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. 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.