Evolution of Fluoride Shuttle Battery Reactions of BiF₃ Microparticles in a CsF/LiBOB/Tetraglyme Electrolyte: Dependence on Structure, Size, and Shape

Abstract

Fluoride shuttle batteries (FSBs) are possible candidates of next-generation batteries with high energy densities that operate by defluorination of metal fluorides and fluorination of metals. Understanding and control of FSB reactions are important for the development of FSBs with high performance. In the present work, evolution of FSB reactions of orthorhombic and cubic BiF3 (o-BiF3 and c-BiF3, respectively) microparticles partly embedded in a gold plating film (BiF3/gold) in a CsF/LiBOB (lithium bis­(oxalato)­borate)/tetraglyme electrolyte was studied by in situ Raman microscopy. Three-dimensional morphology of embedded BiF3 microparticles was also studied by laser scanning confocal microscopy (LSCM) to elucidate the detailed shape dependence of defluorination. Evolution of FSB reactions was found to be different from that in an ionic-liquid electrolyte reported previously. When the voltage of BiF3/gold vs a Pb counter electrode was decreased from OCV (about 0.5 V), direct defluorination of o-BiF3 and c-BiF3 started at 0.25 and 0.05 V, respectively, which are lower than those in the ionic-liquid electrolyte. The direct defluorination mechanism was always predominant at all CsF concentrations studied from saturation to zero. The sizes of most of the BiF3 microparticles that were defluorinated were less than 3 μm. However, partial defluorination of large (>10 μm) BiF3 (insulator) microparticles often occurred at protruded positions far from the contours (i.e., far from the gold current collector). The results suggest that defluorination occurred at positions with small radii of curvatures, probably due to stronger electric fields. On the other hand, defluorination of o-BiF3 particles with sizes of less than 3 μm started from their contours and proceeded toward their central positions. The observed phenomena have implications for controlling the rate-determining steps in FSB reactions that depend on electrolytes and the sizes and shapes of particles and for designing electrodes and electrolytes for superior FSBs.

Fluoride ion batteries – past, present, and future

“…116 This resulted in a better capacity retention during the first three cycles (~210 mAh g -1 and ~100 mAh g -1 at the first and third cycles, respectively) as compared to that with BiF3 as the electrode material. 117,118 In another study, 119 PbF2 has been investigated using a similar liquidbased electrolyte but with the addition of TPhBX as the anion acceptor. There, the effects of pulverization of PbF2 and carbon additive were elaborated and the authors conclude that a mixture of PbF2 and carbon, which is pulverized by ball milling, can increase the capacity of the cell.…”

Influence of Graphene Oxide Surface on the Synthesis and Structure of BiOxFy at Ambient Conditions

“…As shown in Figure S3 (Supporting Information), for Bi1F3@GO, there is only one broad peak ≈150 cm -1 , which is assigned to cubic BiF 3 . [44,45] However, the Raman spectrum of Bi1F1@GO and Bi1F3@GO is completely different. For Bi1F1@GO, there are three new peaks appeared at 90.47, 119.47, and 308.384 cm -1 .…”

Evolution of fluoride shuttle battery reactions and three-dimensional morphology changes of BiF3 microparticles in an ethylene carbonate-based liquid electrolyte

“…o-BiF 3 microparticles were supplied by Fluorochem Ltd. c-BiF 3 microparticles were prepared by annealing o-BiF 3 microparticles at 150 °C in vacuum for 12 hours. 34,35 Structures of o-BiF 3 and c-BiF 3 are shown in Fig. 1(a) and (b), respectively.…”

“…28,29 We have conducted in situ Raman spectroscopy during battery reactions including FSB reactions. 16,[30][31][32][33][34][35][36] Recently, in situ Raman mapping was conducted almost simultaneously with in situ threedimensional (3D) morphology analysis by laser scanning confocal microscopy (LSCM) and the method was found to be a a Office of Society-Academia Collaboration for Innovation, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. E-mail: yamanaka.toshiro.8n@kyoto-u.ac.jp b Graduate School of Global Environmental Studies, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan † Electronic supplementary information (ESI) available: Experimental details, ionic conductivity of the electrolyte, height profiles of particles, CV measurements of the electrolyte.…”

“…28,29 We have conducted in situ Raman spectroscopy during battery reactions including FSB reactions. 16,[30][31][32][33][34][35][36] Recently, in situ Raman mapping was conducted almost simultaneously with in situ threedimensional (3D) morphology analysis by laser scanning confocal microscopy (LSCM) and the method was found to be a powerful method for determining reaction mechanisms and rate-determining steps. 36 In the present work, evolution of FSB reactions of orthorhombic and cubic BiF 3 (o-BiF 3 and c-BiF 3 ), respectively (see Fig.…”