Formation and propagation of fluorine-deficient ... - ACS Publications

Subscriber access provided by Washington University | Libraries

Article

Formation and propagation of fluorine-deficient phases in large LaF single crystals during electrochemical defluorination 3

Toshiro Yamanaka, Hirofumi Nakamoto, Takeshi Abe, Koji Nishio, and Zempachi Ogumi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02068 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Formation and Propagation of Fluorine-Deficient Phases in Large LaF3 Single Crystals during Electrochemical Defluorination Toshiro Yamanaka,†* Hirofumi Nakamoto,† Takeshi Abe,‡ Koji Nishio,† Zempachi Ogumi†

　 †Office

of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan,

‡Graduate

School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

KEYWORDS: Battery; Electrode materials; LaF3; ionic diffusion; Fluoride ion

ABSTRACT: 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

ACS Paragon Plus Environment

1

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

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 fluorinedeficient 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 3×10-9cm2/s to 7×10-9cm2/s and above 4×10-7cm2/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.

1. INTRODUCTION Ionic diffusion and related phase evolution in solids affect the performances of various components in devices, including solid electrolytes in gas sensors, batteries, thermoelectric converters, capacitors and solid oxide fuel cell, and also active materials in batteries. There is a growing demand for the development of superior batteries to solve energy problems. One of the most desired characteristics of batteries is high energy density. Fluoride shuttle batteries (FSBs) utilize the following reactions of polyvalent metals, and FSBs are expected to be future batteries with high energy density.1-11 MFx + xe- → M + xF- (at the positive electrode, in which M is metal) M’ + yF- → M’Fy + ye- (at the negative electrode, in which M’ is metal) Diffusivities of fluoride ions (or fluoride vacancies) in solid electrolytes and active materials in FSBs are key properties for achieving high battery performance. Thus, it is important to understand


2

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


how ions and vacancies behave in these materials in order to obtain guiding principles for the development of better solid electrolytes and active materials. Methods for in situ observation of such phenomena are required. LaF3 is a well-known ionic conductor that has been extensively studied

1,12-14

and it has been

used for electrolytes in gas sensors.15 Recently, LaF3 has been used as a solid electrolyte for FSBs 4-11

and it is also expected to become active material for FSBs that utilize fluorination of La and

defluorination of LaF3. It is known that doping of a small amount of Eu enhances the conductivity of fluoride ions in LaF3.12,16 In the present work, the distribution of phases with dilute fluorine vacancies at surfaces of single crystals of pure LaF3 and Eu(0.9 mol%)-doped LaF3 (2×3×4 mm in size) was observed by in situ wide-view scanning Raman spectroscopy17,18 during electrochemical defluorination. Peaks in Raman spectra were assigned to vibrational modes through analysis by first principles calculation, and it was found that peaks corresponding to the vibration of F atoms between La planes sensitively decreased during defluorination, indicating desorption of F atoms between La planes. By using this phenomenon, the formation and diffusion of fluorine-deficient phases and the effects of Eudoping were studied. The method used in the present study to elucidate the dynamics of fluorinedeficient phases will provide guiding principles for the development of superior electrolytes and active materials.

2. EXPERIMENTAL Figure 1a shows the atomic structure of the LaF3 (001) surface with three-fold symmetry. The large blue spheres represent La atoms. There are fluorine atoms placed at three sites, F1, F2 and F3,19 as shown by bright pink, dark pink and red spheres, respectively. Figure 1b shows a side view


3


Page 4 of 21

(right to left) of the structure shown in Fig. 1a. In this view, two planes of La atoms can be clearly seen. F3 atoms are located in the La plane, while F1 atoms are between the La planes. F2 atoms are close to La planes, but they are not in the La planes.

Figure 1. Atomic structure of LaF3 and setup for defluorination of a single crystal of LaF3. The (001) plane (a) and side view of the (001) plane (b) (right to left in (a)). (c) shows an illustration of the experimental setup. Figure 1c shows the setup for electrochemical defluorination of an LaF3 single crystal (Oxide Corporation).17 The size of the crystal was 2.2×3.2×4.2 mm. The crystal was cut using diamondbased cutters by the manufacturer to avoid undesired reactions. A separator film made of porous polypropylene was inserted between a counter electrode made of an acetylene black composite


4

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and a current collector made of a fine stainless mesh painted with slurry consisting of Fe powder and polyimide varnish. The current collector was attached to the (001) plane of the crystal because the conductivity of F- along the [001] axis is thought to be larger than that perpendicular to the [001] axis.13,20,21 The separator film was wetted by an electrolyte solution, which was a mixture of triglyme, LiPF6 and LiF (molar ratio of 20:5:1).22 Through measurements including cyclic voltammetry (CV) of defluorination/fluorination of LaF3/La and CeF3/Ce, the stability of the electrolyte in the voltage range of fluorination of La and defluorination of LaF3 was confirmed. Defluorination was induced by applying potential (Vc) to the current collector against the counter electrode. The detailed structure of the cell was described in supporting information of a previous paper.23 Wide-view Raman mapping was conducted on the (1-20) plane with a 532-nm laser beam of 40 μm in size with a scan step of 20 μm and a spatial resolution of 20 μm. Since an LaF3 single crystal is transparent, the detection depth is about 40 μm (close to the size of the beam), which is determined by the confocal setting of the apparatus. The area of scanning was about 600 μm × 2300 μm, as indicated by a blue rectangle in the right panel of Fig. 1c. Scanning in this area took about 16 hours. Vibrational modes and Raman spectra of LaF3 were calculated by Materials Studio CASTEP code, which is a first-principles density functional theory (DFT) method. The on the fly generated (OTFG) norm-conserving pseudopotential was used for electron-ionic core interactions. Generalized Gradient Approximation (GGA)-PBE was used to approximate exchange and correlation energies. A method by Koelling and Harmon was used to take into account the effect of theory of relativity for states of valence electrons.


5


Page 6 of 21

Figure 2. Raman spectrum of LaF3 powder and analysis by first principles calculation. Experimental (a) and calculated (b and c) Raman spectra. (d) and (e) show atomic motions of vibrational modes corresponding to peaks A and B, respectively, shown in (a) and (b).


6

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. RESULTS AND DISCUSSION A Raman spectrum of LaF3 powder was measured and the obtained spectrum was compared with a spectrum calculated by the first-principles DFT method to assign peaks in the experimental spectrum to individual vibrational modes. Since the experimentally obtained spectra for powder are averaged over incident angles and polarization of excitation lights relative to crystal orientations, the experimentally obtained spectra can be easily compared with the calculated Raman spectra in which incident angle and polarization of excitation light are not taken into account. Figure 2a shows an experimentally obtained Raman spectrum of LaF3 powder. There are four main peaks labeled by A, B, C and D and other small peaks. The positions of the peaks agree with those in previously reported Raman spectra24 indicated by blue and red closed circles. The symmetries of vibrational modes for previously reported peaks were assigned to A1g (red) and Eg (blue).24 The features of the four main peaks in Fig. 2a were reproduced by the calculated Raman spectrum shown in Fig. 2b. Figure 2c shows the same calculated spectrum as that in Fig. 2b with a magnified scale, and the features below 230 cm-1 agree with those in Fig. 2a. Figures 2d and 2e show atomic motions for vibrational modes corresponding to peaks A (mode A) and B (mode B), respectively. The motions are indicated by yellow-green arrows, and the sizes of arrows are proportional to the amplitudes of atomic motions. In mode A, mainly F atoms between La planes (F1) move, while mainly F atoms in La planes or close to La planes (F3 and F2, respectively) move in mode B. In mode C mainly F2 and F3 atoms move, while mainly F1 atoms move in mode D (See supporting information). For vibrational modes with low frequencies, motions of heavy La atoms become significant. Motions of atoms in some other vibrational modes are shown in supporting information.


7


Page 8 of 21

Figure 3. Raman spectra of a (1-20) plane of an LaF3 single crystal before (a) and during (b) defluorination. Figure 3a shows a Raman spectrum of the (1-20) plane of a single crystal of LaF3. The main peaks A, B, C and D are observed but relative heights of the peaks are different from those in the Raman spectrum for LaF3 powder shown in Fig. 2a. Figure 3b shows a Raman spectrum of the (120) plane of a single crystal of LaF3 during defluorination. It can be seen that the heights of peak A and peak D relative to that of peak B decreased during defluorination. This suggests that preferential desorption of F1 atoms between La planes (bright pink spheres in Figs. 1a, 1b, 2d and 2e) occurred during defluorination. The binding energies of F1 atoms to La atoms are expected to be lower than those of F2 and F3 atoms to La atoms since the distances between La atoms and F1 atoms are greater than those between La atoms and F2 atoms and between La atoms and F3 atoms, being consistent with preferential desorption of F1 atoms. The area of peak A was used in the following mapping. Figure. 4 shows in situ wide-view Raman mapping conducted during defluorination of a single crystal of LaF3. Scanning from the left side (y = 0 μm) to the right side was conducted every day, and the rate of scanning along the y axis was about 140 μm/hour. The position of the (001) plane (Fig. 1c) corresponds to y = 0 μm in Fig. 4. Figures 4a and 4b show results of Raman mapping for the first and second days when the voltage of the current collector vs the acetylene black


8

Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. In situ wide-view Raman mapping of a (1-20) plane of an LaF3 single crystal during defluorination. Results for the 1st day at OCV (a), 2nd day at OCV (b), 3rd day at Vc = -3.2 V (c), and 4th day at Vc = -3.2 V (d). (e) and (f) show side views (down to up in (b) and (d)) of the mapping shown in (b) and (d), respectively.

composite (Fig. 1c), Vc, was kept at the open circuit voltage (OCV). The two images are similar. On the third day, when scanning was started, Vc was changed to -3.2 V to induce defluorination from the (001) plane (y = 0 μm), and Vc was kept at -3.2 V during the third and fourth days. (Voltage and current profiles are shown in Fig. S12 in supporting information.) Results of CV data of a composite electrode of LaF3 showed peaks of fluorination and defluorination at about -2.2 V


9


Page 10 of 21

and -3 V, respectively, indicating that the potential of La/LaF3 is -2.6 V. In the present study, the voltage of the current collector that was attached to LaF3 single crystals was kept at -3.2 V during defluorination. Therefore, the overpotential to induce defluorination is about -0.6 V. Only one (001) plane out of the six planes of the rectangular parallelepiped LaF3 crystal was in contact with an electrode. Therefore, no overpotential was applied to induce ionic transport inside the crystal. The obtained mapping image for the third day shown in Fig. 4c is still similar to those for the first and second days. Figure 4d shows the results for the fourth day taken at Vc = -3.2 V. It can be seen that peak areas in the vicinity of the (001) plane decreased. Figures 4e and 4f show side views of results of mapping shown in Figs. 4b and 4d, respectively. The results indicate that an Fdeficient phase formed over about 250 μm from the (001) plane. The relation between the distance of migration, L, diffusion coefficient, D, and time, t, is often expressed by the formula L=(2 Dt)1/2.25 When the positions around y = 250 μm were scanned on the fourth day, the scanning took about 26 hours since Vc was changed to -3.2 V. Thus, the diffusivity of the fluoride-deficient phase is estimated to be 3.3×10-9cm2/s. Also, analysis of measurements for another crystal resulted in values of diffusivity between 3×10-9cm2/s and 7×10-9cm2/s (See section 5 in supporting information.). Figures 5a and 5b show Raman spectra of the (001) planes of non-doped (same as the spectrum in Fig. 3a) and Eu (0.9%)-doped LaF3 single crystals, respectively. In the spectrum of doped LaF3, three high peaks that were not observed for the non-doped crystal were observed in the range of 650 cm-1 to 900 cm-1. These peaks were not observed in Raman spectra obtained with 1064 nm excitation. Thus, the origins of these peaks are thought to be resonance Raman scattering or photoluminescence. Figure 5c shows the same Raman spectrum as that shown in Fig. 5b with a magnified vertical scale. The peaks observed for the non-doped crystal can be seen as indicated by


10

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


red dashed lines. The spectra for non-doped and doped crystals from 220 cm-1 to 400 cm-1 are shown in Figs. 5d and 5e, respectively. Both spectra show peaks A, B, C and D, which were defined in the spectra shown in Figs. 2a and 3a. However, the heights of peaks A and D relative to that of peak B in Fig. 5e are lower than those in Fig. 5d. The results suggest that vacancies were created at the sites of F1 atoms. The formation of vacancies by the doping of Eu is consistent with the fact that europium atoms exist in Eu-doped LaF3 in the form of Eu2+.16,26

Figure 5. Raman spectra of (1-20) planes of pure and Eu(0.9 mol%)-doped LaF3 single crystals. Pure (a) (same as Fig, 3a) and Eu-doped (b and c) LaF3 single crystals. (d) and (e) show zoomed Raman spectra of pure and Eu-doped crystals, respectively, around peaks A, B, C and D (see Fig. 3a).


11


Page 12 of 21

Figure 6. In situ wide-view Raman mapping of a (1-20) plane of an Eu-doped LaF3 single crystal during defluorination. Results for the 1st day at OCV (a), 2nd day at OCV (b), 3rd day at Vc = -3.2 V (c), 4th day at Vc = -3.2 V (d) and 5th day at Vc = -3.2 V (e). The left sides (y =0 μm to 150 μ


12

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


m) of the mappings shown in (a), (b), (c), (d) and (e) are zoomed in (f), (g), (h), (i) and (j), respectively.

Figure. 6 shows in situ wide-view Raman mapping conducted during defluorination of a single crystal of Eu (0.9%)-doped LaF3. Figures 6a and 6b show results for the first and second days at the OCV. The two images are similar. On the third day, Vc was changed to -3.2 V to induce defluorination, and Vc was kept at -3.2 V during the third to fifth days. (Voltage and current profiles are shown in Fig. S13 in supporting information.) Scanning was started at the same time as Vc was changed. Figure 6c shows results for the third day. It can be seen that peak areas in the vicinity of the (00-1) plane (y = 2150 μm) decreased. When scanning in the vicinity of the (00-1) plane was conducted, scanning took about 16 hours from the start of defluorination. Therefore, the results indicate that fluorine-deficient phases propagated more than 2150 μm within 16 hours. Thus, the diffusivity of the fluoride-deficient phase is estimated to be 4×10-7cm2/s. Figures 6d and 6e show results for the fourth and fifth days. Peak areas in the vicinity of the (00-1) plane at y ≒ 2150 μ m did not change significantly. On the other hand, peak areas around the (001) plane (y ≒ 0 μm) where defluorination occurred decreased. Results around the (001) plane (0 μm < y < 150 μm) from the first day to the fifth day are zoomed in Figs. 6f, 6g, 6h, 6i and 6j, clearly showing that decreases in peak areas around the (001) plane occurred on the fourth day and fifth day. The results indicate that fluorine-deficient phases preferentially segregate in the vicinity of (001) and (00-1) planes. Past works in which NMR line shape analysis was conducted indicated that only F1 fluoride ions are mobile at room temperature, which is responsible for ionic conduction.27,28 Thus, it is thought


13


Page 14 of 21

that F1 fluoride ions are extracted during defluorination. This is consistent with the fact that the areas of peak A and peak D in the Raman spectrum, both of which correspond to the vibration of mainly F1 fluoride ions, decreased during defluorination, indicating desorption of F1 fluoride ions. The areas of peak A and peak D relative to that of peak B in the Raman spectrum of Eu-doped LaF3 (Fig. 5e) are smaller than that in the Raman spectrum of pure LaF3 (Figs. 5d), suggesting that Eu-doped LaF3 contains vacancies at F1 sites. In the case of doping of divalent Sr in LaF3 (La1xSrxLaF3),

vacancies of fluorine atoms are preferentially located at the F1 sublattice at 0.03 < x

Formation and propagation of fluorine-deficient ... - ACS Publications

Recommend Documents