Fluoride-Ion Batteries: A Review of Recent Advances and Future Opportunities

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window.3 It is commonly known that quaternary ammonium cations undergo Hofmann elimination, during which β-hydrogen is withdrawn by strong bases such as OH− and F− ions.4,5 Therefore, to use ILs as an electrolyte for FIB, it is necessary to improve their stability against F− ion.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.AcknowledgmentThis 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

Fluoride ion batteries (FIBs) are among interesting electrochemical energy storage systems that are being considered as alternatives to lithium-ion batteries (LIBs). FIB offers high specific energy and energy density, thermal stability, and safety. Despite the advantages posed by the FIBs, several challenges need to be addressed to realize its full potential. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries. So far rechargeable FIBs have been demonstrated only at an elevated temperature like 150 °C and above. Here, for the first time, we demonstrate room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte. The high ionic conductivity of tetragonal BaSnF4 (3.5 × 10–4 S cm–1) enables the building of RT FIB. We built fluoride ion batteries using Sn and Zn as anodes and BiF3 as a cathode. We have investigated the electrochemical properties of two different electrochemical cells, Sn/BaSn...

Recently reversible batteries based on fluoride ions as a charge carriers have attracted some attentions as an alternative electrochemical energy storage system to conventional lithium ion batteries (LIBs). Fluoride is the most stable anion with a high mobility and therefore, fluoride ion batteries (FIBs) can theoretically provide a wide electrochemical potential window. Moreover, FIBs are capable of being built in an all solid-state modification. Previously, electrochemical fluoride ion cells based on conversion-based electrode materials have been built. However, the state of the art of the FIBs suffer from poor cycling performance in lack of well-developed cell components including the electrode materials. In the current study, intercalation-based cathode materials have been investigated as an alternative approach to make electrode materials for FIBs. In this respect, various compounds with mainly Ruddlesden-Popper-type structure including LaSrMO4 (M = Mn, Co, Fe) and La2MO4+d (M = Co, Ni) as well as Schafarzikite-type compounds of Fe0.5M0.5Sb2O4 (M = Mg, Co) have been subjected to electrochemical measurements including galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy and the structural changes upon electrochemical fluorination/de-fluorination were analyzed by X-ray Diffraction (XRD). LaSrMnO4 has been fluorinated/de-fluorinated via electrochemical method confirming successful intercalation/de-intercalation of the fluoride ions, but showed problems for long-term operation. In contrast, La2NiO4+d showed to be the most promising intercalation-based cathode material (for FIB) in terms of cycling stability (>220 cycles and 60 cycles for cutoff capacities of 30 and 50 mAh/g, respectivly) with a nearly 100% Coulombic efficiency (average Coulombic efficiency of 97.68% and 95.44% for cutoff capacities of 30 and 50 mAh/g, respectively). This is the highest cycle life that has been reported so far for a FIB. One of the major challenges of the proposed FIB systems was found in avoiding oxidation of the conductive carbon which has been mixed with the electrodes to improve the electronic conductivity. This decomposition of the carbon matrix results in a remarkable increase in the impedance of the cell and can significantly impair the cycle life and discharge capacity. However, the critical charging conditions which could be determined by cyclic voltammetry and electrochemical impedance spectroscopy have a major impact on preserving the conductivity of the cell. In addition, the effect of volume change in the conversion-based anode materials has been studied showing that the overpotentials arising from the volume change can significantly influence the cycling behavior of the battery system (due to absence of well-developed intercalation-based anode materials for FIBs, conversion-based counter electrodes have been used as anode materials).

Solid-state synthesis and ion transport characteristics of the β-KSbF4 for all-solid-state fluoride-ion batteries

The case for fluoride-ion batteries

Studies on fluoride ion conductivity of the mechanochemically synthesized β-KSbF4 for all-solid-state fluoride-ion batteries

Fast ion transport in mechanochemically synthesized SnF2 based solid electrolyte, NH4Sn2F5

Fluoride ion batteries (FIBs) are a recent alternative all-solid-state battery technology. However, the FIB systems proposed so far suffer from poor cycling performance. In this work, we report La2NiO4.13 with a Ruddlesden-Popper type structure as an intercalation-based active cathode material in all solid-state FIB with excellent cycling performance. The critical charging conditions to maintain the conductivity of the cell were determined, which seems to be a major obstacle towards improving the cycling stability of FIBs. For optimized operating conditions, a cycle life of about 60 cycles and over 220 cycles for critical cut-off capacities of 50 mAh/g and 30 mAh/g, respectively, could be achieved, with average Coulombic efficiencies between 95 – 99%. Cycling of the cell is a result of fluorination/de-fluorination into and from the La2NiO4+d cathode, and it is revealed that La2NiO4.13 is a multivalent electrode material. Our findings suggest that La2NiO4.13 is a promising high energy cathode for FIBs.

Rechargeable batteries with high energy density have been required for recent applications such as battery electric vehicles. Fluoride ion batteries (FIBs) can achieve high energy density by using monovalent fluoride ions as charge carriers and multi-electron reactions of electrode active materials, and have attracted much attention as a candidate to surpass the performance of the current lithium-ion battery (LIB). However, FIB using the metal/metal fluoride conversion reaction, which has a high potential for energy density, has not been fabricated with good reversibility as the current LIB. On the other hand, the FIB using a crystal capable of intercalation (insertion-extraction) reaction of fluoride ions as the active material has been developed [1]. Recently, an all-solid-state fluoride ion battery using an oxyfluorosulfide (Sr2F2Fe2OS2; SFFOS) as the intercalation-type positive active material has been reported, and the battery has operated with high capacity and good reversibility at 413 K [2]. In this study, SFFOS was evaluated as an active material for liquid-based FIB. In particular, the thermal effect on the intercalation reaction of SFFOS in ionic liquid-based electrolyte solution was measured.The SFFOS was synthesized from four kinds of powder, SrF2, SrO, Fe and S, with molar ratio of 1:1:2:2 referring to the previous papers [2, 3]. These powders were mechanically ground under argon and pressed at 6 MPa to form a pellet. The resulting pellet was vacuum sealed in a quartz tube and sintered at 1073 K for 36 hours to obtain the SFFOS. PbSnF4 as the counter electrode material was prepared from PbF2 and SnF2 by mechanical milling at 600 rpm for 6 h using a planetary ball mill, and then calcined at 673 K for 1 h under an argon atmosphere. The electrolyte solution was prepared by dissolving anhydrous tetramethylammonium fluoride (TMAF) in an ionic liquid, N,N,N-trimethyl-N-propylammonium-bis(trifluoromethanesulfonyl)amide (TMPA-TFSA), at various concentrations such as 0.075 mol/dm3 (the molar ratio of TMAF:TMPA-TFSA=1:50). The working and counter electrodes were prepared by mixing of SFFOS or PbSnF4 with acetylene black and PVdF, respectively. The galvanostatic intercalation reaction of fluoride ions into SFFOS was measured using a three-electrode cell with a Pb|PbF2 reference electrode at 298, 373, and 423 K.The galvanostatic intercalation reaction (corresponding to a charging reaction) of the SFFOS/AB/PVdF composite electrode showed the charging plateau at about 0.5 V vs. Pb|PbF2. The following deintercalation (discharging) reaction, the discharge plateau was observed around 0 V. The intercalation/deintercalation plateaus were similar to those of the previously reported all-solid-state FIB, well [2].

Fluorine is highly electronegative (+2.84 V vs. SHE) element and possess low weight. Fluoride is monovalent, and it has high charge density. Consequently, it enables high ionic conductivity similar to Li+ in certain solids. However, fluorine is a corrosive and toxic gas, which forbids it as one of the electrodes. Nevertheless, fluoride ions can be transported between a metal and a metal fluoride through a fluoride transporting electrolyte, and fluoride-ion batteries (FIB) can be realized. FIB offers high specific energy, energy density, thermal stability, and safety compared to state of the art lithium-ion batteries (LIBs). Primary batteries based on fluoride transport were demonstrated earlier [1]. Recently we have demonstrated first rechargeable FIB that was operable at 150 °C [2]. Several challenges need to be addressed to realize its full potential of FIBs. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries [3-5]. Recently, for the first time, we have demonstrated room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte [6]. BaSnF4 shows high fluoride conductivity of 3.5x10–4 S cm–1, but limited electrochemical stability window. To be compatible with BaSnF4, we have utilized low electropositive Sn, Zn as anode materials to demonstrate RT FIB. However, to enable cells with high operating potentials, the electrolyte should be compatible with highly electropositive metals (e.g., Mg, Ce). Toward this, we developed a new interlayer electrolyte that is compatible with highly electropositive metals like Ce and Mg. Further, utilizing the interlayer electrolyte, we demonstrate the functioning of first high potential RT FIB.

The increasing demand for energy storage resulted in improving the performance of Lithium ion batteries (LIB) and a continuing search for alternative battery technologies. Reversible batteries based on a fluoride anion shuttle (fluoride ion battery) are an interesting alternative to LIBs [1]. Fluoride ion batteries can theoretically achieve a high-energy density above 5000 Wh.L-1 , which is e.g. 50% above the theoretical capacity of a Li air cell [2]. However, research in the field of fluoride ion batteries is at an early stage of development, needing large improvements to meet the requirements for application. Understanding the electrochemical reactions occurring in the battery electrodes during cycling is essential to improve the performance and cyclic stability of fluoride ion batteries. Battery research groups worldwide, are try to directly observe the structural and chemical evolution of battery components in real space and to correlate this with the corresponding ex-situ cyclic behavior. In-situ analysis comprises complex sample environment systems, requiring careful development of experimental aspects to enable correlation with the true operating conditions. In-situ TEM is one of the few techniques that can provide direct structural and compositional information of micron-sized batteries during cycling. These in-situ studies often necessitate unique sample preparation techniques. An additional aspect is electron beam damage of battery materials, which modify the system and complicate the data interpretation. Here, beam damage challenges and sample preparation possibilities along with our strategies to identify an optimum system for in-situ TEM studies will be presented. We selected a fluoride ion battery system for this study.Ball milling of a mixture of (1−y)LaF3 and yBaF2 was employed to prepare a La0.9Ba0.1F2.9 solid electrolyte. This electrolyte was initially studied for its structure, composition and stability towards the electron beam. A mixture of Cu (90%), as an active material, and C (10%) was used as a cathode. The anode, in case of a half-discharged-state, was prepared from a mixture of Mg, MgF2, La0.9Ba0.1F2.9 (for ionic conductivity), and C (for electronic conductivity). Cathode, anode and electrolyte were pressed together to form a pellet. A focused ion beam system was used to prepare a thin cross-section of the complete battery and electrically contacted on an MEMS based device at the edge of the electrodes (Fig. 1a). For electrochemical measurements, an Aduro sample holder was used in the Titan 80-300 TEM. Variations in morphology, structure and composition of the electrodes, electrolyte and their interfaces were characterized using TEM, STEM, and SAED during electrochemical cycling (Fig. 1b). The HRTEM images, SAED studies and STEM-Map of the cathode at the interface indicate the formation of CuF2 phase after charging (Fig. 1c,d), which was not present in the as-prepared state. The sample preparation, and the changes in the morphology, structure and composition of the La, Ba/ La0.9Ba0.1F2.9/BiF3 and the Mg/ La0.9Ba0.1F2.9/CuF2 systems will be presented and discussed.

Recently, fluoride ion batteries have been attracting attentions because of their potential for large capacity far beyond that of conventional lithium ion batteries. However, fluoride ion batteries are still not into practical use. One of the problems with fluoride ion batteries is low ionic conductivity in the electrolyte. At this moment, no solid-state fluoride ion conductors, showing high ionic conductivity at room temperature together with a wide electrochemical window, have been developed. Among fluoride ion conductors, Tysonite-type (La,Ba)F3 (LBF) is known to have a wide electrochemical window and exhibit relatively high conductivity near room temperature. However, its conductivity is still needed to be improved. It was reported that the conductivity of a single crystalline LBF was approximately ten times higher than that of the polycrystalline ones. This indicated that microstructural factors in the polycrystalline LBF, such as the density and/or the grain boundary, deteriorate the ionic conduction. Thus, it is important to understand influences of the microstructures on the ionic conductivity for improving the ionic conductivity of LBF. From the above backgrounds, in this study, dense LBF samples with different grain sizes were prepared by using the spark plasma sintering (SPS) method, and their bulk and grain boundary conductivities were evaluated in order to clarify influences of the microstructures on the ionic conductivity. We first evaluated the relation between the microstructures and the ionic conductivity in Tysonite-type La0.93Ba0.07F2.93 by using the samples sintered at 700 to 1000 ºC. The dense samples, having the relative density higher than 95%, could be obtained by the SPS method when the sintering temperature was 800 ºC or higher. The average grain size was about 0.4, 0.5, and 1 μm in the samples sintered at 800, 900, and 1000 ºC, respectively. The conductivity increased with increasing the sintering temperature. From the AC impedance spectroscopy measurements, it was found that the bulk conductivity was almost independent of the sintering temperature, whereas the apparent grain boundary conductivity increased with increasing the sintering temperature. This indicated that the grain growth due to the high sintering temperature decreased the number of grain boundary, thus the apparent grain boundary resistance. Based on above results, we sintered the samples at 1100 and 1200 ºC, aiming further grain growth. The average grain size of the sintered samples became larger, about 20 and 60 μm by sintering at 1100 and 1200 ºC, respectively. However, the conductivity unexpectedly decreased with increasing the sintering temperature. From AC impedance spectroscopy measurements, a significant decrease in the grain boundary conductivity was observed with increasing the sintering temperature, while the bulk conductivity was almost the same regardless of the sintering temperature. In SEM observation, many pores were observed at the grain boundaries. These pores were considered as a main cause for the deterioration of the grain boundary conductivity. In order to suppress the pore formation at the grain boundaries, the sintering condition was re-examined. By decreasing the rate of rising temperature during the sintering process, the pore formation could be suppressed even for the sintering at 1200 ºC. The conductivity of the sample sintered at 1200 ºC was improved by decreasing the rate of raising temperature. For instance, the conductivity of the sample sintered at 1200 ºC with the slow rate, 2 ºC·min-1, was almost comparable with that sintered at 1000 ºC with the fast rate, 50 ºC·min-1. Throughout this work, it was concluded that the densification and the grain growth would be effective for the enhancement of ionic conductivity in Tysonite-type La0.93Ba0.07F2.93. Acknowledgement: This work was partly supported by JST. K.M appreciate Hatano Foundation for the support to his travel.

An anion flow battery has recently emerged as an option to store electricity with high volumetric energy densities. In particular, fluoride ions are attractive for these batteries because they have the smallest size among anions, which is beneficial for charge transport. To date, reported fluoride ion batteries either operate with an ionic liquid, organic electrolyte or solid-state electrolyte at high temperatures. Herein, an aqueous fluoride ion flow battery is proposed that consists of bismuth fluoride as the anode, 4-hydroxy-TEMPO (TEMPO) as the cathode, and NaF salt solution as the aqueous electrolyte. During the charging process, bismuth fluoride electrochemically releases fluoride ions with the formation of bismuth metal, while TEMPO captures the fluoride ions. A reversible and stable discharge capacity of 89.5 mAh g−1 was achieved at 1000 mA g−1 after 85 cycles. The fluoride ion battery possesses excellent rate performance. To the best of our knowledge, this is the earliest demonstration that fluoride ion batteries can work in aqueous solutions, which can be used for future clean energy applications.

A flexible tysonite-type La0.95Ba0.05F2.95@PEO-based composite electrolyte for the application of advanced fluoride ion battery

Fluoride ion conductors are developed for use as solid-state electrolytes in fluoride ion batteries which are one of promising candidates for next-generation storage batteries. Ba-doped LaF3 (La0.9Ba0.1F2.9: LBF) is mainly used as a solid-state electrolyte in fluoride ion batteries. However, room temperature conductivity of LBF is considerably low, on the order of 10−6 S cm−1 and it is still unclear the optimal elements to be doped to LaF3. In this study, we have explored La0.9Sr x Ba0.1−x F2.9 system (x = 0, 0.01, 0.025, 0.05, 0.1), in which Ba in LBF is substituted for Sr and investigated the composition dependence of ionic conductivity. We elucidate that the higher concentration of Sr without Ba can significantly improve the ionic conductivity, and the maximum ionic conductivity of La0.9Sr0.1F2.9 is 1.5 × 10−5 S cm−1 at room temperature, which is one order of magnitude larger than that of LBF. The higher ionic conductivity of LSF is due to the larger grain size and higher sintering density of LSF compared to LBF, which results in lower grain boundary resistance. The LSF total ionic conductivity of 10−4 S cm−1 can be achieved at 350 K, which significantly lowers operating temperature of fluoride ion batteries down to 350 K.