Reactivity and Mechanisms in Fluoride Shuttle Battery Reactions: Difference between Orthorhombic and Cubic BiF₃ Single Microparticles

Abstract

BiF3 is a strong candidate for a cathode material in fluoride shuttle batteries (FSBs), which utilize defluorination of metal fluorides and fluorination of metals. Much interest has been shown in FSBs as next-generation batteries with high energy densities. Elucidation and control of reactivity and mechanisms in FSB reactions are crucial for the development of FSBs with high performances. In the present work, structural transformation, reactivities, and mechanisms in FSB reactions of BiF3 microparticles partly embedded in a gold plating film (BiF3/gold, mainly orthorhombic BiF3 (o-BiF3) containing small amounts of cubic BiF3 (c-BiF3)) were studied by in-situ Raman microscopy. Reactivities and mechanisms in defluorination were found to be different for o-BiF3 and c-BiF3. In an ionic-liquid-based electrolyte, gradual transformation of o-BiF3 into cubic c-BiF3 occurred. When the voltage of BiF3/gold (cathode) vs a Pb counter electrode (anode) was decreased from OCV (0.7 V) to 0.05 V step by step, direct defluorination of the surfaces of only o-BiF3 started from their contours at 0.45 V and then extended to their center parts and was mostly completed at 0.2 V. Then defluorination of c-BiF3 started at a voltage below 0.2 V by both direct defluorination and dissolution–deposition mechanisms. In the former 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. The rate of direct defluorination of c-BiF3 was much slower than that of o-BiF3. Defluorination of c-BiF3 by the dissolution–deposition mechanism was dominant and fast when the excitation beam was strong, probably due to a thermal effect. Such an observation is important for the development of electrodes and electrolytes with proper solubility to better utilize reactions by the two mechanisms to realize FSBs with high performance.

Fluoride ion batteries – past, present, and future

“…Apart from using Bi/BiF3 as a cathode in an all-solid-state modification, it has also been used as the cathode in a liquid electrolyte system. In 2019 Yamanaka et al reported 131 on using orthorhombic-BiF3 microparticles in a gold plating film as a cathode material against Pb wire as the counter electrode. Then the structural transformation and reaction mechanisms within BiF3 as the active electrode material were studied by means of in-situ Raman microscopy.…”

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

“…However, the mapping of the electrolyte did not change significantly in this defluorination process, as shown in Figure 2c−f, also suggesting the direct mechanism. During these processes, nonelectrochemical transformation of o-BiF 3 to c-BiF 3 , which was previously observed in an ionic-liquid electrolyte, 33 was not observed (see also Figure S1 in the Supporting Information. ).…”

“…These results indicate that defluorination of c-BiF 3 is slower than that of o-BiF 3 , being consistent with previously reported results obtained by using an ionic-liquid electrolyte. 33 However, defluorination of both o-BiF 3 and c-BiF 3 occurred at lower voltages (below 0.3 and 0.15 V, respectively) than those in the ionic-liquid electrolyte (0.45 and 0.2 V, respectively). 33 In addition, the current corresponding to defluorination for (o, c)-BiF 3 /gold (Figure 2b) was one order lower than that in the ionic-liquid electrolyte (Figure 5b in ref 33).…”

Nanoscale Defluorination Mechanism and Solid Electrolyte Interphase of a MgF2 Anode in Fluoride-Shuttle Batteries

“…Fluoride-shuttle batteries, which utilize fluoride ion (F – ) transfer between electrodes, are one of the candidates as next-generation batteries due to their potential to achieve higher energy density than the lithium-ion batteries. , The concept of this battery was reported in 1970s. , Reddy and Fichtner in 2011 demonstrated the first reversible cycling performance in an all-solid-state fluoride-shuttle battery, and it has attracted significant attention ever since. − There are many types of metal/metal fluorides, thus making it possible to achieve a high voltage cell by exploring suitable combinations of cathode and anode materials with an appropriate fluoride ion conductivity electrolyte. Then, in a defluorination process based on a conversion reaction of metal/metal fluorides in the electrodes, multiple electron reactions can be promoted by moving single-charged anions of F – in electrolytes.…”