What Happens Structurally and Electronically during the Li Conversion Reaction of CoFe2O4 Nanoparticles: An Operando XAS and XRD Investigation

The need to identify lithium battery anodes consisting of new materials exhibiting high energy density and good cycling stability has intensified the research on reversible so-called conversion reactions between lithium and oxidic spinels such as, e.g., CoFe2O4. Operando nondestructive synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were applied to directly determine the complex reaction mechanisms occurring during discharge and charge processes. Distinct reaction mechanism steps related to different electrochemical features of nanosized CoFe2O4 as the anode material are elucidated. The different discharge steps include uptake of a small fraction of Li into an amorphous layer covering the particles, the transformation of the spinel structure into a NaCl-like structure, and the formation of nanosized metallic Co and Fe embedded in a Li2O matrix. The metals are oxidized to form Co2+ and Fe3+ during charging. The reaction Fe3+ ↔ Fe0 is observed for many discharge/charge processes,...