Redox Potential Paradox in NaxMO₂ 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, NaMO 2 (M: transition metal), are the most promising cathode materials owing to their high theoretical capacity with much more stable nature than Li 1– x MO 2 system, factors influencing the redox potential have not yet been fully understood. Here, we identify redox potential paradox, E (Ni 3+ /Ni 2+ ) > E (Ni 4+ /Ni 3+ ), in an identical structural framework, namely, NaTi 4+ 0.5 Ni 2+ 0.5 O 2 and NaFe 3+ 0.5 Ni 3+ 0.5 O 2, which is induced by transition of the oxides from Mott–Hubbard to negative charge-transfer regimes. The origin of the unusually low E (Ni 4+ /Ni 3+ ) 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.