The increasingly growing demand of lithium-ion batteries (LIBs) for electromotive and stationary storage application has raised concerns about the future, long-term availability and cost of the critical raw materials employed in LIB production, such as cobalt, nickel, lithium, graphite and copper . In this scenario, sodium-ion batteries (SIBs) have the potential to become the next green, low cost, environmentally friendly electrochemical storage system for large-scale stationary applications and light electromobility. Indeed, the SIB technology would not be limited by the abundance of the employed raw materials and their cost .
However, to meet the requirements for practical application, development and optimization of novel electrode materials and electrolytes is required. Several materials have been proposed with outstanding electrochemical performance, however, the instability of the electrode-electrolyte interphase layers at both, cathode and anode side, represents the biggest challenge to be tackled for a successful development of high performing SIBs. At the cathode side, layered transition metal oxides have been identified as one of the most promising candidates in view of their high specific capacity and ease of synthesis . It has been widely reported that the P2-type materials offer better electrochemical performance in terms of long-term stability and rate capability compared to their O3- or P3-type analogs .
Among the P2-type layered oxides, Na2/3Ni1/3Mn2/3O2 exhibits a theoretical capacity of 173 mAh g-1. However, its complex electrochemical behavior is characterized by several structural changes upon cycling inevitably leading to rapid capacity fading . The main processes associated with the material degradation have been identified as i) the irreversible P2-O2 phase transition at high potentials above 4.2 V, most likely linked to oxygen release, ii) the instability of the cathode-electrolyte interphase (CEI) and iii) the manganese dissolution process occurring due to the presence of Jahn-Teller active Mn3+ .
In this work we attempt to disclose degradation mechanisms occurring at the CEI and propose a degradation proof strategy in order to mitigate the above-mentioned issues: an inert Al2O3 surface coating for the P2-Na2/3Ni1/3Mn2/3O2 cathode.
A comprehensive ex-situ surface chemistry analysis of un-coated and Al2O3 coated P2-Na2/3Ni1/3Mn2/3O2 cathodes at different states of charge upon the first cycle has been carried out by means of in-house and synchrotron X-ray based techniques. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) are used to elucidate the CEI evolution upon electrochemical cycling and combined with further electrochemical performance. The understanding of the CEI’s chemical composition and the formation mechanism, as well as the effect of Al2O3 coating opens the door for a rational design of enhanced cathode materials for application in SIBs.
 C. Vaalma, D. Buchholz, M. Weil, S. Passerini, Nat. Rev. Mater. 3 (2018) 18013.
 I. Hasa, S. Mariyappan, D. Saurel, P. Adelhelm, A. Y. Koposov, C. Masquelier, L. Croguennec, M. Casas-Cabanas, J. Power Sources 482 (2021) 228872.
 M. H. Han, E. Gonzalo, G. Singh, T. Rojo, Energy Environ. Sci. 8 (2015) 81.
 I. Hasa, D. Bucholz, S. Passerini, J. Hassoun, ACS Appl. Mater. Interfaces 7 (2015) 5206.
 Z. Lu, J.R. Dahn, J. Electrochem. Soc. 148 (2001) A1225.