Fluoride Shuttle Batteries Research Articles

Redox Behavior of a CuF2/Cu Electrode for an All-Solid-State Fluoride Shuttle Battery

Introduction The popularization of electrified vehicles connected to the smart grid community systems with stationary energy storage has attracted much attention as one of the solutions to global environmental problems. The critical technologies for such systems are rechargeable batteries with energy densities higher than that of conventional Li-ion batteries (LIBs). Among various candidates, all-solid-state fluoride-ion batteries (FIBs) operated by fluoride-ion transport [1] are promising owing to an ultra-high theoretical energy density over 5000 Wh/L [2].The FIBs currently we refer to as Fluoride Shuttle Batteries (FSBs) [3] were previously reported as a primary battery [4]. For the development of rechargeable FSBs, both electrolytes with high fluoride-ion conductivity and active materials that undergo reversible fluorination/de-fluorination reactions are crucial. Fluorite- and tysonite-type fluorides have been investigated as potential solid electrolytes since around the 1970s [ 5 , 6]. Recently, our group reported a series of PbxSn2-xF4 (x = 0.96~1.52) compounds [7] with a unique structure consisting of two alternating layers; a double Pb layer and a triple-layer, each flanked by a single Sn layer. The highest conductivity of 1.1 × 10-1 S/cm at 140 °C was achieved for fluorite-type Pb1.2Sn0.8F4 (x = 1.2). As for tysonite-type fluoride, Reddy et al. reported a possible rechargeable FIB using a La0.9Ba0.1F2.9 solid electrolyte [1] in combination with the various metal fluoride active materials such as CuF2, BiF3, and KBiF4. Among many metal fluorides, CuF2/Cu is a fascinating redox couple as the cathode active material owing to its large theoretical capacity (843 mAh/g) and high electro-motive-force (3.5 V vs. Li/Li+) [8]. However, full utilization of their theoretical capacity has not yet been demonstrated. In this study, we investigated the possible rate-determining steps in the CuF2/Cu cathode reaction using Pb1.2Sn0.8F4 (x = 1.2) with a high fluoride-ion conductivity. Experimental The Pb1.2Sn0.8F4 electrolyte was prepared by solid-state reaction using a mixture of PbF2 and SnF2 powders [7]. The fluoride-ion conductivity was measured by electrochemical impedance spectroscopy. The electrochemical cells were prepared by pressing into a 10 mm diameter pellet. Here, the cathode material was assembled using Cu, Pb1.2Sn0.8F4, and C in a 25:70:5 wt% ratio to facilitate both electronic and ionic conduction. Charge/discharge measurements were performed at 140 °C. Structural and electrochemical characterizations were conducted using TEM/SAED and operando XAFS. Results and Discussion The initial charge/discharge curves at 140 °C for the Cu cathode versus the PbF2 anode are shown in Fig. 1. The Cu cathode with Pb1.2Sn0.8F4 electrolyte exhibited a large charge (discharge) capacity of 522 (342) mAh/g. This capacity corresponds to 62 (41) % of the theoretical specific capacity even though the ionic conductivity of Pb1.2Sn0.8F4 was approximately 1000 times higher than that of La0.9Ba0.1F2.9 [1]. The Cu K-edge XAFS measurements during the charging process revealed the oxidation of Cu from Cu0 to Cu2+, and vice versa during the discharging process. The isosbestic points observed for all the spectra indicated that the conversion reaction involved the two phases, in agreement with the results of our DFT calculations [9]. In the TEM/SAED patterns of the cathode after charge, the distinct diffraction spots related to the CuF2 phase were observed on the surface of the metallic Cu particle. The fluoride-ion conductivity of CuF2 was 10-11 S/cm at 140 °C, which was very low compared to Pb1.2Sn0.8F4 and La0.9Ba0.1F2.9. These results suggest that the solid electrolyte may not be a dominant factor in the reaction on the cathode side, and alternatively smooth fluoride-ion diffusion inside the Cu active material plays a key role for redox behavior. As a next step, it may be effective to make the Cu active material a nano-composite with other fluorides, which is currently underway. Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan under the RISING and RISING2 projects.

(Invited) Novel Fluoride Batteries, Fluoride Shuttle Batteries, and Their Positive Electrodes

Progress of battery technology has opened new applications, which requires development of batteries of high performances, especially high energy density. There are two directions to improve the energy densities, i.e., enhancing cell voltage and increasing capacity of active materials. High cell voltage meets, however, the difficulty of stability of electrolyte.Batteries based on rocking-chair type reactions are limited in their capacity due to requirement of host lattice. On the other hand, conversion type active materials including traditional dissolution-deposition type ones can give higher capacity by using multi electron reactions. Anion mediating system may be able to be operated various metal electrodes using the common charge carrier. Among anion mediating systems, fluoride mediating systems, fluoride shuttle batteries (FSBs), give high capacity because metal fluoride is generally low formal weight.The authors’ group has focused on developing FSBs based on reactions of metal fluoride and metal, MFm/M. Metal fluoride are usually insulator, while some metal fluorides show ionic conductivity for F-. In this talk BiF3 and CuF2 are focused as positive active material. Electrolyte is a key material for activation of MFm/M electrodes. Solid electrolyte, liquid electrolyte including IL, and polymer electrolyte have been examined.Addition of anion acceptors to organic electrolyte enhances charge and discharge performances of BiF3/Bi electrode. The selection of adding anion acceptors is critical for each MFm/M electrode. The addition of anion acceptor, fluorobis(2,4,6-trimethylphenyl)borane, in tetragryme-CsF electrolyte increases remarkably the capacity and cyclability of BiF3/Bi electrode1). Copper fluoride is known as a typical insulator and when Cu electrode is oxidized to CuF2 only the skin-layer of Cu metal, ca. 6 nm, is oxidized to CuF2 and is passivated using lanthanum fluoride solid electrolyte operated at 140 oC. The addition of BaF2 to CuF2 powder activates CuF2/Cu electrode2). Konishi, T. Minato, T. Abe, Z. Ogumi, J. Electrochem. Soc., 2017 161, A3702.Morita, T. Abe, Z. Ogumi, et al., in preparation

Development of Cu-Based Positive Electrode for an All-Solid-State Fluoride Shuttle Battery

For the development of batteries with higher energy density, we have been developing all-solid-state fluoride shuttle batteries (FSBs) in RISING2 project of NEDO, Japan. Here we report the reversible electrochemical fluorination/de-fluorination of copper-based positive electrode using operando XAFS (X-ray absorption fine structure) measurement during charge/discharge processes. The Operando XAFS measurements were performed at the BL28XU of SPring-8, Japan. Based on the XAFS results, we have tried to improve the utilization of the active material by devising the electrode manufacturing method.The CeF3/Ce0.95Ba0.05F2.95/Cu cell showed 680 mAh/g(Cu) charge capacity and 593 mAh/g(Cu) discharge capacity, which was higher capacity than that of the previous works [1,2]. During the charge process, the operando XAFS spectra showed that pre-edge and white line peak intensity decreased and increased in proportion to the charge capacity, respectively, indicating that Cu converts to CuF2. On the other hand, the changes of XAFS spectra were opposite during the discharge process. These results indicate that this positive electrode is highly reversible for fluorination/de-fluorination reaction of Cu. However, some Cu metal remained at the full charge state because the charge capacity didn’t reach to the theoretical specific capacity (843 mAh/g), which was also confirmed from the XAFS spectrum.Other details will be shown in the conference. Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan under the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING2) project.

Lactone-Based Li+/F- Hybrid Liquid Electrolytes for Room-Temperature Fluoride Shuttle Batteries

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

Operando Analysis of Bi/BiF3 Electrode Reaction in Li+/F- Hybrid Electrolyte By Synchrotron Radiation Diffraction

1.IntroductionIn recent years, the development of innovative batteries that exceed the performance of lithium-ion batteries has been desired. One of the candidates is a fluoride shuttle battery (FSB). It is an anion-based, rechargeable batteries operating under fluoride shuttling, and rests upon the foundation of a redox-active fluoride ion transporting electrolyte of high electrochemical stability. To realize the FSB, we have developed a lactone-based Li+/F- hybrid electrolyte, in which Li+ and F- ions that normally cannot coexist in organic solvents form some complexes soluble in GBL. The thus reformed electrolyte, together with its delicate and unique anion accepting (AA) function, largely expands the negative potential window to or beyond the standard redox potential of Li+/Li. In this study, in order to clarify the electrochemical behaviors of BiF3/Bi electrode combined with the electrolyte, the operando analysis was carried out by the synchrotron radiation diffraction (SRD) measurement.2. ExperimentalThe BiF3 powder mixed with acetylene-black and PVdF was used as the working electrode. The activated carbon and Ag metal were used as the counter and reference electrodes, respectively. The Al-laminated half-cell composed of above-mentioned electrodes was assembled using a newly developed hybridized electrolyte. Thus prepared cell was pre-discharged (0th cycle) in advance in the potential range of -1.5 V – 1.0 V at ca. 0.04C. Then, several charge/discharge cycles were repeated with the same conditions. SRD was measured with BL28XU beam line at Super Photon ring-8, Japan during the whole charge/discharge process. High-energy X-ray with a wavelength of 0.049592 nm was used to obtain 2D diffraction patterns. The exposure time was set to 10 s, and several thousand diffraction profiles were obtained during the discharge/charge process. In order to analyze the obtained enormous profiles data in conjunction with the charge/discharge curves and their differential curves (V-dQ/dV), an analysis software called “Profile Chaser” was newly developed. By using the software, all the profiles of electrode during charge/discharge processes are loaded automatically in the computer. The profiles were successively displayed on a monitor as a series of profiles in conjunction of compositions estimated from the charge/discharge curves, so that the profile change was traced from the monitor.3. Results and discussionsFig.1 shows the SRD profile changes in the discharge process of 0th and 1st cycles. In both cycles, the intensity of 111 diffraction of BiF3 decreased and that of 012 diffraction of Bi increased as the discharge reaction proceeded. However, the formation of BiF3 was hardly observed in the charge process. It indicates that the bismuth fluoride formed in the charge process is amorphous or solved in the electrolyte. Figure 1

Defluorination/Fluorination Reaction of BiF3 Nanoparticles Prepared Via Sol–Gel Method for Fluoride Shuttle Battery

Introduction Rechargeable batteries with high energy density have been required for recent applications such as hybrid electric vehicles and large-scale energy storage. Rechargeable batteries based on fluoride shuttle (FSBs) have attracted much attention as a candidate above, owing to their high theoretical energy density by selecting appropriate metal/metal fluoride couples as active materials of negative- and positive-electrode[1-3]. One of the important things of the battery is adoption of the good conductive media for fluoride species, that is, an electrolyte[4]. Another is reversible fluorination/defluorination of active materials at the interface with electrolyte. Previously, we have demonstrated archetypal reversible FSB using a liquid electrolyte containing organic fluoride at room temperature[3]. However, this battery needed to improve the utilization and cyclability of its active material. Since the common metal fluoride shows low electronic- and ionic-conductivity, the active materials should be minified for FSBs. In this study, metal fluoride nanoparticle, e.g. BiF3, was prepared via sol–gel method using a trifluoroacetate precursor for improvement of the charge/discharge reactions in FSB. Bismuth trifluoride having 302 mAh/g in theoretical capacity is considered as a candidate for positive active material[1-6]. Then, we studied reversible defluorination/fluorination reaction of thus prepared BiF3 nanoparticles in a liquid electrolyte of FSB at room temperature, and valence change of bismuth species in the consequent electrode was evaluated using synchrotron ex-situ XANES measurement.Experimental Bismuth trifluoride nanoparticle was prepared from bismuth trifluoroacetate precursor via decarboxylation at 453 K in an oleylamine solution under argon atmosphere. At that time, specific amount of trifluoroacetic acid and/or trifluoroacetic anhydride were added as a supplementary fluorine source. After the nanoparticles were precipitated via addition of toluene and ethanol sequentially, the resulting precipitation was centrifuged and then re-dispersed in toluene containing oleic acid as a dispersant. Finally, nanoparticles were obtained after stripping of ligand using a trimethyloxonium tetrafluoroborate. Size of the resulting nanoparticles can be controlled by precursor concentration, decarboxylation reaction time and its temperature. Electrochemical measurement was carried out using a three-electrode cell with a VSP-300 BioLogic potentiostat/galvanostat in an argon-filled glove box at room temperature. The cell was composed of a carbon cloth as a counter electrode, a silver wire as a quasi-reference electrode, and an ionic liquid containing an organic fluoride as a liquid electrolyte. The liquid electrolyte was prepared by dissolving N-methyl-N-propylpiperidinium fluoride (MPPF) or N,N,N-trimethyl-N-neopentylammonium fluoride (NpF) in N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide (TMPA-TFSA) with molar ratio of 1:10. Nanoparticles-supported carbon cloth electrode as a working electrode was prepared by soaking the cloth in BiF3-nanoparticles/acetonitrile dispersion. The prepared nanoparticles via the sol–gel method and the defluorination/fluorination products in electrochemical measurements were evaluated using XRD, XPS, SEM-EDS, and so on in our laboratory. Furthermore, valence change of bismuth species was evaluated using synchrotron ex-situ XANES measurement at SPring-8, Japan.Results and Discussion Thus obtained nanoparticles via above-mentioned procedure had a few tens of nanometers in diameter and showed low crystallinity. Moreover, it was suggested that small amount of oxygen would be doped into the nanoparticles according to elemental analyses. Then, non-doped BiF3 nanoparticles having hexagonal crystalline structure had been acquired after further fluorination-treatment. Consequent BiF3 nanoparticles-supported carbon cloth showed ca. 240 mAh/g (80%) as an initial discharge capacity, and retained ca. 100 mAh/g for more than five cycles at room temperature. Furthermore, in ex-situ bismuth L3-edge XANES spectra, valence of bismuth species changed from trivalent to zero (metal) during the defluorination reaction and similar reverse change was elucidated in fluorination reaction. In this way, the size of the active material was minified by the preparation method, thereby improving the utilization and cyclability in FSB. This method will be also applied for other positive- and negative-active material having high gravitational theoretical capacity.Acknowledgements This work was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Electrochemical Performance of BiF3‐BaF2 Solid Solution with Three Different Phases on a Fluoride Shuttle Battery System

AbstractSolid solution of bismuth(III) fluoride (BiF3) and barium(II) fluoride (BaF2) with a variety of crystal structures was prepared, and their electrochemical performances were evaluated on a fluoride shuttle battery (FSB) system. BiF3‐BaF2 based solid solution (Bi1‐xBaxF3‐x (x=0) with orthorhombic phase, x=0.2 with hexagonal phase, and x=0.4 with cubic phase) were successfully obtained with the mechanical alloy method. The change from Bi1‐xBaxF3‐x (x=0) (orthorhombic) to x=0.2 (hexagonal) improved the initial reversible capacity and the capacity retention during cycling. The change from x=0.2 (hexagonal) to 0.4 (cubic) led to further improvement in capacity retention during cycling; however, the initial reversible capacity decreased due to the substitution of excess amount of barium that did not contribute to the redox reaction. For the BiF3‐based compound, controlling the crystal structure and substitution of inactive element affected the FSB performance.

Reactivity of the anion acceptor in electrolyte: An important factor in achieving high electrochemical performance of a lead (II) fluoride electrode in a fluoride shuttle battery

The electrochemical reaction in a fluoride shuttle battery (FSB) requires an organic solvent containing a supporting electrolyte salt. To dissolve an insoluble inorganic fluoride compound (e.g., cesium fluoride (CsF)) in an organic solvent (e.g., bis[2-(2-methoxyethoxy)ethyl]ether [tetraglyme, G4]), an anion acceptor (AA) (e.g., triphenylborane (TPhB) or triphenylboroxine (TPhBX)) is typically added. This study investigates the effect of AA type and CsF/AA molar ratio on the electrochemical performance of a lead (II) fluoride (PbF2) FSB electrode. The PbF2 electrodes were cycled in an FSB with G4 containing CsF (0.45 mol dm−3) and TPhB (0.5 mol dm−3) as the electrolyte. The capacity was high during the first cycle, but drastically decreased during subsequent cycling. X-ray diffraction (XRD) and atomic absorption spectrometry (AAS) revealed Pb formation during the discharging process, but PbF2 formation and Pb dissolution during the charging process. The loss of active material through Pb dissolution caused the capacity degradation. Changing the AA type from TPhB to TPhBX promoted Pb dissolution, thereby aggravating the capacity retention problem. In contrast, increasing the CsF concentration from 0.45 mol dm−3 to its saturation concentration effectively preserved the capacity during cycling by suppressing the Pb dissolution, as confirmed in the AAS results. However, increasing the CsF concentration deteriorated the discharge and charge capacities of the PbF2 electrode. Changing the AA type from TPhB to TPhBX in the electrolyte containing saturated CsF increased the discharge and charge capacities during the first cycle, and maintained a high capacity in the subsequent discharging and charging processes, as supported in the XRD results. Overall, the AA type and CsF concentration in the electrolyte strongly affected the electrochemical performance of the PbF2 electrodes. Thus, an FSB with high electrochemical performance requires judicious selection of both AA type and CsF/AA ratio.

Experimental Visualization of Interstitialcy Diffusion Pathways in Fast-Fluoride-Ion-Conducting Solid Electrolyte Ba0.6La0.4F2.4

All-solid-state fluoride shuttle batteries (FSBs) have the potential to become the next generation of rechargeable batteries. However, there are gaps in the fundamentals of developing all-solid-sta...

Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study.

Fluoride-shuttle batteries (FSBs), which are based on fluoride-ion transfer, have attracted attention because of their high theoretical energy densities. The fluorination and defluorination reactions at the electrodes are the possible rate-determining steps in FSBs, and understanding the mechanism is important to achieve smooth charge/discharge. In this study, we discuss the thermodynamically favored pathways for the fluorination and defluorination reactions and compare the reactions through the solid-solution and two-phase-coexistent states by density functional theory (DFT) calculations. The free energies of the solid-solution and two-phase states approximate the energies calculated by DFT, and their accuracy was validated by comparison with experimental formation enthalpies and free energies. The relative formation enthalpies of typical, transition, and relativistic metal (Tl, Pb, and Bi) fluorides are well reproduced by DFT calculations within 0.1, 0.2, and 0.4 eV, respectively. We also show that the reaction pathway can be determined by comparing the formation enthalpies of the metal fluoride H, a fluorine vacancy HV, and an interstitial fluorine defect HI from the simple selection rule. The enthalpy relation of HI > H > -HV observed in all the calculations strongly suggests that fluorination and defluorination in FSB electrodes occur by a two-phase reaction. This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.

Reactivity and Mechanisms in Fluoride Shuttle Battery Reactions: Difference between Orthorhombic and Cubic BiF3 Single Microparticles

BiF3 is a strong candidate for a cathode material in fluoride shuttle batteries (FSBs), which utilize defluorination of metal fluorides and fluorination of metals. Much interest has been shown in F...

Room-Temperature Fluoride Shuttle Batteries Based on a Fluorohydrogenate Ionic Liquid Electrolyte

Fluoride shuttle batteries (FSBs) are attracting considerable interest as next-generation energy storage devices, owing to their superior energy density. We focused on fluorohydrogenate ionic liqui...

Charge and Discharge Reactions of a Lead Fluoride Electrode in a Liquid‐Based Electrolyte for Fluoride Shuttle Batteries:‐The Role of Triphenylborane as an Anion Acceptor‐

AbstractLead fluoride (PbF2) is a promising electrode material for fluoride shuttle batteries (FSBs) owing to its high theoretical capacity (219 mAh g–1). In this study, the discharge and charge capacities of a PbF2 electrode were measured using a bis[2‐(2‐methoxyethoxy)ethyl] ether containing cesium fluoride and triphenylborane as an electrolyte. A high specific capacity was maintained during both the discharge and charge processes in the first cycle, but the capacity decreased from the first charge process to the following discharge process. To clarify the electrochemical reaction mechanism, the dissolution and change in the electronic state of Pb at the PbF2 electrode during the discharge and charge processes were evaluated via atomic absorption spectrometry (AAS) and X‐ray photoelectron spectroscopy (XPS). The results obtained from AAS and XPS indicated that Pb was formed during the discharge process. Conversely, the formation of PbF2 and dissolution of Pb coexisted within the wide range of charge process. The PbF2 could react in the following cycle, but the dissolved Pb was unable to contribute to the following discharge/charge reaction. Therefore, after the initial charge process, the capacity decreased.

Influence of Electrolyte Composition on the Electrochemical Reaction Mechanism of Bismuth Fluoride Electrode in Fluoride Shuttle Battery

Fluoride shuttle battery (FSB) is a promising next-generation battery candidate. In the FSB, metal fluoride and organic solvent containing supporting electrolyte salt and anion acceptor were used a...

Formation and Propagation of Fluorine-Deficient Phases in Large LaF3 Single Crystals during Electrochemical Defluorination

LaF3 is expected to be useful as an electrolyte and an active material of fluoride shuttle batteries (FSBs), which are considered as next-generation batteries with high energy densities. Diffusivity of fluoride ions in solid electrolytes and active materials is a key property for achieving high battery performance of an FSB. In the present work, the formation and propagation of phases with dilute fluorine vacancies at surfaces of single crystals of LaF3 (2.2 × 3.2 × 4.2 mm3 in size) were studied during electrochemical defluorination by using in situ wide-view scanning Raman spectroscopy. The area of the peak at 387 cm–1,which was assigned by first-principles calculations to vibrations of F atoms between La planes, sensitively decreased during defluorination, indicating desorption of F atoms between La planes. During defluorination of pure LaF3, the fluorine-deficient phase extended over 250 μm from the (001) plane where defluorination occurred. During defluorination of Eu(0.9 mol %)-doped LaF3, segregation of the fluorine-deficient phase was observed in the vicinity of the (00–1) plane on the opposite to the (001) plane where defluorination occurred. The diffusivities of fluorine-deficient phases in pure and Eu-doped LaF3 were estimated to be 3 × 10–9 cm2/s to 7 × 10–9 cm2/s and above 4 × 10–7 cm2/s, respectively, which are larger than previously reported diffusion coefficients of F–. The unique method to observe phases with dilute fluorine vacancies and their large scale migration will be useful for the development of superior electrolytes and active materials for FSBs and also other materials.

Fluoride Shuttle Batteries by Liquid Electrolytes

Rechargeable batteries with fluorides as charge transporting ions, which we refer to as fluoride shuttle butteries (FSBs, see Fig. 1), potentially offer a high electrochemical energy storage capability that overwhelms those of the state-of-the-art lithium-ion batteries (LIBs). This is due to the unique properties of monovalent fluoride ions, which are lightest of halides, hardly oxidized, and yet capable of reacting with various metals to form mono- and multi-valent fluoride salts. They thus allow anion-based, multi-electron transfer, redox reactions under wide potential window. Besides, FSBs as such no longer require host lattices (prerequisite in LIBs), which further adds to the high energy density. The fundamental concepts and some successful examples of FSB systems that had been developed in the RISING battery project were addressed by one of the present co-authors (Z. Ogumi) in IBM2017 and IBA2018. We here focus on the recent advance in liquid-type FSBs that came out through the continued RISING2 battery project. It should be noted that none of alkali fluorides are soluble in common buttery-oriented organic solvents no matter how large static dielectric constants they have. Used as alternative electrolytes have thus typically been organic fluoride salts (such as alkylammonium or substituted alkylammonium fluorides) in combination with preferably ionic liquids. Another, more-or-less sophisticated, way out is to utilize so-called anion receptors that make even alkali fluorides substantially soluble in common organic solvents. We here show nonaqueous plain liquid fluoride electrolytes, which, according to our comprehensive and careful analyses, are apparently free from any addenda but somehow solvated Cs+ (or K+) cations and F- anions at a molar concentration of ~50 mM to give total ionic conductivity of typically ~0.8 mS/cm. The simple electrolytes exhibit versatile performance to make a variety of combinations of electrodes (ranging from Au/AuF3 to at least Zn/ZnF2) work reversibly in terms of the expected conversion reaction between the respective metal and its fluoride counterpart.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

Electrochemical performance of a lead fluoride electrode mixed with carbon in an electrolyte containing triphenylboroxine as an anion acceptor for fluoride shuttle batteries

Abstract In fluoride shuttle batteries (FSBs), the addition of an anion acceptor (AA) is required to dissolve the supporting electrolyte salt in an organic solvent. Based on theoretical calculations and practical experiments, the effectiveness of triphenylboroxine (TPhBX) as an AA for FSB was verified. Using an electrolyte with TPhBX as an AA, the specific capacities of the following two types of lead fluoride (PbF2) electrodes were evaluated: (i) PbF2 pulverized using a ball mill, (ii) pulverized PbF2 mixed with carbon using a ball mill. The experimental results indicate that mixing PbF2 with carbon using a ball mill increases the specific capacity of PbF2 electrode.

Evolution of Reactions of a Fluoride Shuttle Battery at the Surfaces of BiF3 Microclusters Studied by In Situ Raman Microscopy.

Fluoride shuttle batteries (FSBs), which utilize defluorination of metal fluorides and fluorination of the resultant metals, are expected to have high energy densities. In situ Raman microscopy was conducted during FSB reactions of a nearly-2D cluster of orthorhombic BiF3 microparticles partly embedded in a gold-plated film (o-BiF3 /gold). At a high overpotential, defluorination of the surface of an o-BiF3 particle (or cluster) was almost completed within approximately 120 s. At a low over potential, defluorination proceeded from the contours of the cluster that was in contact with the gold to the center of the cluster, suggesting that the rate-limiting process was electronic diffusion. Conversely, fluorination proceeded uniformly at the surface of the cluster to form BiF3 with a cubic structure (c-BiF3 ). The results will lead to the establishment of a strategy for efficient use of active materials with low electronic and ionic conductivities.

In Situ Observation of Fluoride Shuttle Battery Reactions with Dissolution-Deposition Mechanisms by Raman Microscopy

Fluoride shuttle batteries (FSBs) are expected to have high energy densities. Evolution of FSB reactions, defluorination and fluorination of a nearly two-dimensional cluster of orthorhombic BiF3 microparticles partly embedded in a gold plating film (o-BiF3/gold) was studied by in-situ Raman microscopy. Solutions of fluorobis(2,4,6-trimethylphenyl)borane (FBTMPhB) and CsF (0.45 M and about 0.5 M (saturation)) in tetraglyme were used as electrolytes. During defluorination in the 0.45 M CsF solution, metallic bismuth appeared not only at positions of the cluster but also at positions of the gold plating, accompanying changes in morphology of the cluster, indicating that a dissolution-deposition mechanism (BiF3 → Bi3+ + 3F−, Bi3+ + 3e− →Bi) was operative in defluorination. Deposition of metallic bismuth preferentially occurred in the vicinity of the cluster, resulting in extension of protruding areas on the surface. BiF3 with a hexagonal structure appeared during fluorination together with change in the morphology of the cluster. With 0.5 M CsF solution, changes in morphology of the cluster during defluorination and fluorination were small, suggesting that the contribution of the dissolution-deposition mechanism was small. The observed changes in distribution and morphology of active materials will be useful information for designing better electrolytes and electrodes in future batteries.

Triphenylboroxine and Triphenylborane as Anion Acceptors for Electrolyte in Fluoride Shuttle Batteries

For liquid-based fluoride shuttle batteries, electrolyte composed of organic solvent and supporting electrolyte salt is developed. To increase the solubility of supporting electrolyte salt in organ...