Na-ion
batteries have been the subjects of intensive studies for
grid-scale energy storage recently. O3-type NaFeO2 is a
promising candidate for the Na-ion cathode materials, though the irreversibility
during Na-ion extraction/insertion seriously hinders its practical
application. The present work demonstrates that partial replacement
of Fe in O3-NaFeO2 with Ni leads to the significant improvement
of the electrochemical properties. The 57Fe Mössbauer
and X-ray absorption spectra show that O3-type NaFeO2–NaNiO2 solid solution forms hybridized frontier orbital of a Fe–O–Ni
bond via ligand-to-metal charge transfer, which plays a dominant role
in the charge–discharge process. The resulting O3-NaFe0.3Ni0.7O2 delivers an initial discharge
capacity of 135 mA·h·g–1, most of which
is in a high-voltage region of 2.5–3.8 V, with a high initial
Coulombic efficiency of 93%, and shows enhanced cycle stability.
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.
We have investigated the local structure of differently charged Na x CoO 2 cathode material as a function of temperature by Co K-edge X-ray absorption fine structure (EXAFS) measurements. We have found that the charge/ discharge process has a direct effect on the bond characteristics of the cathode in the Na-ion batteries. The results reveal that the local Co−O bonds get softer, while the Co−Co bonds hardly show any change during discharge (sodiation). The present study underlines the key role of local atomic displacements in diffusion and the reversibility of ions in cathodes for batteries and points toward the feasibility of Na x CoO 2 to be used as a cathode material. The results are discussed in comparison with the lithiation/delithiation of Li x CoO 2 battery materials.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.