Reactivity and Mechanisms in Fluoride Shuttle Battery Reactions: Orthorhombic Vs Cubic BiF3 Single Microparticles
Authors: Yamanaka Toshiro , Okazaki Ken–ichi , Ogumi Zempachi , Abe Takeshi 
Published: May 2020

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
+ xe- → M + xF- (at the positive electrode, M being metal) (1)
M’ + yF- → M’Fy + ye- (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.
BiF3 is a strong candidate for a cathode material in FSBs. In the present work, structural transformation, reactivities and mechanisms in FSB reactions of BiF3 single microparticles were studied by insitu Raman microscopy. BiF3 microparticles were partly embedded in a gold plating film (BiF3/gold, orthorhombic BiF3 (o-BiF3) and cubic BiF3 (c-BiF3)). By using a Raman cell consisting a BiF3/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-BiF3 and c-BiF3.
Experimental
BiF3/gold was prepared by embedding o-BiF3 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-BiF3 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 BiF3.
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 BiF3/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-BiF3 gradually transformed into cubic c-BiF3. When the voltage of the BiF3/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-BiF3 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-BiF3 is electronic conduction at the surface of o-BiF3. Then defluorination of c-BiF3 started at a voltage below 0.2 V by both direct defluorination (eq. (1)) and dissolution-deposition mechanisms (BiF3 → Bi3
+ + 3F−
, Bi3
+ + 3e− → Bi). In the direct mechanism, the nucleus of Bi first appeared near the edge of c-BiF3 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-BiF3 was much slower than that of o-BiF3. Defluorination of c-BiF3 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 BiF3 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 BiF3/gold sample in which both o-BiF3 and c-BiF3 particles were distributed. Only defluorination of o-BiF3 occurred at 0.45 V. The arrows in (b) indicate that defluorination started at the contours of o-BiF3 particles.

Figure 1