Investigation of the conversion mechanism of nanosized CoF2

Nanoparticles (∼20 nm) of tetragonal (P42/mnm) cobalt fluoride (CoF2) has been synthesized by precipitation using cobalt nitrate and ammonium fluoride solution at room temperature, followed by annealing at 400 °C under argon atmosphere. The morphology and structure have been studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The electrochemical lithiation of CoF2 and its structural and valence changes during conversion have been investigated. Electrochemical measurements revealed a discharge specific capacity close to theoretical specific capacity of 553 mAh g−1 and charge specific capacities ranging from 250 to 450 mAh g−1 in the first cycle, depending on the type of conductive carbon addictive used. Lithiation of CoF2 occurs without any intercalation, but via conversion reaction CoF2 + 2Li+ + 2e− → Co(0) + 2LiF. During lithiation, CoF2 is fully reduced to Co(0) and breaks down into smaller particles of ∼2 nm. Upon recharging, CoFx is formed instead of CoF2. XPS and EELS studies show that only the surface of the electrode is reconverted back to CoFx during recharging. In addition, recharging to form CoFx commences at the interface between the cobalt and lithium fluoride. These results offer the experimental evidence explaining the lack of cycle stability of metal fluorides that undergo a conversion reaction with lithium such as cobalt fluoride. In addition, they also provide insight into the reactions during conversion and reconversion. Finally, it is shown that the morphology of the conductive carbon addictives used in electrode fabrication plays a crucial role in determining the capacity retention ability for materials that undergo a conversion reaction with lithium.