Deep Supercooling of Li Salts By Trace Polymer Additives: An Approach to Single Li Ion Conducting Liquid Electrolytes
Authors: Sudoh Taku , Takashi Goto , Ugata Yosuke , Philippi Frederik , Watanabe Masayoshi , Ueno Kazuhide 
Published: December 2024

As a solution to global warming in recent years, the conversion from gasoline-powered vehicles to EVs is being attempted around the world, but the long charging time of the batteries in EVs has delayed their widespread adoption. For realizing the rapid charging of Li secondary batteries, also equipped with EVs, it is necessary to improve the transport properties of electrolytes such as ionic conductivity (σ) and Li+ transference number (t
Li) 1. Therefore, all-solid-batteries with solid electrolytes, which has high σ and t
Li ~ 1, are considered as a promising next-generation rechargeable battery, even though the designing of the electrolyte-electrode interface still remains a challenge 2. On the other hand, in liquid electrolytes (LEs), which has an advantage in interface design, t
Li is still low. Although various studies have reported an enhancement of in LEs by means of changes to anion structures, the use of polyanions, and highly concentrated electrolytes, the reports of t
Li ~ 1 are limited to the use of molten salts in a battery cell 3, 4. However, most of Li salt have both a high melting point and crystallinity, so that it crystallizes easily near room temperature, which is unsuitable for battery adaptation.
To overcome this high crystallinity of Li salt at room temperature, we herein report anti-crystallization of Li salt by adding a small amount of polymers as obstacles to pack crystal structure. As shown in figure, we mixed lithium (fluorosulfonyl)(trifluoromethanesulfonyl)amide (Li[FTA]) and poly(methyl methacrylate) (PMMA) at the molar ratio of 9:1 (95.5 wt% of Li salt), thereby we could obtain the liquid mixture at room temperature. The result of differential scanning calorimetry shows that it remains supercooled state at ambient temperature (deep supercooling), even though 95 wt% of mixture is composed of Li salts. Additionally, the experimental value of 95.5 wt% Li[FTA]/ PMMA mixture was −25 °C and lower than that of Li[FTA] (−5.3 °C) and PMMA (106 °C), respectively (super-plasticizing). This indicates that the presence of slight polymer is not only an obstacle for packing into crystal lattice but also a plasticizer for Li salts. Therefore, this mixture, composed almost entirely of Li salt and liquified by deep supercooling and super-plasticizing effect, is considered as a deeply supercooled salt (DSS).
DSS has ionic conduction at ambient temperature and show t
Li ~ 1 under anion blocking conditions at 60 °C due to the absence of neutral solvents and only a small amount of polymer in electrolytes, which cannot cause valuable salt concentration gradients in measurement time scale. We will report ionic transport and electrochemical properties of DSS for batteries.

Acknowledgement:

This study was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA-Next) of the Japan Science and Technology Agency (JST).

Reference:

1. M. Diederichsen, E. J. McShane and B. D. McCloskey, ACS Energy Letters, 2017, 2, 2563-2575.
2. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba and R. Kanno, Nature Energy, 2016, 1, 16030.
3. Kubota and H. Matsumoto, The Journal of Physical Chemistry C, 2013, 117, 18829-18836.
4. Shigenobu, F. Philippi, S. Tsuzuki, H. Kokubo, K. Dokko, M. Watanabe and K. Ueno, Physical Chemistry Chemical Physics, 2023, 25, 6970-6978.

Figure 1