Improved Thermal Stability of Ionic Liquid through Hydrogen Bond Donor As an Electrolyte for Fluoride-Ion Battery
Authors: Govindarajan Kaviarasan , Sakamoto Ryo , Kukita Susumu , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Fluoride ion batteries (FIBs) have recently gained much attention as a next generation battery system because of their potential to surpass lithium-ion batteries (LIBs) in many aspects such as high energy density and low cost. FIBs with solid or liquid electrolyte have been studied intensively, amongst liquid-based FIBs has an advantage of good interfacial contact between the electrolyte and electrode, and low-temperature operation over all-solid-state FIBs. However, stability has been a major concern for liquid electrolytes. The instability caused by the strong basicity of F− ion attacking most of the commonly used organic solvents leading to side reactions.1 So far bis(2,2,2-trifluoroethyl) ether (BTFE), which has good base resistance, has been proposed as an effective organic solvent for electrolyte.2 However, the boiling point of BTFE is relatively low (64 °C). While looking beyond organic solvents, ionic liquids (ILs) can be a choice as an electrolyte for FIBs owing to their good properties such as nonvolatility and wide electrochemical window.3 It is commonly known that quaternary ammonium cations undergo Hofmann elimination, during which β-hydrogen is withdrawn by strong bases such as OH− and F− ions.4,5 Therefore, to use ILs as an electrolyte for FIB, it is necessary to improve their stability against F− ion. One way to weaken the basicity of F- ion is to solvate them by a complexing agent. Hagiwara et al. reported that imidazolium-based ILs containing ethylene glycol are stable despite the presence of F− ion.6 Bond formation between F− ion and hydrogen in the hydroxy groups of ethylene glycol and imidazolium-based cation prevents the F− ion from attacking the β-hydrogen of the cation's alkyl chain. However, there have been no reports of FIBs using ILs containing solvated F− ion as electrolytes. In this study, we investigated the thermal stability of choline bis(trifluoromethanesulfonyl)amide (N111(2OH) TFSA), an IL with hydrogen bond donor functional group, against F− ions. Furthermore, we performed charge-discharge tests on liquid-based FIBs using N111(2OH) TFSA containing fluoride salt as an electrolyte.
N111(2OH) TFSA (Fig. 1 (a)) was synthesized by ion exchange of N111(2OH)
Cl with K TFSA. The melting point of the synthesized N111(2OH) TFSA was estimated to be 38 °C from DSC measurement and it was solid at room temperature. 0.4 mol kg−1 tetramethylammonium fluoride (TMAF) in N111(2OH) TFSA was found to be stable up to about 150 °C (Fig. 1(c)). TGA measurements of N,N,N-trimethyl-N-propylammonium (N1113 TFSA) (Fig. 1(b)) without hydroxy groups were also performed under the same conditions for comparison. A solution of 0.4 mol kg−1 TMAF in N1113 TFSA showed a gradual weight loss right after the measurement was started, with a significant weight loss at about 120 °C (Fig. 1(c)). These results suggest that N111(2OH)
TFSA with the hydroxy group exhibits higher thermal stability toward F− ion. This was attributed to the hydrogen bonding of the hydroxy group to F− ion, which weakened the basicity of F− ion. Finally, charge-discharge tests were performed in a three-electrode cell using BiF3 working electrode, Pb counter electrode, and Ag/Ag+ reference electrode. Figure 1 (d) shows charge-discharge curves of BiF3 electrode with 0.4 mol kg−1 TMAF in N111(2OH)
TFSA as an electrolyte at 60 °C. The first discharge capacity of the BiF3 electrode was 300 mAh g−1, which represents 99 % of the theoretical capacity of BiF3 (302 mAh g−1) with a columbic efficiency of 85%. This result suggests that the BiF3 was reversibly defluorinated/fluorinated with 0.4 mol kg−1 TMAF in N111(2OH) TFSA.
Acknowledgment
This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
References
(1) V. K. Davis, et al., Mater. Chem. Front., 2019, 3, 2721-2727.
(2) V. K. Davis, et al., Science, 2018, 362, 1144–1148.
(3) K. Okazaki, et al., ACS Energy Lett., 2017, 2, 1460–1464
(4) S. Raiguel, et al., Green Chem., 2020, 22, 5225-5252.
(5) H. Sun, and S. G. Dimagno, J. Am. Chem. Soc., 2005, 127, 2050-2051.
(6) Z. Chen, et al., J. Phys. Chem. Lett., 2018, 9, 6662−6667

Figure 1