Elucidation of the sodium kinetics in layered P-type oxide cathodes

Sodium-ion intercalation oxides generally possess high compositional diversity according to their different stacking sequences. The sodium diffusion pathway in layered P-type materials used in sodium-ion batteries is open, which can increase their rate capability by directly transmitting Na⁺ between adjacent triangular prismatic channels, rather than passing through an intermediate tetrahedral site in O-type structure. However, how the structure chemistry of the P-type oxides determines their electrochemical properties has not been fully understood yet. Herein, by comparing the crystalline structures, electrochemical behaviors, ion/electron transport dynamics of a couple of P-type intercalation cathodes, P2-Na_2/3Ni_1/3Mn_2/3O₂ and P3-Na_2/3Ni_1/3Mn_2/3O₂ with the same compositions, we demonstrate experimentally and computationally that the P2 phase delivers better cycling stability and rate capability than the P3 counterpart due to the predominant contribution of the faster intrinsic Na diffusion kinetics in the P2 bulk. We also point out that it is the electronic conductivity that captures the key electrochemistry of layered P3-type materials and makes them possible to enhance the sodium storage performance. The results reveal that the correlation between stacking structure and functional properties in two typical layered P-type cathodes, providing new guidelines for preparing and designing alkali-metal layered oxide materials with improved battery performance. [Figure not available: see fulltext.]