An Abnormal 3.7 Volt 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 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_0.7Ni_0.35Sn_0.65O₂ 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.