Basics: Conversion Reaction Materials for Energy Storage

Conversion reaction compounds are a new class of materials which exhibit high charge storage density in Li-ion battery applications. Upon exposure to Li, these compounds are reduced from a high oxidation state to their metallic state, thereby storing two or three electrons per formula unit. This represents a 2- or 3-fold increase in charge storage capacity over conventional intercalation batteries. 

Nanophase transition metal fluorides and oxides such as FeF2 and CoO have exhibited good performance in electrochemical studies, but the atomistics and phase evolution of the conversion reaction are not well understood. In order to characterize these materials without contamination from atmosphere or electrolytes, we have grown thin films of FeF2 and CoO in ultra-high vacuum and exposed them to atomic Li to induce a conversion reaction. The electronic structure and phase of the films before and after lithiation have been investigated with ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPS), and x-ray photoemission spectroscopy (XPS). 

Using these techniques, we have observed the reduction of the conversion compounds as a function of Li exposure. We have also used transmission electron microscopy (TEM) to show that both FeF2 and CoO react with Li to form metal nanoparticles embedded in a Li-rich matrix. Additionally, angle resolved XPS (ARXPS) of epitaxially grown CoO and FeF2, has been used to show that the conversion reaction front proceeds uniformly through the films from the surface, in some cases leaving unreacted regions behind.

       Recent Highlights: Temperature Dependence of the Li-CoO Reaction

Polycrystalline CoO thin films were grown and exposed to atomic Li in vacuum. XPS of the film before and after a series of Li exposures shows a gradual increase in Li content concomitant with the formation of metallic Co and Li2O, as expected from electrochemical studies. However, an additional peak in the O-1s spectrum suggests the formation of an additional Li-oxide species due to the parasitic reaction of Li2O with residual O2 or H2O gas in the vacuum chamber. This compound is believed to be Li2O2, which is known to inhibit Li diffusion at room temperature in Li-air batteries. Consequently, when the film is lithiated at high temperature (150oC), the amount of CoO conversion is proportional to the amount of Li deposited, whereas lithiation at room temperature (25oC) results in a diminished degree of conversion. These results suggest that any conversion battery whose products include Li2O is likely suffer from kinetic limitations at room temperature.

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