Solid Polymer Electrolyte Embedded with Ionic Liquid for Lithium-Ion Battery Operating at Ambient Temperature
Authors: You Duck-Jae , Yoo Jeeyoung , Ahn Yong-keon , Cho Sanghun , Chung Sung Yun , Kim Youn Sang 
Published: February 2020

The application of lithium-ion battery (LIB) is expanding from small electrical devices to electrical vehicles, which are required high energy, power and safety. Energy and power density were improved by developing cathode and anode. However, there are huge potential safety hazards when commercialized LIB employing organic solvent is applied electrical vehicles. The flammable organic solvents are considered the source of thermal runaway reaction of LIB. The solution to these problems is to replace the liquid electrolyte with solvent-free solid electrolyte for high safety and stability. Solid polymer electrolytes (SPEs) have been studied for potential use in lithium-ion batteries over the four decades.[1] They are composed of Li-salts dissolved in solid, coordinating polymers, such as poly (ethylene oxide) (PEO). However, Li+-conducting SPEs show poor ionic conductivity and interfacial properties. In the case of PEO, below 65 oC, ionic conductivity decreases dramatically as a result of crystallization, which severely restricts ion transport.[2] Thus, most PEO-based SPEs are not suitable for the electrical device which operating at ambient temperature. In this study, we made the high-performance SPEs for lithium ion batteries operating at room temperature. Furthermore, these electrolytes have good compatibility with electrodes by using direct casting on electrode. The electrolytes are composed of cross-linked polymer, lithium salts (Bis(trifluoromethane)sulfonimide lithium salt, LiTFSI) and ionic liquid (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EMITFSI). The cross-linked polymer can be prepared by reacting polyethylene glycol and 3-glycidoxypropyltrimethoxysilane. EMITFSI are selected for high ion mobility. The addition of ionic liquids cause the increase of ionic conductivity due to the enhancement of ion dissociation and improvement of ion mobility.
Electrochemical impedance spectroscopy (EIS) and galvanostatic charging/discharging tests were performed with lithium iron phosphate (LiFePO4)/SPE/Li foil cells at the room temperature. The bulk resistance was decreased according to the amount EMITFSI whose value from 698.5 Ω to 105.3 Ω. Also, charge transfer resistances were decreased from 2507.4 Ω to 560 Ω. These results show that EMITFSI can enhance the ion conductivity.
With increasing amount of EMITFSI, specific capacities were increased. This is mainly due to an increase of the lithium ion mobility when EMITFSI was increasing. The specific capacity of prepared cell with EMITFSI was 75.815 mAh/g and 16.538 mAh/g at 0.1C, 0.5 C respectively. The specific capacity of LiFePO4/SPE/Li Cell using SPE without ionic liquid was 15 mAh/g at 0.1 C. And capacities were recovered after high current rate charging/discharging test. Therefore, we believe that suggested SPEs act as a stepping stone to the realization of high-performance all-solid lithium-ion batteries operating at room temperature.
[1] D.E. Fenton, J.M. Parker, P.V. Wright, Polymer 14 (1973) 589
[2] K.P. Barteau, M. Wolffs, N.A. Lynd, G.H. Fredrickson, E.J. Kramer, C.J. Hawker, Macromolecules 46 (2013) 8988-8994.

Figure 1