An Abnormal 3.7Volt O3-Type Sodium-Ion Battery Cathode

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 3V (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 Na0.7Ni0.35Sn0.65O2 material displays a highest redox potential of 3.7V (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.