P2‐Na0.67AlxMn1−xO2: Cost‐Effective, Stable and High‐Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2‐stacking Na0.67MnO2 materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn4+/Mn3+ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition‐metal layers to decrease the number of Mn3+, we obtain the low cost pure P2‐type Na0.67AlxMn1−xO2 (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al‐doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid‐state NMR techniques. Our results reveal that Al‐doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na+ layer thus producing a remarkable rate capability of 96 mAh g‐1 at 1200 mA g‐1. Al the better: Sodium layered P2‐stacking Na0.67MnO2 materials have shown great promise for sodium‐ion batteries. Undesired Jahn–Teller effect of the Mn4+/Mn3+ redox couple lead to cracks, amorphization and rapid capacity decay. These problems are overcome by introducing Al to decrease the number of Mn3+ Jahn–Teller centers, giving pure P2‐type Na0.67AlxMn1−xO2 materials.