title: Exploring Oxygen Activity in the High Energy P2-Type Na0.78Ni0.23Mn0.69O2Cathode Material for Na-Ion Batteries.
abstract: Large-scale electric energy storage is fundamental to the use of renewable energy. Recently, research and development efforts on room-temperature sodium-ion batteries (NIBs) have been revitalized, as NIBs are considered promising, low-cost alternatives to the current Li-ion battery technology for large-scale applications. Herein, we introduce a novel layered oxide cathode material, Na0.78Ni0.23Mn0.69O2. This new compound provides a high reversible capacity of 138 mAh g-1and an average potential of 3.25 V vs Na+/Na with a single smooth voltage profile. Its remarkable rate and cycling performances are attributed to the elimination of the P2-O2 phase transition upon cycling to 4.5 V. The first charge process yields an abnormally excess capacity, which has yet to be observed in other P2 layered oxides. Metal K-edge XANES results show that the major charge compensation at the metal site during Na-ion deintercalation is achieved via the oxidation of nickel (Ni2+) ions, whereas, to a large extent, manganese (Mn) ions remain in their Mn4+state. Interestingly, electron energy loss spectroscopy (EELS) and soft X-ray absorption spectroscopy (sXAS) results reveal differences in electronic structures in the bulk and at the surface of electrochemically cycled particles. At the surface, transition metal ions (TM ions) are in a lower valence state than in the bulk, and the O K-edge prepeak disappears. On the basis of previous reports on related Li-excess LIB cathodes, it is proposed that part of the charge compensation mechanism during the first cycle takes place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient. We believe that by optimizing and controlling oxygen activity, Na layered oxide materials with higher capacities can be designed.
authors.affiliation: Department of NanoEngineering, University of California San Diego , La Jolla, California 92037, United States.
authors.forename: Chuze
authors.initials: C
authors.lastname: Ma
authors.affiliation: Department of NanoEngineering, University of California San Diego , La Jolla, California 92037, United States.
authors.forename: Judith
authors.initials: J
authors.lastname: Alvarado
authors.affiliation: Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.|Department of Materials Science and Engineering, Iowa State University , Ames, Iowa 50011, United States.
authors.forename: Jing
authors.initials: J
authors.lastname: Xu
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.|Department of Materials Science and Engineering, University of California Berkeley , Hearst Mining Memorial Building, Berkeley, California 94720, United States.
authors.forename: Raphaële J
authors.initials: RJ
authors.lastname: Clément
authors.affiliation: Department of NanoEngineering, University of California San Diego , La Jolla, California 92037, United States.
authors.forename: Moses
authors.initials: M
authors.lastname: Kodur
authors.affiliation: Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.
authors.forename: Wei
authors.identifier: 0000-0002-2878-1297
authors.initials: W
authors.lastname: Tong
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Clare P
authors.initials: CP
authors.lastname: Grey
authors.affiliation: Department of NanoEngineering, University of California San Diego , La Jolla, California 92037, United States.
authors.forename: Ying Shirley
authors.identifier: 0000-0001-8936-8845
authors.initials: YS
authors.lastname: Meng