title: Passivation Layer and Cathodic Redox Reactions in Sodium-Ion Batteries Probed by HAXPES.
abstract: The cathode material P2-Nax Co2/3 Mn2/9 Ni1/9 O2, which could be used in Na-ion batteries, was investigated through synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Nondestructive analysis was made through the electrode/electrolyte interface of the first electrochemical cycle to ensure access to information not only on the active material, but also on the passivation layer formed at the electrode surface and referred to as the solid permeable interface (SPI). This investigation clearly shows the role of the SPI and the complexity of the redox reactions. Cobalt, nickel, and manganese are all electrochemically active upon cycling between 4.5 and 2.0 V; all are in the 4+ state at the end of charging. Reduction to Co(3+), Ni(3+), and Mn(3+) occurs upon discharging and, at low potential, there is partial reversible reduction to Co(2+) and Ni(2+). A thin layer of Na2 CO3 and NaF covers the pristine electrode and reversible dissolution/reformation of these compounds is observed during the first cycle. The salt degradation products in the SPI show a dependence on potential. Phosphates mainly form at the end of the charging cycle (4.5 V), whereas fluorophosphates are produced at the end of discharging (2.0 V).
authors.affiliation: LCME, FST Marrakesh, University Cadi Ayyad, Av. Abdelkrim Khattabi, Box 511, 40000, Marrakech, Morocco.
authors.forename: Siham
authors.initials: S
authors.lastname: Doubaji
authors.affiliation: Department of Physics and Astronomy, Uppsala University, Box 516, 751 20, Uppsala, Sweden.
authors.forename: Bertrand
authors.initials: B
authors.lastname: Philippe
authors.affiliation: LCME, FST Marrakesh, University Cadi Ayyad, Av. Abdelkrim Khattabi, Box 511, 40000, Marrakech, Morocco.|Center for Advanced Materials, Université Mohammed VI Polytechnique, Lot 660-Hay Moulay Rachid, Ben Guerir, Morocco.
authors.forename: Ismael
authors.initials: I
authors.lastname: Saadoune
authors.affiliation: Helmholtz-Zentrum Berlin für Materialien und Energie, Albert Einstein Strasse 15, 12489, Berlin, Germany.
authors.forename: Mihaela
authors.initials: M
authors.lastname: Gorgoi
authors.affiliation: Department Of Chemistry-Ångström Laboratory, Uppsala University, Box 538, 721 21, Uppsala, Sweden.
authors.forename: Torbjorn
authors.initials: T
authors.lastname: Gustafsson
authors.affiliation: Center for Advanced Materials, Université Mohammed VI Polytechnique, Lot 660-Hay Moulay Rachid, Ben Guerir, Morocco.
authors.forename: Abderrahim
authors.initials: A
authors.lastname: Solhy
authors.affiliation: Department Of Chemistry-Ångström Laboratory, Uppsala University, Box 538, 721 21, Uppsala, Sweden.
authors.forename: Mario
authors.initials: M
authors.lastname: Valvo
authors.affiliation: Department of Physics and Astronomy, Uppsala University, Box 516, 751 20, Uppsala, Sweden.
authors.forename: Håkan
authors.initials: H
authors.lastname: Rensmo
authors.affiliation: Department Of Chemistry-Ångström Laboratory, Uppsala University, Box 538, 721 21, Uppsala, Sweden. kristina.edstrom@kemi.uu.se.
authors.forename: Kristina
authors.initials: K
authors.lastname: Edström
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D011211:Electric Power Supplies; D004563:Electrochemistry; D004566:Electrodes; D017895:Manganese Compounds / Q000737:chemistry*; D009532:Nickel / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry; D056951:Photoelectron Spectroscopy; D012964:Sodium / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D017895:Manganese Compounds; D010087:Oxides; D003035:Cobalt; D009532:Nickel; D012964:Sodium
keywords: electrochemistry; interfaces; photoelectron spectroscopy; reaction mechanisms; sodium