Room-Temperature, Rechargeable Solid-State Fluoride-Ion Batteries

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.

With increasing demand on energy supply together with reducing fossil sources, many efforts have been put into the research and development of environmentally benign, high energy density and low-cost rechargeable battery systems as an alternative. Fluoride Ion Battery (FIB), which is based on fluoride ion electrochemistry rather than lithium ion (in case of lithium ion battery), has emerged as a potential alternative. The element fluorine possesses an outstanding potential compared to Li owing to its high electronegativity and, thus, electrochemical stability, relatively low weight, more natural abundance and comparably low cost. By choosing an appropriate pair of electrodes comprising metal fluoride and metal together with a suitable fluoride-containing electrolyte, a high voltage electrochemical FIB cells can be built [1, 2]. Recently the first proof of concept for rechargeable fluoride ion batteries based on all-solid-state setup has been demonstrated [2]. Alkaline earth metals have recently attracted much attention for their employment in energy storage technologies due to potentially high energy density, low cost and environmental friendliness. Hence, in this study Ca and Ba pure metals were chosen and tested as anode in rechargeable FIB system. CaxBa1-xF2 (x ~ 0.5), a highly ion conducting metastable fluoride compound, was selected and synthesized via high energy mechanical milling route as fluoride ion conducting electrolyte. Powder X-ray diffraction confirms the formation of solid solution and electrochemical impedance measurement of CaxBa1-xF2 shows ionic conductivity in the order of 10-4 S.cm-1 at elevated temperature around 200 0C, which is in agreement with earlier report [3]. The assembly of all-solid-state electrochemical FIB cells was carried out with Ca or Ba as anode, selected metal fluorides (BiF3, CoF3, and CuF2) as high voltage cathode, and CaxBa1-xF2 (x ~ 0.5) as electrolyte. The cells were assembled in the charged state and electrochemically tested at ~ 200 0C. Initial results showed that the specific capacities of Ca/BiF3 and Ca/CuF2 cells for the first discharge are about 260 mAh/g and 350 mAh/g, respectively. These capacities are about 86% (in case of BiF3) and 66% (in case of CuF2) of their theoretical specific capacities and higher than those of Ce/BiF3 and Ce/CuF2 systems reported previously [2]. Electrode analysis together with cycling behaviour reveal the fluoride ion transfer between anode and cathode, as well as the cyclability of alkaline earth metal based FIB system. Further structural and electrochemical characterization will be presented and discussed.

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

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].

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

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).

Last developments in polymers for wearable energy storage devices

Testing Mg as an anode against BiF3 and SnF2 cathodes for room temperature rechargeable fluoride ion batteries

Solid-state fluoride-ion batteries (FIBs) attract significant attention worldwide because of their high theoretical volume, energy density, and high safety. However, the large interfacial resistance caused by the point-point contact between the electrolyte and the electrode seriously impedes their further development. Using liquid-phase therapy to construct a conformal interface is a good choice to eliminate the influence of inadequate contact between the electrode and the electrolyte. In this study, a β-PbSnF4 solid-state electrolyte with high room-temperature ionic conductivity is prepared, and a trace amount of the liquid electrolyte (LE) between the electrode and the electrolyte is introduced in order to minimize the interfacial resistance and enhance the cycle life. The Allen-Hickling simulations show that the introduction of an interfacial wetting agent (LE) can significantly reduce the energy barrier of charge transfer and mass transfer processes at the interface and reciprocate FIBs an enhanced interfacial reaction kinetics. As a result, the initial discharge capacity of the fabricated FIBs is 210.5 mAh g-1 and the capacity retention rate is 82.6% after 50 cycles at room temperature, while the initial discharge capacity of the unmodified battery is only 170.9 mAh g-1 and the capacity retention rate is 22.1% after 50 cycles. Therefore, interfacial modification with a trace amount of LE provides a significant exploration for the improvement of FIB performances.

The case for fluoride-ion batteries

Fluoride-ion batteries are promising “next-generation” electrochemical energy storage devices, and thus, the room-temperature rechargeable fluoride-ion batteries (FIBs) have attracted tremendous attention due to their high theoretical volume energy density and high safety. However, a series of problems including high interface impedance and poor ionic conductivity at room temperature prevent further development and commercial application of FIBs. Herein, rare-earth element Eu3+-doped BaSnF4 solid solutions [Ba1–xEuxSnF4+x (0 ≤ x ≤ 0.06)] are designed and prepared to improve the performance of BaSnF4 solid electrolytes for room-temperature FIBs. It has been found that the as-prepared Ba0.98Eu0.02SnF4.02 solid-state electrolyte can achieve a better ionic conductivity of 3.8 × 10–4 S cm–1 at room temperature after a calcination process at 300 °C for 2 h, which is the improvement of an order of magnitude in comparison with the original samples. In addition, the FIBs based on Ba1–xEuxSnF4+x (0 ≤ x ≤ 0.04) solid-state electrolytes (Sn/Ba0.98Eu0.02SnF4.02/BiF3) show a discharge capacity of 106 mAh g–1 at 1st cycle and 72 mAh g–1 at 20th cycle. Moreover, the Sn/Ba1–xEuxSnF4+x/BiF3 (0 ≤ x ≤ 0.04) batteries also exhibit good cycling stability and rate performance. Therefore, the addition of Eu3+ can better improve the ionic conductivity of the original solid electrolyte material, which provides a new strategy for the preparation and modification of fluoride-ion electrolytes in FIBs chracterization chracterization.

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.

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

In the search for novel battery systems with high energy density and low cost, fluoride ion batteries have recently emerged as a further option to store electricity with very high volumetric energy densities. Among metal fluorides, CuF2 is an intriguing candidate for cathode materials due to its high specific capacity and high theoretical conversion potential. Here, the reversibility of CuF2 as a cathode material in the fluoride ion battery system employing a high F− conducting tysonite‐type La0.9Ba0.1F2.9 as an electrolyte and a metallic La as an anode is investigated. For the first time, the reversible conversion mechanism of CuF2 with the corresponding variation in fluorine content is reported on the basis of X‐ray photoelectron spectroscopy measurements and cathode/electrolyte interfacial studies by transmission electron microscopy. Investigation of the anode/electrolyte interface reveals structural variation upon cycling with the formation of intermediate layers consisting of i) hexagonal LaF3 and monoclinic La2O3 phases in the pristine interface; ii) two main phases of distorted orthorhombic LaF3 and monoclinic La2O3 after discharging; and iii) a tetragonal lanthanum oxyfluoride (LaOF) phase after charging. The fading mechanism of the cell capacity upon cycling can be explained by Cu diffusion into the electrolyte and side reactions due to the formation of the LaOF compound.

Rechargeable lithium-ion batteries (LIBs) involving lithium metal oxides, liquid electrolyte and graphite have been widely used in portable electronic devices due to their relatively high energy density and long cycle life. These desirable features make LIBs very attractive as the power source for electronic devices, hybrid electric vehicles (HEVs) and electric vehicles (EVs) applications [1, 2]. For future EV applications, higher energy density of LIBs up to 360 Wh kg-1 is required. Currently, the energy density of the state-of-the-art LIBs using conventional graphite anode, LiFePO4 (denoted as LFP) or LiNi0.5Co0.2Mn0.3O2 (NCM523) cathodes and 1-1.2 M LiPF6 in organic carbonate electrolytes provide practically achievable energy densities of up to around 200-260 Wh kg−1 [3]. When commercial graphite anodes are used, LiNi0.8Co0.15Al0.05O2 (NCA), LiNi0.8Co0.1Mn0.1O2 (NCM811), LiNi0.5Mn1.5O4 (LNMO) and LiNiPO4 (LNP) cathode based batteries with high-voltage provide energy densities of 354, 338, 351 and 414 Wh kg-1, respectively. However, LIBs using these high-voltage cathode materials and the organic carbonate electrolytes exhibit quite low thermal stability and tend to catch fire or even explode when abnormal charge/discharge cycling or accidental penetration of cells occurs, which greatly limits the automotive applications. When replacing graphite with a Li metal anode, the energy densities of all battery systems can be enhanced significantly due to the highest theoretical specific energy density (3860 mAh g-1) among all anode materials for rechargeable LIBs. Nevertheless, commercial LIBs are prone to cause safety problems due to the safety concern arising from Li dendrite growth in liquid organic electrolytes [4-6].The promising solid-state LIBs offer high thermal stability (i.e., low risk in catching fire), high energy density, wide electrochemical stability window and less environmental impact. A competent electrolyte is the key component of solid-state LIBs. The solid-state electrolyte materials are mainly classified as solid polymer electrolytes (SPEs), inorganic solid electrolytes (ISEs), and organic/inorganic composite electrolytes. ISEs include oxide-based and sulfide-based materials [7, 8], which show very high ionic conductivity (10-2 – 10-3 S cm-1). Furthermore, the lithium ion transference number is close to 1. However, the major limitation factors of practical solid-state LIB applications are the large interfacial impedance between electrode and ISE and the difficulty of processing [9]. Considering processability, mechanical flexibility, interfacial compatibility and electrochemical stability, one prefers SPEs to the inorganic ceramic electrolytes. Nevertheless, SPEs have low ion conductivities (10−7 − 10−5 S cm−1 near room temperature) and most of the Li+ transference numbers are lower than 0.5 [10, 11]. The major requirements for SPEs include high ionic conductivity and transference number at room temperature, wide electrochemical potential window, high mechanical strength and excellent thermal stability. However, the ion conductivity is the most important (> 10-4 S cm-1 at room temperature desired) and should be considered first. The coordinating groups of a good polymeric host are expected to interact with Li+ and facilitate dissociation.In this study, we prepared various novel acrylonitrile-based polymers (e.g., acrylonitrile/acrylate copolymer and polymer with two pendant groups b-cyano ethyl ether (-O-CH2CH2-CN) sulfonate alkyl ether (-O-(CH2)3SO3Li). The corresponding SPEs comprising acrylonitrile-based polymer and ca. 50 wt.% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with high ionic conductivity (up to 10-3 S cm-1) at room temperature, high ion transfer number (up to 0.45) and large electrochemical potential window (oxidation stability > 5 V vs. Li+/Li) achieved. The selected SPEs were used as the separator in solid-state batteries with LiFePO4 as the cathode and Li foil as the anode; and long-term cycle stability of solid-state LIB was achieved. The polymers and corresponding SPEs were characterized by using DSC, SEM, XRD and FTIR measurements. Ionic conductivities of SPEs were determined from electrochemical impedance spectroscopy results. The linear sweep voltammetry technique was adopted to measure the oxidation stability window of SPE, and the Evans-Vincent-Bruce method was used to determined ion transfer number.