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

Abstract Sodium layered P2‐stacking Na 0.67 MnO 2 materials have shown great promise for sodium‐ion batteries. However, the undesired Jahn–Teller effect of the Mn 4+ /Mn 3+ 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 Mn 3+ , we obtain the low cost pure P2‐type Na 0.67 Al x Mn 1− x O 2 ( 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 .