Mechanisms of and three-dimensional morphology changes in fluoride shuttle battery reactions of PbF2 microparticles

Abstract

Fluoride shuttle batteries (FSBs), which are based on defluorination of metal fluorides and fluorination of metals, are expected to become next-generation batteries with high energy densities. To achieve high performance, elucidation and control of the reaction mechanisms are necessary. In this work, the evolution of FSB reactions of orthorhombic and cubic PbF₂ (o-PbF₂ and c-PbF₂, respectively) supported on gold foil in a CsF/LiBOB (lithium bis(oxalato)borate)/tetraglyme electrolyte was studied. By conducting in situ laser scanning confocal microscopy (LSCM) and in situ Raman microscopy in the same areas of the sample in turn, changes in the color and three-dimensional morphology of the particles and in the chemical distribution were elucidated. When the potential of o-PbF₂vs. Pb was decreased from OCV (0.3 V) to −0.25 V step by step, Pb microparticles appeared at 0 V at positions far from o-PbF₂ (defluorination by the dissolution–deposition mechanism). Next, defluorination by desorption of F⁻ started at −0.25 V at the protruded positions on the o-PbF₂ particles. At the same time, deposition of Pb started at the contours of o-PbF₂ particles and extended outward, indicating that desorption of F⁻ induced dissolution of PbF₂. On the other hand, defluorination of c-PbF₂ proceeded mainly by the direct desorption mechanism without inducing dissolution. Contrary to expectation, the apparent volumes of PbF₂ particles increased during desorption of F⁻, which sometimes resulted in the irreversible agglomeration of neighboring particles. These findings are important for understanding the mechanisms of changes in the morphology of active materials and for designing electrodes and electrolytes to achieve high performance.