Abstract: 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. BiF 3 , 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 BiF 3 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 BiF 3 -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 BiF 3 nanoparticles having hexagonal crystalline structure had been acquired after further fluorination-treatment. Consequent BiF 3 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 L 3 -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. References [1] M.A. Reddy and M. Fichtner, J. Mater. Chem. , 21 , 17059 (2011). [2] F. Gschwind, G. Rodriguez-Garcia, D.J.S. Sandbeck, A. Gross, M. Weil, M. Fichtner, N. Hörmann, J. Fluorine Chem. , 182 , 76 (2016). [3] K. Okazaki, Y. Uchimoto, T. Abe, Z. Ogumi, ACS Energy Lett. , 2 , 1460 (2017). [4] V.K. Davis et al., Science , 362 , 1144 (2018). [5] T. Yamanaka, K. Okazaki, T. Abe, K. Nishio, Z. Ogumi, ChemSusChem , 12 , 527 (2019). [6] T. Yamanaka, K. Okazaki, T. Abe, Z. Ogumi, ACS Appl. Energy Mater. , DOI:10.1021/ acsaem.9b01803.

Abstract: Introduction Fluoride shuttle batteries (FSBs) are attracting interest as candidates for a next generation battery with high energy density.[1-3]FSBs utilize the following redox reactions. MF x + x e - → M + x F - (at the positive electrode, M being metal) (1) M’ + y F - → M’F y + y e - (at the negative electrode, M’ being metal) (2) To realize high battery performance, it is necessary to elucidate and control reactivity and mechanisms in FSB reactions. BiF 3 is a strong candidate for a cathode material in FSBs. In the present work, structural transformation, reactivities and mechanisms in FSB reactions of BiF 3 single microparticles were studied by insitu Raman microscopy. BiF 3 microparticles were partly embedded in a gold plating film (BiF 3 /gold, orthorhombic BiF 3 (o-BiF 3 ) and cubic BiF 3 (c-BiF 3 )). By using a Raman cell consisting a BiF 3 /gold cathode, a Pb wire anode and an ionic liquid-based electrolyte,[2] reactivities and mechanisms in defluorination were found to be different for o-BiF 3 and c-BiF 3 . Experimental BiF 3 /gold was prepared by embedding o-BiF 3 microparticles (Fluorochem Ltd.) in a gold plating film on a gold foil. [4,5] Briefly, a gold foil and gold plating solution for deposition of 24 K gold were put into a small vessel, and then a small amount of o-BiF 3 powder was put in the solution. Then a platinum wire was placed in the solution and a current was applied between the wire and the gold foil. The amount of deposited gold corresponded to that of the gold plating film with a thickness of 1 μm without BiF 3 . An organic fluoride (1-methyl-1-propylpiperidinium fluoride: MPPF) was dissolved in an ionic liquid (N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide: TMPATFSA) at a molar ratio of 1:10 and the resultant solution was used as an electrolyte.[2] The electrochemical cell had a quartz window and a Pb wire as a counter electrode. The distance between BiF 3 /gold and the window was 30 μm to 4 μm, and the space between them was filled with the electrolyte. The cell was sealed with Kalrez and placed in flowing Ar during Raman microscopy measurements. Raman microscopy was conducted with an NRS-4500 Raman spectrometer (JASCO Corporation) and 532 nm laser light. Results At OCV (0.7 V), o-BiF 3 gradually transformed into cubic c-BiF 3 . When the voltage of the BiF 3 /gold cathode vs the Pb anode was decreased from OCV to 0.05 V step by step, direct defluorination (eq. (1)) of the surfaces of only o-BiF 3 started from their contours at 0.45 V (Figs. 1a and 1b) and then extended to their center parts and was mostly completed at 0.2 V. The results suggest that the rate-limiting process of direct defluorination of o-BiF 3 is electronic conduction at the surface of o-BiF 3 . Then defluorination of c-BiF 3 started at a voltage below 0.2 V by both direct defluorination (eq. (1)) and dissolution-deposition mechanisms (BiF 3 → Bi 3 + + 3F − , Bi 3 + + 3e − → Bi). In the direct mechanism, the nucleus of Bi first appeared near the edge of c-BiF 3 microparticles, and the nucleus grew for defluorination to proceed over the whole surface, suggesting that the rate limiting process is formation of the nucleus of Bi. The direct defluorination of c-BiF 3 was much slower than that of o-BiF 3 . Defluorination of c-BiF 3 by the dissolution-deposition mechanism was fast and dominant at a high power of the excitation beam, probably due to a thermal effect. The results of the present work provide important implication for the development of electrodes and electrolytes with proper solubility of BiF 3 and for better utilization of reactions by the two mechanisms in order to realize FSBs with high performance. ACKNOWLEDGMENT This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under contract from the Research & Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2). REFERENCES [1] F. Gschwind, G. Rodriguez-Garcia, D. Sandbeck, A. Gross, M. Weil, M. Fichtner, N. Hörmann, J. Fluorine Chem. 182 , 76 (2016). [2] K.-I. Okazaki, Y. Uchimoto, T. Abe, Z. Ogumi, ACS Energy Lett. 2 , 1460 (2017). [3] V. K. Davis et al., Science 362 , 1144 (2018). [4] T. Yamanaka, K. Okazaki, T. Abe, K. Nishio, Z. Ogumi, ChemSusChem , 12 , 527 – 534 (2019). [5] T. Yamanaka, T. Abe, K. Nishio, Z. Ogumi, J. Electrochem. Soc., 166 , A635-A640 (2019). Figure 1. Results of Raman mapping of an area on a BiF 3 /gold sample in which both o-BiF 3 and c-BiF 3 particles were distributed. Only defluorination of o-BiF 3 occurred at 0.45 V. The arrows in (b) indicate that defluorination started at the contours of o-BiF 3 particles. Figure 1

Abstract: Since a fullerene was discovered in 1985, fullerene has attracted much attentions [1]. A unique feature of the fullerene that has the spherical structure with hollow interior allows a fullerene to accommodate any other atoms and molecules. Especially, the fullerenes containing other atoms which is called as endohedral fullerene have been expected to be applied in medical uses, electronics systems and so on [2] . In many cases, endohedral fullerenes have been synthesized by ion implantation, co-vaporization of carbon and metal atoms [3], or ion plasma irradiation technique for non-metal material [4] . However, these procedures have remained several issues. One of the critical issues is low production rate for endohedral fullerene synthesize due to difficulties in controlling a physical and a chemical property during synthesizing process. To increase the production efficiency of endohedral fullerene, it is required to optimize elementary process between fullerenes and injected atoms. Under these situations, we recently developed a novel procedure to synthesize endohedral fullerenes that induce the collision of vaporized fullerenes and ablated particles produced by the laser ablation of a solid material. Here, a Q-switched Nd:YAG Laser (Continuum, Powerlite DLS8000) with a wavelength of 1064 nm, a pulse-width of 9 ns, a laser energy of up to 1 J, and a pulse-frequency of up to 10Hz was used. Laser light was focused to be 50 m in diameter on the target surface. In this procedure, N@C60 was synthesized using the ablation of boron nitride material. From the previous studies [4], it is quite important for encapsulated particles to have about 100 eV. Optical emission spectrum of ablated particles by a polychromator (Bunkokeiki, CT-1000) with an intensified CCD detector (Princeton Instruments, PI-Max) was measured, and Doppler shift of the spectrum was estimated. The result showed that the kinetic energy of nitrogen ions was in the range of 20 to 65 eV by controlling the energy of the incident laser beam. The synthesis of N@C60 was conducted with a nitrogen kinetic energy of 65 eV, fullerenes vaporizing temperature of 800 degree Celsius, and a total reaction time of one hour. By optimizing the kinetic energy of ablated particles, synthesis of N@C60 was succeeded [5] . Existence of N@C60 in the toluene solution was confirmed by an electron spin resonance measurement (Elexsys E500, Bruker) without purification by high performance liquid chromatography [6]. The purity of produced N@C60 was two orders of magnitude higher than that achieved using alternative physical synthetic methods that use solid materials, such as arc discharge and co-evaporation methods. However, the purity must be increased by at least two orders of magnitude for the commercial use. Therefore, it is necessary to enhance the purity by optimizing experimental conditions such as laser energy, laser wavelength, amount of ion flux with appropriate energy, repetition rate of laser pulses, laser duration time, area of the laser ablation, and so on. In the meeting, novel synthesis procedure for endohedral-fullerenes using laser ablation plasma from solid material and vaporized fullerenes will be presented. Acknowledgments This study was financially supported by JSPS KAKENHI Grant No. 25287157 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. [1] H. W. Kroto, et.al., Nature 318, 162-163 (1985). [2] H. Shinohara, Rep. Prog. Phys. 63, 843-892 (2000). [3] Alexey A. Popov, et.al., Chem. Rev. 113, 5989-6113 (2013). [4] S.C. Cho, et.al., J. Appl. Phys. 117, 123301 (2015). [5] H. Itagaki, et al., AIP Advances 9-7 (075324), 1-7 (2019). [6] T.A. Murphy, et al., Phys. Rev . Lett. 77-6, 1075 (1996).