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Reaction Mechanism and Surface Film Formation of Conversion Materials for Lithium- and Sodium-Ion Batteries: An XPS Case Study on Sputtered Copper Oxide (CuO) Thin Film Model Electrodes
Charge storage based on conversion
reactions is a promising concept
to store electrical energy. Many studies have been devoted to conversion
reactions with lithium; however, still many scientific questions remain
due to the complexity of the reaction mechanism combined with surface
film formation. Replacing lithium by sodium is an attractive approach
to widen the scope of conversion reactions and to study whether the
increase in ion size changes the reaction mechanisms and whether the
cell performance benefits or worsens. In this study, we use thin film
electrodes as a additive-free model system to study the conversion
reaction of CuO with sodium (CuO/Na) by means of electrochemical methods,
microscopy, and X-ray photoelectron spectroscopy. The reaction mechanism
and film formation are being discussed. Some important differences
to the analogue lithium-based system (CuO/Li) are found. Whereas CuO
has been reported as charge product in CuO/Li cells, charging is incomplete
in the case of CuO/Na and only Cu<sub>2</sub>O is formed. As an important
finding, oxygen appears to be redox active and Na<sub>2</sub>O<sub>2</sub> forms during charging from Na<sub>2</sub>O. Moreover, surface
film formation due to electrolyte decomposition is much more severe
as compared to CuO/Li. Depth profiling is used to probe the inner
composition of the surface film, revealing a much thicker surface
film with more inorganic components as compared to the lithium system.
It is also found that the surface film disappears to a large extent
during charging