Enhancing structural flexibility in P2-type Ni-Mn-based Na-layered cathodes for high power-capability and fast charging/discharging performance

P2-type Ni-Mn-based Na-layered cathodes suffer from severely large structural changes, such as the direct P2-O2 phase transition, occurring during charging to the high voltage region, resulting in the poor power-capability with large overpotential, as well as the diminished cycle-performance. In this study, through a combination of first-principles calculations and various experiments, we demonstrate that enhanced structural flexibility through Co-Al co-substitution provides smooth and continuous structural changes in the P2-type Ni-Mn-based Na-layered cathode without the direct phase transition, enabling the highly improved electrochemical performances. P2-type Na_0.67[Ni_0.35Co_0.1Mn_0.5Al_0.05]O₂ delivers a high discharge capacity of approximately ∼156.31 mAh g⁻¹ and an energy density of ∼551.71 Wh kg⁻¹ at 10 mA g⁻¹, outperforming P2-type Na_0.67[Ni_0.35Mn_0.65]O₂. These performance differences are especially pronounced during fast charging/discharging process, highlighting the enhanced power-capability and Na⁺ diffusion kinetics due to improved structural flexibility. Moreover, smooth and continuous structural changes enable improved cycle performance, including reduced voltage decay during prolonged cycling, for P2-type Na_0.67[Ni_0.35Co_0.1Mn_0.5Al_0.05]O₂. These results highlight that introducing structural flexibility is one of the most efficient ways to enhance power-capability and fast-charging/discharging performance in P2-type Ni-Mn-based Na-layered cathodes, while also improving cyclability.