Redox Potential Paradox in Na_xMO₂ for Sodium-Ion Battery Cathodes

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, NaMO₂ (M: transition metal), are the most promising cathode materials owing to their high theoretical capacity with much more stable nature than Li_1-xMO₂ system, factors influencing the redox potential have not yet been fully understood. Here, we identify redox potential paradox, E(Ni³⁺/Ni²⁺) > E(Ni⁴⁺/Ni³⁺), in an identical structural framework, namely, NaTi⁴⁺_0.5Ni²⁺_0.5O₂ and NaFe³⁺_0.5Ni³⁺_0.5O², which is induced by transition of the oxides from Mott-Hubbard to negative charge-transfer regimes. The origin of the unusually low E(Ni⁴⁺/Ni³⁺) 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.