Charge-Discharge Characteristics on Low Viscosity Phosphonium Ionic Liquid Electrolytes for Lithium Ion Battery

Abstract

We evaluated low viscosity Ionic Liquids (ILs) based on quaternary phosphonium cations as electrolytes for Lithium Ion secondary Battery (LIB) in this work. The secondary batteries are required more safety and higher energy density per volume and/or mass for electric vehicles (EVs) to equipped to convenient driving technology, i.e. IOT, electric sensing technology and devices. LIB is one of the promising batteries for EVs, and is mainly used for cellular phones and portable devices for its large capacity and light weight. Carbonate solvents are commercially used as an electrolyte in LIBs, but they are highly inflammable for a short circuit and/or thermal runaway. Hence we focused on ILs as an electrolyte for LIB because of their properties (e.g. incombustibility, hard volatility, wide potential window and high ion conductivity, etc.). Since the discovery of water stable 1-ethyl-3-methylimidazolium tetrafluoroborate 1), ILs have attracted attention. The ILs which are based on mainly imidazolium cations, pyridinium cations and quaternary ammonium cations, etc., have been studied in various fields including electrolytes for LIB 2, 3). On the other hand, ILs based on quaternary phosphonium cations have more unique properties, especially, i.e. lower viscosity than quaternary ammonium cations system 4), while a few electrochemical properties as electrolyte for LIB are reported 5, 6). Moreover, they have higher thermal stability and wider potential window than ammonium cation systems, so their properties are advantageous for safety. High discharge capacities of near the theoretical capacity of LiCoO2 and coulomb efficiencies of ca. 100% were obtained during Charge-discharge cycle tests on P111X-FSA system which consist of trimethylphoshine (TMP) based phosphonium cations (P111X + ; X = 1O1(methoxymethyl group), Al(allyl group) etc.) and bis(fluorosulfonyl)amide anion (FSA-). Moreover, the discharge rate capability of P111(1O1)-FSA was quite higher than another functionalized ILs, while the ion conductivity was not so higher. This is indicated that some physical parameters, e.g. transport numbers of Li+ in ILs or electrode-electrolyte interfacial phenomenon would affect to the rate characteristics. [References] 1) S. Wilkes, and M.J. Zaworotko, J. Chem. Soc. Chem. Commun., 0, 965-967 (1992). 2) N. Papageourgiou, Y. Athanassov, M. Armand, P. Bonhote, H. Pettersson, A. Azam, and M. Gratzel, J. Electrochem. Soc., 143(10), 3099-3108 (1996). 3) H. Sakaibe, H. Matsumoto, and K. Tatsumi, J. Power Sources, 146, 693-697 (2005). 4) K. Tsunashima, F. Yonekawa, and M. Sugiya, Electrochem. Solid-State Lett., 12(3), A54-57 (2009). 5) K. Tsunashima, Y. Sakai, and M Matsumiya, Electrochem. Commun., 39, 30-33 (2014). 6) X. Lin, R. Kavian, Y. Lu, Q. Hu, Y. Shao-Horn, and M. W. Grinstaff, Chem. Sci., 6, 6601-6606 (2015).