Enabling Na ⁺ /Vacancy Disordering and Superior Redox Kinetics of P2-Type Layered Cathode by Tuning Cationic Ratio

The P2-type layered oxide cathodes, especially for the Ni/Mn-based counterparts with the merits of high voltage, high capacity, and air stability, show promise for sodium-ion batteries (SIBs). However, a Na+/vacancy-ordered superstructure in these P2-type oxides easily occurs, which corresponds to the obvious charge/discharge platforms and lowers the Na+ chemical diffusion coefficients. In this work, a series of P2-type oxides Na2/3NixMn1–xO2 (x = 1/4, 1/3, and 2/5) with different Ni/Mn ratios (1/3, 1/2, and 2/3) are prepared via coprecipitation and subsequent solid-state reactions. It is found that the reduced Ni/Mn ratio of 1/3 suppresses the Na+/vacancy ordering of P2-type oxides. Density functional theory calculations reveal that the charge-sparse regions formed by clustered Ni atoms regulate Na+ adsorption sites, and the lattice site mismatch caused by the low Ni/Mn ratio hinders the formation of Na+/vacancy ordering. The Na+/vacancy disordering in the optimized Na2/3Ni1/4Mn3/4O2 significantly improves the Na+ diffusion coefficients as well as high-rate performance and maintains above 60% capacity retention even at 20C compared with that at 0.2C. Besides, the excellent cycling stability is also achieved, presenting a capacity retention of 97.3% after 200 cycles. These findings offer meaningful insight into enabling Na+/vacancy disordering and superior Na+ kinetics of P2-type cathodes for high-power SIBs.