Charge–Discharge Behavior of Bismuth in a Liquid Electrolyte for

Charge−Discharge Behavior of Bismuth in a Liquid Electrolyte for Rechargeable Batteries Based on a Fluoride Shuttle Ken-ichi Okazaki,*,† Yoshiharu Uchimoto,‡ Takeshi Abe,§ and Zempachi Ogumi† †

Office of Society−Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji 611-0011, Japan Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan § Graduate School of Global Environmental Studies, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡

S Supporting Information *

ABSTRACT: Rechargeable batteries based on fluoride transfer have attracted attention because of the possibility of achieving high energy densities surpassing those of current lithium-ion batteries. Although the batteries of this type, fluoride shuttle batteries (FSBs), have been developed using solid electrolytes, most operate at relatively high temperatures, greater than 423 K. In addition, in attempts to fabricate FSBs using liquid electrolytes, only the discharge reactions have been investigated, and they still suffer from serious issues of reversibility. In the present study, we have prepared a fluoride-conducting liquid electrolyte by dissolving an organic fluoride in a room-temperature ionic liquid, yielding a FSB electrolyte with a high fluoride concentration (0.35 mol dm−3) and conductivity (2.5 mS cm−1). By using this electrolyte, we have demonstrated a rechargeable FSB working at room temperature that is constructed from Bi|BiF3 and PbF2|Pb couples as the positive and negative electrodes, respectively. the defluorination of metal fluoride at the positive electrode and the fluorination of metal at the negative electrode, simultaneously proceed during the discharge process as follows:

R

echargeable batteries with energy densities higher than those of commercially available lithium-ion batteries (LIBs)1−3 are required for applications such as hybrid electric vehicles. In addition, the efficient storage of energy from fluctuating renewable energy sources, such as wind or solar power, in energy-dense batteries is crucial for improving global energy resources and reducing environmental impact. Recently, batteries based on anion transfer, such as that of chloride4 and fluoride,5−9 have attracted attention because by selecting appropriate active materials for the positive and negative electrodes high-energy-density batteries can be prepared. The fluoride battery is a promising candidate because fluoride compounds have high theoretical discharge capacities. For example, the theoretical gravimetric discharge capacities based on the reaction of each metal fluoride/metal combination for positive electrode materials such as bismuth trifluoride (BiF3), copper fluoride (CuF2), and iron trifluoride (FeF3) are estimated to be 302, 528, and 712 mAh g−1, respectively.2,3 This type of fluoride battery is not always based on the insertion/extraction of fluoride ions at the positive and negative electrode, as in LIBs. Some authors call that “fluoride ion batteries”. In fluoride batteries, the conversion between a metal fluoride and metal (i.e., the fluorination/defluorination of metals) occurs at the electrodes. Furthermore, both reactions, © 2017 American Chemical Society

MFx + x e− → M + x F−

M′ + x F− → M′Fx + x e−

(at the positive electrode)

(at the negative electrode)

In this way, fluoride ions carry charge between the positive and negative electrodes. Consequently, the authors call these types of batteries “fluoride shuttle batteries (FSBs)”. An electrochemical cell based on a similar concept was first reported about 40 years ago using a thin solid film as a fluoride conductor.10−12 However, the development of electrochemical cells based on the fluoride shuttle did not developed significantly until Anji Reddy and Fichtner proposed it again.6 In their study, an electrochemical cell was constructed using a solid electrolyte, LaF3/BaF2 composite (La1−xBaxF3−x, 0 < x < 0.15), and metal fluorides and metallic cerium as the positive and negative electrodes, respectively. Most investigations into FSBs have been carried out using solid electrolytes13−17 and at Received: April 13, 2017 Accepted: May 25, 2017 Published: May 25, 2017 1460

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Letter

http://pubs.acs.org/journal/aelccp

Letter

ACS Energy Letters temperatures greater than 423 K.6,7,16 In contrast, scientific papers of the fabrication of FSBs using liquid electrolytes are scarce,8,9 although some patents have been applied for.18,19 In the former studies, only the discharge reactions (i.e., the defluorination of the metal fluoride at the positive electrode) have been focused on, and they still suffer from serious reversibility issues.8,9 In the latter, the electrochemical cell was assembled from metal fluorides (PbF2 or BiF3) and polyaniline (PANI) as the negative and positive electrode in the discharged state using room-temperature ionic liquids (ILs) as the liquid electrolyte.18 In the work, although some charge−discharge curves were presented, no structural analysis about the products of the positive and negative electrodes during charge and discharge processes was provided. Particularly, the redox process of PANI with fluoride ions remains unclear. Therefore, it seems weak as evidence for the FSB. In this study, an IL-based liquid electrolyte was used as the FSB electrolyte. ILs have characteristic physicochemical properties, such as high thermal and chemical stabilities, high ionic conductivities, negligible volatilities, nonflammability, and the ability to dissolve many substances.20,21 For instance, ILs provide a superior medium for the electrodeposition20 of metals and semiconductors and for electrochemical capacitors.22 This is because the wide electrochemical potential window of ILs overcomes the limitations of common aqueous or organic media. In particular, for battery applications, the nonvolatility and nonflammability of ILs increase the safety of LIBs, and some ILs show high electrochemical stability in a wide potential range of more than 4 V; therefore, ILs have attracted much attention for use as LIB electrolytes.23 Indeed, the chloride-ion battery4 and the electrochemical cell in the patent18 mentioned above used an IL as the electrolyte. In this Letter, a BiF3|Bi electrode was examined as a positive electrode of the FSB in a liquid electrolyte composed of an organic fluoride and an IL. The liquid electrolyte containing fluoride ions was prepared by dissolving an organic fluoride (1-methyl-1-propylpiperidinium fluoride: MPPF) in an IL (N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide: TMPATFSA), at a molar ratio of 1:50 or 1:10. The conductivities of MPPF/TMPA-TFSA = 1:50 and 1:10 were 3.0 and 2.5 mS cm−1, respectively, determined at 298 K by AC impedance measurements using a symmetric cell. A bismuth needle or chip was used as the working (WE) and positive electrodes. A surface-fluorinated lead plate was used as the counter and reference electrodes (CE and RE) and the negative electrode. The electrode preparation method is described in detail in the Experimental details section of the Supporting Information. Hereafter, the PbF2-deposited Pb electrode is denoted “PbF2| Pb”. Figure 1 shows the cyclic voltammograms (CVs) of the bismuth and platinum electrodes in MPPF/TMPA-TFSA (1:50 in molar ratio) at 298 K. The corresponding electrode potential relative to Li/Li+ and estimated based on the fluorination potential of Pb at +2.49 V versus Li/Li+ is shown on the upper x-axis. In the case of the platinum electrode (the gray lines with circles), no redox current was observed between 0 and +0.7 V versus Pb|PbF2. Therefore, the electrolyte is stable, that is, no significant degradation of MPPF and TMPA-TFSA occurs in this potential window. In contrast, noticeable anodic and cathodic currents were observed in the CV curves of the bismuth electrode. During the first scan in the positive direction, the anodic current increased sharply from +0.43 V,

Figure 1. CVs of the bismuth and platinum electrodes in ∼0.07 mol dm−3 MPPF/TMPA-TFSA at a scan rate of 1.0 mV s−1. Blue lines: bismuth. Gray lines with circles: platinum.

and a peak was observed at around +0.5 V. In addition, the peak current gradually decreased, and its potential shifted in the positive direction with increasing cycle number. The shift and decrease of the anodic peak can be explained by an increase in the ohmic resistance. This may result in a low discharge efficiency (see below). During oxidation, the metallic luster of the bismuth electrode surface became dull-colored. During the reverse scan, a broad cathodic peak rising at around +0.41 V was observed, and a shoulder peak at 0.25 V was also observed, but its origin is not clear yet. This result indicates that the bismuth electrode has been reversibly oxidized and reduced in this electrolyte, although the surface color did not return to the original color. To clarify the progress of the oxidation and reduction of bismuth, we prepared the oxidized and reduced products by using a potential step between 0 and +0.5 V, voltages at which the anodic and cathodic reactions proceed sufficiently. Figure S1 in the Supporting Information shows the transient current of the bismuth electrode during the potential step in the MPPF/TMPA-TFSA electrolyte. During the oxidation step, the depth of oxidized bismuth was estimated to be 6 μm. The efficiency of the reduction of the oxidized product based on the charge was estimated to be 74.8%. Thus, some oxidized species remain during the reduction at 0 V. The surface morphology and composition of the oxidized bismuth was investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Figure 2 shows typical SEM images and the corresponding EDX results for the bismuth electrodes before and after oxidation at +0.5 V for 12 h. The SEM images reveal that the surface after oxidation is rough and contains many cracks compared to that of the pristine electrode. EDX analysis showed that the oxidized bismuth is composed of bismuth and fluorine. Although a peak corresponding to oxygen at 0.53 keV seems to remain, no peaks corresponding to lead, nitrogen, or sulfur were observed. This fact indicates that the detected fluorine was derived from neither the electrolyte solution nor PbF2 from the CE and RE. The crystal structures of the oxidized and reduced bismuth electrodes obtained from the potential step were evaluated by X-ray diffraction (XRD), and the XRD patterns are shown in Figure 3. For the pristine bismuth electrode, the diffraction peaks agree well with those of the rhombohedral phase bismuth metal (JCPDS card No. 85-1331; not shown here). After the 1461

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fluorination/defluorination of bismuth in the MPPF/TMPATFSA electrolyte. Furthermore, these results indicate that the fluorination/defluorination of bismuth occurs reversibly in the electrolyte at room temperature. Figure 4 shows the galvanostatic charge/discharge curves of the Bi/PbF2 system using the MPPF/TMPA-TFSA (1:50)

Figure 2. SEM images of the bismuth electrodes before and after oxidation at +0.5 V versus Pb|PbF2 for 12 h (a,b) and the corresponding results of the EDX analyses (c,d). The asterisks indicate Al from the SEM specimen stage.

Figure 4. Charge and discharge curves of the bismuth electrode in ∼0.07 mol dm−3 MPPF/TMPA-TFSA at room temperature. A PbF2-deposited Pb plate was used as a negative electrode. The charge and discharge current densities were 64 and 32 μA cm−2, respectively.

electrolyte. The cell was assembled from a bismuth needle and a PbF2|Pb plate as the positive and negative electrodes, respectively, and they were immersed in the electrolyte in a beaker-type cell. The voltage plateaus in the charge and discharge curves were observed at approximately 0.47 and 0.2 V, respectively. The former value corresponds to the fluorination potential of bismuth estimated from PbF2|Pb-RE, and the latter is consistent with the peak potential for the reduction of BiF3, as shown in Figure 1. Furthermore, the XRD patterns of the charge and discharge products (not shown here) coincide well with those shown in Figure 3. The first charge capacity was ∼0.2 mAh, which is equivalent to a depth of approximately 30 μm from the surface. This relatively deep reaction depth is discussed in detail later. Here, the charge/ discharge behavior of the positive bismuth electrode when a lead plate without a PbF2 layer was used as a negative electrode in the same cell is shown in Figure S2 (Supporting Information). A very low charge capacity, less than 0.02 × 10−3 mAh, was obtained in this system because of the lack of reduction reactions on the negative electrode within the tested voltage range. These results indicate that the coupled defluorination of PbF2 and fluorination of the bismuth must occur simultaneously during the charging process. That is, in the Bi/PbF2 system, fluoride ions are shuttled between both electrodes through the liquid electrolyte. The charge/discharge cycling tests was carried out using a flat cell constructed with a bismuth chip and a PbF2|Pb disk as the positive and negative electrodes, respectively. The electrolyte-impregnated separators were sandwiched between the electrodes and pressed together by a spring. The obtained charge/discharge profiles are shown in Figure 5. Two-step plateaus were observed in the first charge and discharge curves. However, the origin of the two steps is currently unclear. Meanwhile, the first charge capacity reached approximately 0.30 mAh. This value is comparable to the result obtained using a needle-shaped bismuth electrode (Figure 4) considering the surface area in contact with the separator or immersed in the

Figure 3. XRD patterns of the bismuth electrodes obtained after oxidation at +0.5 V versus Pb/PbF2 for 12 h (a) and subsequent reduction at 0 V for 15 h (b). Plane indices of the hexagonal BiF3 phase are depicted in (a).

oxidation process, several peaks assigned to the hexagonal phase of BiF324−27 appeared, and some of the metallic bismuth phase is present, as shown in Figure 3a. The characteristic planes of the hexagonal BiF3 phase are shown in Figure 3a, and all of the diffraction peaks in the XRD pattern in Figure 3a could be assigned to the BiF3 and the original metallic bismuth. Therefore, the SEM-EDX and XRD results clearly indicate that the fluorination of bismuth to BiF3 occurred at +0.5 V in the MPPF/TMPA-TFSA electrolyte. Furthermore, the XRD pattern of the subsequently reduced electrode is shown in Figure 3b. After the reduction process, the intensities of the peaks corresponding to BiF3 were drastically reduced, and the peaks of the rhombohedral bismuth phase reappeared. Thus, most of the BiF3 produced at +0.5 V was reduced and defluorinated, yielding metallic bismuth at 0 V. Here, on the basis of the thermodynamic data, the theoretical fluorination potential of bismuth to BiF3 was estimated to be +0.39 V versus Pb|PbF2.27 Considering the uncertainty of the activities, the anodic and cathodic peaks observed in the CV profile of bismuth shown in Figure 1 can be attributed to the 1462

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cell from a crystallographic and morphological point of view; however, the isolated BiF3 that was formed led to the low charge−discharge efficiency. Both the charge and discharge capacities gradually reduced as the number of cycles increased. Thus, it is necessary to improve the discharge process in this cell. As described above, the detachment of the BiF3 from the base bismuth metal results in the formation of inactive BiF3 (dead BiF3). This issue may be overcome by using a composite electrode of nanosized active material and conductive additives. Although the XRD patterns did not show indications of amorphous products or an extremely thin layer of bismuthoxyfluoride derivatives,7 the influence of formation of these products on a bismuth surface on the low cycle efficiency cannot be excluded. In summary, we successfully demonstrated the reversible fluorination/defluorination of bismuth, which is one of the candidates for the positive electrode of a FSB, by using a fluoride conductive liquid electrolyte prepared by dissolving an organic fluoride in an IL. In addition, a room-temperature, rechargeable FSB was also demonstrated in the form of a cell constructed from a bismuth electrode connected to another metal−fluoride/metal couple, such as PbF2|Pb, as a negative electrode through the liquid electrolyte, although the cyclability must be improved. Our results indicate that the coupled defluorination (fluorination) of the positive electrode and fluorination (defluorination) of the negative electrode must occur simultaneously to achieve discharge (charge) of the FSB. We are currently working on further improvements to this FSB.

Figure 5. Cycling test of the bismuth electrode in ∼0.35 mol dm−3 MPPF/TMPA-TFSA at room temperature. Both the charge and discharge current densities were 20 μA cm−2.

electrolyte. The charge capacity per surface area of the bismuth needle (Figure 4) and chip (Figure 5) was calculated to be around 1.3 and 1.2 mAh cm−2, respectively. The Coulombic efficiency (=discharge capacity/charge capacity) for the first cycle (shown in Figure 5) was estimated to be 0.56, lower than that of the bismuth needle. However, the BiF3 produced in the charging process was partially defluorinated during the discharging process. In addition, the value at the first cycle is close to the value in Figure 4. The surface SEM images after charge and discharge of the first cycle are shown in Figure 6.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00320. Experimental section and supplemental figures S1 and S2 showing the transient current during the oxidation and reduction of bismuth and charge/discharge behavior of the Bi/Pb system without a PbF2 layer (PDF)

Figure 6. SEM images of the bismuth electrode after charge (a) and subsequent discharge (b) in a ∼0.35 mol dm−3 MPPF/TMPATFSA at room temperature. The inset shows a magnified image of the charged bismuth.

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The inset presents a magnified image of the surface after charging. The morphology of the surface suggests the exfoliation of layers from the surface, one after the other. Pristine bismuth metal and the charged product, BiF3, crystallize in the rhombohedral and hexagonal crystal systems, respectively, although both crystal systems belong to the hexagonal family. Therefore, our results suggest that fluoride ions have been inserted between the atomic layers of rhombohedral bismuth from the grain boundaries at the surface of the electrode. This insertion resulted in expansion of the grains, leading to exfoliation of the surface. Consequently, a new, pristine bismuth surface appeared under the exfoliated layer at the cost of the isolation of the formed BiF3. Thus, the fluorination of bismuth proceeded deep into the surface. In contrast, after discharge, a large part of the surface appears smooth again, as shown in the left part of Figure 6b, and no fluorine was detected in the EDX analysis of this region. However, as shown in Figure 6b, a minor roughened area at the right part of the image is still present, and fluorine was detected in this region, indicating that some BiF3 was electrically isolated from the bismuth by the exfoliation. Thus, the fluorination/ defluorination of bismuth electrode occurred reversibly in the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ken-ichi Okazaki: 0000-0003-0800-712X Author Contributions

K.O. performed the experiments and wrote the manuscript. Z.O. proposed the concept. All of the authors discussed the results and reviewed the final version of the manuscript before submission. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Ms. Tomomi Yamamoto for her technical assistance during the experiments. This work was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING and RISING2)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. 1463

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(23) Sakaebe, H.; Matsumoto, H. N-Methyl-N-Propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13-TFSI) - Novel Electrolyte Base for Li Battery. Electrochem. Commun. 2003, 5, 594−598. (24) Bervas, M.; Badway, F.; Klein, L. C.; Amatucci, G. G. Bismuth Fluoride Nanocomposite as a Positive Electrode Material for Rechargeable Lithium Batteries. Electrochem. Solid-State Lett. 2005, 8, A179−A183. (25) Bervas, M.; Mansour, A. N.; Yoon, W. S.; Al-Sharab, J. F.; Badway, F.; Cosandey, F.; Klein, L. C.; Amatucci, G. G. Investigation of the Lithiation and Delithiation Conversion Mechanisms of Bismuth Fluoride Nanocomposites. J. Electrochem. Soc. 2006, 153, A799−A808. (26) Amatucci, G. G.; Pereira, N.; Badway, F.; Sina, M.; Cosandey, F.; Ruotolo, M.; Cao, C. Formation of Lithium Fluoride/Metal Nanocomposites for Energy Storage through Solid State Reduction of Metal Fluorides. J. Fluorine Chem. 2011, 132, 1086−1094. (27) Ko, J. K.; Halajko, A.; Parkinson, M. F.; Amatucci, G. G. Electronic Transport in Lithiated Iron and Bismuth Fluoride. J. Electrochem. Soc. 2015, 162, A149−A154.

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DOI: 10.1021/acsenergylett.7b00320 ACS Energy Lett. 2017, 2, 1460−1464