Dual Fluoride Salt-Based Hydrate-Melt Electrolyte: Advancing Aqueous Fluoride Shuttle Battery
Authors: Pradhan Anusha , Kondo Yasuyuki , Saguchi Katsuhiko , Gorai Naoki , Kawashima Kenji , Katayama Yu , Yamada Yuki 
Published: December 2024

Introduction

Fluoride shuttle battery (FSB) is an attractive post lithium-ion battery (LIB) candidate owing to its high theoretical energy density due to the use of multi-electron conversion reactions on transition metal fluorides as high as 5000 Wh L−1. However, its utilization of charge-discharge reactions at room temperature (RT) has been hindered by the scarcity to adopt adequately conductive electrolytes which enable reversible fluorination, and most of the reported FSBs employ solid-state electrolytes at operating high-temperatures1. Few reports on the liquid electrolytes suggested the employment of a saturated organic solution to control the metal fluoride dissolution for improving its cycling performance2. However, inorganic fluoride salts do not readily dissolve into organic solvents, and concentrated electrolytes were not obtained despite the addition of anion receptors, which hinders the fluoride conductivity3. Therefore, the use of organic solvents carries many demerits such as poor ionic conductivity, flammability, toxicity and so on. In this context, concentrated aqueous electrolytes could be a great substitute to address these issues owing to a) the toxic-free nature of water, b) the high solubility of inorganic fluoride salts in water, c) electrochemical stability enhancement at the electrodes due to the minimal free solvent molecules, d) facile room temperature transport properties4. Recently, a concentrated 35 mol kg-1 CsF electrolyte with a potential window of 3.1 V and ionic conductivity of around 50 mS cm–1 was reported5. However, there are still some challenges in terms of reversibility of metal fluoride electrodes.
Here we report a fluoride hydrate melt with the highest F- concentration of 45.2 mol kg-1, which corresponds to a F-:water molar ratio of 1:1.2. We used a eutectic mixture of CsF-RbF, inspired by our previous report on eutectic dual salt-based hydrate-melts for aqueous LIBs4. We studied the electrochemical properties of the CsF-RbF hydrate melt and applied it to an aqueous electrolyte for FSBs.

Experiment

CsF and RbF were mixed with water in several molar ratios to find the highest solubility for synthesizing hydrate-melts. Linear sweep voltammetry (LSV) tests were conducted using Pt metal, Pt mesh, and Ag/AgCl as the working, counter and reference electrodes, respectively. The ionic conductivity was evaluated by electrochemical impedance spectroscopy using symmetric cells with Pt electrodes. To explore the RT cycling performance of Cu electrodes in the hydrate-melt electrolytes, cyclic voltammetry (CV) of the Cu (negative electrode) – CuF2 (positive electrode,) with Ag/AgCl as the reference electrode was conducted. Both the Cu electrode and CuF2 electrode were prepared by mixing Cu or CuF2, acetylene black, and PTFE with a weight ratio of 85:10:5.

Results and Discussion

We discovered a novel RT hydrate-melts of fluoride salts with minimal water, which was denoted as CsF0.6RbF0.4. 1.2H20 with the concentration of 45.2 mol kg-1 (Figure 1a). In the LSV study, the oxidation and reduction potential of the hydrate-melt were observed to be 2.19 V and -1.4 V vs. Ag/AgCl, respectively (Figure 1b), which gave a wide potential window of 3.49 V on the Pt electrode. In addition, the ionic conductivity of the CsF-RbF hydrate-melt was found to be 20.5 mS cm-1 at 25 0C. As compared to lithium-based hydrate meltat similar high concentrations (potential window: 2.7 V, ionic conductivity: 3 mS cm-1)4, the CsF-RbF hydrate melt shows both wide potential window and high ionic conductivity.
The CV profile exhibited reversible fluorination of Cu at -0.25 V and defluorination at around -0.9 V. This could also be a hint where the dissolution of the active material is inhibited despite many cycles, as the concentration is high enough and amount of free water is insufficient for further dissolution. Thus, the application of the novel hydrate-melt electrolyte elucidates the intersection of enhanced solubility, non-flammability, low toxicity, wider potential window, high ionic conductivity and active material dissolution prevention for the FSBs.

References

M. A. Reddy, M. Fichtner, J. Mater. Chem., 2011, 21 (43), 17059-17062.

H. Konishi, T. Minato, T. Abe, Z. Ogumi, Z, J. Appl. Electrochem. 2018, 48, 1205-1211.

K. I. Okazaki, Y. Uchimoto, T. Abe, Z. Ogumi, ACS Energy Lett., 2017, 2 (6), 1460-1464.

Y. Yamada, K. Usui, K. Sodeyama, S. Ko, Y. Tateyama, A. Yamada, Nat. Energy,2016,
1 (10), 1-9.

O. Alshangiti, G. Galatolo, G.J. Rees, H. Guo, H, J.A. Quirk, J.A. Dawson, M. Pasta, ACS Energy Lett., 2023, 8 (6), 2668-2673.

Figure 1