Lactone-Based Li+/F- Hybrid Liquid Electrolytes for Room-Temperature Fluoride Shuttle Batteries
Authors: Kawasaki Mitsuo , Morigaki Ken-ichi , Kano Gentaro , Kawauchi Shigehiro , Minato Taketoshi , Fujimoto Hiroyuki , Ogumi Zempachi , Abe Takeshi 
Published: May 2020

The fluoride shuttle batteries (FSBs) represent one of the potentially promising beyond-lithium-ion innovative battery systems, operating with a redox-active fluoride-ion transporting electrolyte. The low-weight, anion-based, multi-electron-transfer reactions at the anode and cathode, with a wide range of options on metals and their fluoride counterparts as the active materials, offer the opportunity to hopefully breakthrough the energy density limitation of the state-of-the-art lithium ion batteries.
Of the two options of FSBs, all-solid type and liquid-type cells, we have focused on the latter format that would probably enable a much easier room-temperature operation than does the all-solid based cell, if only we could succeed in the development of a redox-active fluoride-ion (or its complexed forms) transporting liquid electrolyte that concomitantly ensures a high electrochemical stability over a widest possible potential window. To this end, we have recently established a novel method to fabricate a plain fluoride-ion electrolyte in a high boiling lactone solvent (γ-butyl lactone; GBL), denoted as CsF(KF)/GBL, where either CsF or KF is dissociated without the help of any extra addenda. This is believed to be by virtue of the acidic α-hydrogen atoms next to the carbonyl group of GBL. As shown in Figure 1, F- ions in GBL may be stabilized in the configuration where they are coordinated by two GBL molecules via the interactions with α-hydrogens on both sides. This structure prevents the formation of HF (by the α-hydrogen abstraction by the F- ion), thus resulting in the sufficient stability and the long (many months or even years) shelf-life of CsF(KF)/GBL as the liquid electrolyte for room-temperature FSBs.
Nevertheless, we subsequently realized that while the high reactivity of the GBL-solvated F- ions brought about good reversible fluorination and defluorination reactions for almost arbitrary metals from Ag down to Zn in the redox series, it also caused a highly irreversible reductive sub-reaction that could never be traced back to the simple electrochemical reduction of GBL on its own. What we then came up with is the Li+/F- hybrid liquid electrolyte, where the Li+ ions mixed in a large excess of an equimolar composition with respect to the F- ions form some complexes, Li
x
F(

x

-1)+, that are soluble in GBL, and thus prevent the precipitation of LiF.
The lactone-based Li+/F- hybrid liquid electrolyte prepared in the above-noted manner largely extended, especially, the high negative potential window to or beyond the standard redox potential of Li+/Li. The Li+/F- hybrid liquid electrolyte was also accompanied by a unique function as an AA (anion acceptor) that facilitates a very minor but yet substantial dissolution of various metal fluorides as the active materials in the proximity of the electrodes under the respective redox operations. This, together with the expanded potential window, greatly improved the charge/discharge capacities and reversibility of both positive and negative electrodes in the practical composite forms employing nano-scale or sub-nanoscale particles of the relevant active materials. We are demonstrating the charge/discharge behaviors of some typical electrodes in the Li+/F- hybrid electrolyte, such as Cu/CuF2, Bi/BiF3, Al in combination with a highly disordered phase of AlF3, and so forth.

Acknowledgement:
This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery 2 (RISING2), financially supported by New Energy and Industrial Technology Development Organization (NEDO).

Figure 1