Strong Anionic Repulsion for Fast Na Kinetics in P2-Type Layered Oxides

An intriguing mechanism for enabling fast Na kinetics during oxygen redox (OR) is proposed to produce high-power-density cathodes for sodium-ion batteries (SIBs) based on the P2-type oxide models, Na_2/3[Mn_6/9Ni_3/9]O₂ (NMNO) and Na_2/3[Ti_1/9Mn_5/9Ni_3/9]O₂ (NTMNO) using the “potential pillar” effect. The critical structural parameter of NTMNO lowers the Na migration barrier in the desodiated state because the electrostatic repulsion of O(2p)-O(2p) that occurs between transition metal layers is combined with the chemically stiff Ti⁴⁺(3d)-O(2p) bond to locally retain the strong repulsion effect. The NTMNO interlayer distance moderately decreases upon charging with oxygen oxidation, whereas that of NMNO decreases at a much faster rate, which can be explained by the dependence of OR activity on the coordination environment. Fundamental electrochemical experiments clearly indicate that the Ti doping of the bare material significantly improves its rate capability during OR, and detailed electrochemical and structural analyses show much faster Na kinetics for NTMNO than for NMNO. A systematic comparison of the two cathode oxides based on experiments and first-principles calculations establishes the “potential pillar” concept of not only improving the sluggish Na kinetics upon OR reaction but also harnessing the full potential of the anionic redox for high-power-density SIBs.