Sodium-ion batteries are attractive energy storage media owing to the abundance of sodium, but the low capacities of available cathode materials make them impractical. Sodium-excess metal oxides Na2MO3 (M: transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. However, the general strategies for enhancing the capacity of Na2MO3 are poorly established. Here using two polymorphs of Na2RuO3, we demonstrate the critical role of honeycomb-type cation ordering in Na2MO3. Ordered Na2RuO3 with honeycomb-ordered [Na1/3Ru2/3]O2 slabs delivers a capacity of 180 mAh g−1 (1.3-electron reaction), whereas disordered Na2RuO3 only delivers 135 mAh g−1 (1.0-electron reaction). We clarify that the large extra capacity of ordered Na2RuO3 is enabled by a spontaneously ordered intermediate Na1RuO3 phase with ilmenite O1 structure, which induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na2MO3 cathodes.
Herein, we investigate the formation of a cathode electrolyte interphase (CEI) by electrolyte oxidation on a LiNi x M 1−x O 2 (x > 0.5; M, transition metal) layered oxide (Ni-rich) cathode and compare this phenomenon with a Li-rich layered oxide (Li-rich) cathode. Our investigations focused on two electrochemical properties, the potential and kinetics of electrolyte oxidation, studied using hard X-ray photoelectron spectroscopy (HAXPES), soft X-ray absorption spectroscopy, and density functional theory calculations. HAXPES revealed that a thicker CEI formed on the Ni-rich cathode compared to that on the Li-rich cathode, despite the operation potential of the Ni-rich cathode being lower than that of the Li-rich cathode. Thus, the Ni-rich cathode induces the CEI formation through active oxidation of the electrolyte during charge−discharge cycles. The electronic state of the Ni-rich cathode indicates that the antibonding hybrid orbital of the transition metal 3d−O 2p corresponds to the lowest unoccupied molecular orbital energy level, that is, the hole, and lies near the highest occupied molecular orbital energy level of the electrolyte. In addition, the hole concentration in the charged state was found to be significantly increased, in comparison to other active materials, which promotes oxidization of the electrolyte.
Raising the operating potential of the cathode materials in sodium-ion batteries is a crucial challenge if they are to outperform state-of-the-art lithium-ion batteries. Although the layered transition metal oxides, NaMO 2 (M: transition metal), are the most promising cathode materials owing to their high theoretical capacity with much more stable nature than Li 1−x MO 2 system, factors influencing the redox potential have not yet been fully understood. Here, we identify redox potential paradox, E(Ni 3+ /Ni 2+ ) > E(Ni 4+ /Ni 3+ ), in an identical structural framework, namely, NaTi 4+ 0.5 Ni 2+ 0.5 O 2 and NaFe 3+ 0.5 Ni 3+ 0.5 O 2 , which is induced by transition of the oxides from Mott−Hubbard to negative charge-transfer regimes. The origin of the unusually low E(Ni 4+ /Ni 3+ ) is the surprisingly large contribution (over 80%) of oxygen orbital to the redox reaction, of which the primary effect on the electrochemical property is demonstrated for the first time, providing a firm platform to design better cathodes for advanced sodium-ion batteries.
Rechargeable zinc−air batteries are considered as one of the possible candidates to replace conventional lithium-ion batteries. One of the requirements for effective battery operation is an oxygen evolution reaction (OER) that needs to be generated in a highly alkaline electrolyte. The A 2 BB′O 5 brownmillerite-type Ca 2 FeCoO 5 electrocatalyst having a 57 Pbcm symmetry exhibits very high electrocatalytic activity toward OER in 4 mol dm −3 KOH. Our studies show that the electrocatalyst undergoes bulk amorphization upon OER and adequately activates catalytically active domains. The synchrotron radiation studies using the extended X-ray absorption fine structure (EXAFS) technique show that the central structural unit found in the polarized Ca 2 FeCoO 5 is a cluster of edge-sharing CoO 6 octahedra. The electrochemical data indicate that OER preferentially takes place on the edge-sharing CoO 6 octahedra catalytic centers reconstructed in the brownmillerite-type electrocatalyst. The EXAFS second shell peaks at an interatomic distance of 2.8 Å are the fingerprints of the catalytically active domains.
The characteristics of CO2 adsorption sites on a nitrogen-doped graphite model system (N-HOPG) were investigated by X-ray photoelectron and absorption spectroscopy and infrared reflection absorption spectroscopy. Adsorbed CO2 was observed lying flat on N-HOPG, stabilized by a charge transfer from the substrate. This demonstrated that Lewis base sites were formed by the incorporation of nitrogen via low-energy nitrogen-ion sputtering. The possible roles of twofold coordinated pyridinic N and threefold coordinated valley N (graphitic N) sites in Lewis base site formation on N-HOPG are discussed. The presence of these nitrogen species focused on the appropriate interaction strength of CO2 indicates the potential to fine-tune the Lewis basicity of carbon-based catalysts.
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