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

Abstract Layered O3‐type sodium oxides (NaMO 2 , 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 Na 0.7 Ni 0.35 Sn 0.65 O 2 material displays a highest redox potential of 3.7 V (vs. Na + /Na) among the reported O3‐type materials based on the Ni 2+ /Ni 3+ couple, by virtue of its increased Ni−O bond ionicity through reduced orbital overlap between transition metals and oxygen within the MO 2 slabs. This study provides an orbital‐level understanding of the operating potentials of the nominal redox couples for O3‐NaMO 2 cathodes. The strategy described could be used to tailor electrodes for improved performance.