Operando Observations of Reversible α-BiF 3 Conversion in Liquid Electrolyte by Synchrotron Radiation Diffraction and 7 Li Nuclear Magnetic Resonance

Abstract

The electrochemically-driven conversion reaction of BiF 3 with Li + ions is of interest for potential application as a positive electrode system in conversion type rechargeable batteries. Here, we use synchrotron radiation diffraction and 7 Li nuclear magnetic resonance (NMR) for complementary operando observations of reversible or quasi-reversible conversion reactions of α -BiF 3 nanocomposite electrode at room temperature. The nanocomposite electrode was fabricated by ball milling and subsequent heat treatments. A pronounced XRD peak broadening indicated a disordered fine crystalline state in the electrode. The operando synchrotron radiation diffraction measurements revealed a quasi-reversible smooth crystallographic transformation between α -BiF 3 and metallic Bi. The operando 7 Li NMR measurement quantitatively showed reversible conversion reactions. The series of in-situ 7 Li NMR spectra contained sharp signals from Li + ions in the electrolyte and a broad signal associated with a solid phase of LiF. These signals followed a stoichiometric relationship that conformed to a three-electron BiF 3 conversion pathway.