Investigation of the Electrode/Electrolyte Interface of Fe2O3 Composite Electrodes: Li vs Na Batteries

We have investigated the properties of the electrode/electrolyte interfaces of composite electrodes based on nanostructured iron oxide cycled in Li- and Na-half cells containing analogous electrolytes (i.e., LiClO4 or NaClO4 in ethylene carbonate:diethyl carbonate (EC:DEC)). A meticulous nondestructive step-by-step analysis of the first discharge/charge cycle has been conducted via soft X-ray photoelectron spectroscopy using synchrotron radiation. In this way, different depths were probed by varying the photon energy (hν) for both electrochemical systems. The results of this thorough study clearly highlight the differences and the similarities of their respective solid electrolyte interface (SEI) layers in terms of formation, composition, structure, or thickness, as well as their conversion mechanisms. We specifically point out that the SEI coverage is more pronounced, and a homogeneous distribution rich in inorganic species exists in the case of Na, compared to the organic/inorganic layered structure observed for the Li system. The SEI formation gradually occurs during the first discharge in both Li- and Na-half cells. For Na, a predeposit layer is formed directly by simple contact of the electrode with the electrolyte. Despite using similar electrolytes, the nature of the cation (Li+ or Na+) has significant impact on the overall composition/structure of the resulting SEI.