XAS study of the reversible reactivity mechanism of micro- and nanostructured electrodeposited Cu2O thin films towards lithium

Highly uniform electrodeposited Cu2O electrodes for Li-ion batteries, with variable particle size, morphology and thickness, have been studied in the charged and discharged steps as a function of the number of cycling processes, by means of ex situ X-ray absorption spectroscopy (XAS) at the Cu K edge. Both, fingerprint analysis of the X-ray Absorption Near Edge Structure spectra (XANES) and the radial atomic distribution obtained by the analysis of the Extended X-ray Absorption Fine Structure (EXAFS) spectra, have allowed us to quantify the bimodal distribution (Cu and Cu2O) of the electrodes. These results correlate qualitatively well with their different electrochemical response, establishing a deep insight into the reversible reactivity mechanism of these films towards lithium in Li batteries. Our results demonstrate that the reversibility of the reduction process upon the number of cycles depends on the thickness, as it was suggested in previous works but also on the morphology of the electrodes. Higher reversibility corresponds to smaller thickness electrodes. For similar thickness, the reversibility is higher for porous and nanometric particle electrodes than for compact and micrometric particle electrodes. The thinnest film exhibited an increase of capacity after several recycling steps, which agrees with the increase of the Cu2O content.