Layered O3-type sodium oxides (NaMO2 , M=transition metal) commonly exhibit an O3-P3 phase transition, which occurs at a low redox voltage of about 3 V (vs. Na+ /Na) during sodium extraction and insertion, with the result that almost 50 % of their total capacity lies at this low voltage region, and they possess insufficient energy density as cathode materials for sodium-ion batteries (NIBs). Therefore, development of high-voltage O3-type cathodes remains challenging because it is difficult to raise the phase-transition voltage by reasonable structure modulation. A new example of O3-type sodium insertion materials is presented for use in NIBs. The designed O3-type Na0.7 Ni0.35 Sn0.65 O2 material displays a highest redox potential of 3.7 V (vs. Na+ /Na) among the reported O3-type materials based on the Ni2+ /Ni3+ couple, by virtue of its increased Ni-O bond ionicity through reduced orbital overlap between transition metals and oxygen within the MO2 slabs. This study provides an orbital-level understanding of the operating potentials of the nominal redox couples for O3-NaMO2 cathodes. The strategy described could be used to tailor electrodes for improved performance.
Keywords: O3 phase; P3 phase; cathodes; high voltage; orbital hybridizations.
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