li TFSI）.In contrast,DPIL-GPE-6（Li TFSI）has weaker crystallinity and lower glass transition temperature（Tg）,which can provide more lithium ion conduction path and free space.Lower Tg and the structure of double cation center also gives DPIL-GPE-6（Li TFSI）more excellent self-healing performance and toughness.At 60℃,the electrochemical window of optimized DPIL-GPE-6（Li TFSI）is greater than 4.5V（vs Li⁺/Li）,the ionic conductivity is 3.47×10^-4 S cm^-1,and the ion migration number is 0.46.Li Fe PO₄/Li battery was assembled with DPIL-GPE-6（Li TFSI）as electrolyte.The first cycle specific capacity of Li Fe PO₄/Li battery was 147.1 m Ah g^-1 at 0.1 C and 60℃.After 50 cycles,the capacity retention rate was 94%,and the coulomb efficiency reached 96%.Then DPIL-GPE-6（Li TFSI）was used to assemble lithium metal symmetrical battery.The polarization voltage remained stable after300 cycles,and it was proved by SEM that it effectively inhibited the formation of lithium dendrite.（2）With the preparation of single and double cation center six-arm branched polymerized ionic liquids as matrix,adding Na PF₆ and 1-ethyl-3-methylimidazolium bis（trifluoromethanesulfonimide）to obtain gel electrolyte PIL-GPE-6（Na PF₆）and DPIL-GPE-6（Na PF₆）.The crystallinity of DPIL-GPE-6（Na PF₆）was lower than that of PIL-GPE-6（Na PF₆）,and DPIL-GPE-6 showed better thermal stability,mechanical properties and efficient self-healing properties.At 60℃,the electrochemical window and ionic conductivity of DPIL-GPE-6（Na PF₆）were 4.7 V and 8.71×10^-5 S cm^-1,respectively.The specific discharge capacity of the Na₃V₂（PO₄）₃/Na battery assembled with DPIL-GPE-6（Na PF₆）can reach 63.5 m Ah g^-1,but the discharge capacity decays quickly after ten cycles,and the cycle stability is not ideal.It is necessary to continue to explore the improvement of the composition of gel electrolyte,the influence of positive and negative materials and the assembling process of sodium battery.In summary,the gel electrolyte based on DPIL-6 has superior comprehensive properties and has great potential for solid-state secondary battery applications.