Preparation And Electrochemical Properties Of Ionogel Electrolytes For Secondary Lithium Batteries Towards High-temperature Operation

In energy storage applications such as portable consumer electronics and electric vehicles,secondary lithium batteries have dominated the market due to their high energy density and long cycle life.However,the safe operating temperature range of secondary lithium batteries is often restricted to-20 to 55℃.Long-term operation beyond this temperature range will result in significantly shorted cycle life or even safety issues,which fails to meet the needs of many applications（such as space exploration,military,and oil industry）at extreme temperatures.To further improve the safety and expand the operating temperature range of lithium batteries,replacing the highly volatile and flammable organic electrolytes with more stable electrolyte systems has long been considered as one of the most effective solutions.Ionogels inherit nonvolatility,nonflammability,and superior chemical and electrochemical stability of their constituent ionic liquids.They exhibit excellent safety comparable to inorganic solid-state electrolytes while avoiding their disadvantages,including mechanical fragility and large interfacial resistance with the electrodes.Despite these advantages,two major difficulties impede the practical application of ionogels.First,high ionic conductivity and good mechanical properties always can not be achieved simultaneously.The ionic conductivity of ionic liquids will be reduced by one or two orders of magnitude after gelation.Second,the low Li⁺transference number of ionogels severely limits the rate capability of batteries.In this paper,a series of ionogels with outstanding mechanical strength,high ionic conductivity,and high Li⁺transference number were designed and synthesized through optimizing ionic liquid loadings,designing matrix structures,functionalize matrix,and regulating ionic liquids.The application of these ionogels significantly improves the safety and service life of secondary lithium batteries under extreme environmental conditions.（1）High ionic liquid mass loading is beneficial in enhancing the ionic conduction of ionogels,but usually at the expense of reduced mechanical strength.In this effort,an organic-inorganic double network structure that has a significant advantage of mechanical strength was introduced to the design of ionogels.As expected,ionogels with high ionic liquid mass loading and good mechanical performance were achieved.The unique multiscale-pore structured double network enables the ionogel to retain a high Young’s modulus of 152 kPa even when the ionic liquid mass loading was increased to 92.4%.With the use of this ionogel,the LiFePO₄/Li cell delivers a specific capacity of 133 mAh g^-1 at 0.5C and remains 97%of the initial capacity after 300 cycles,the safe operating temperature of the pouch-type LiFePO₄/Li cell was extended to 150℃.（2）In terms of the low Li⁺transference number of conventional ionogels,a solvate ionic liquid was used to fabricate a double network ionogel,namely solvate ionogel（SIGE）.The findings show that SIGE exhibits a high Li⁺transference number of 0.43.The LiFePO₄/Li cell based on SIGE can deliver a high discharge capacity of 133 mAh g^-1 at 1 C.At an elevated temperature of 55℃,the LiFePO₄/Li cell retains 95.2%of its initial capacity over 500 cycles.Double network endows SIGE with high Young’s modulus and high fracture energy of 85MPa and 5.9 MJ m^-3,respectively.The excellent mechanical strength of SIGE ensures the safety of batteries when subjected to mechanical abuse.The strategy of combing the respective advantages of matrix structure and ionic liquid provides useful insights for developing advanced ionogels.（3）The primary role of gel matrix is to provide mechanical strength,but on the other hand,it also blocks ion migration.Concerning this problem,an ionogel with poly（diallyldimethylammonium）bis（trifluoromethanesulfonyl）imide（PDADMATFSI）electrospun nanofiber membrane as the rigid matrix and poly（trifluoromethyl methacrylate）（PTFEMA）as the flexible matrix was designed and synthesized.The results show that TFSI^-anions located on the PDADMATFSI nanofiber surface can reduce the number of TFSI^-in the Li⁺solvation shell.The side group of the PTFEMA can enhance the degree of ion dissociation within the ionogel.These two matrices synergistically facilitate fast ion transport.As a result,high ionic conductivity of 0.82 m S cm^-1 comparable to its ionic liquid component was achieved.The application of such ionogel enables the Li/Li cell to cycle continuously for>2000 h at 0.2 mA cm^-2.Excellent cycling stability was achieved for the LiFePO₄/Li cell over a wide temperature range from 0 to 90℃.