P2-Na_0.67 Al_x Mn_1-x O₂ : Cost-Effective, Stable and High-Rate Sodium Electrodes by Suppressing Phase Transitions and Enhancing Sodium Cation Mobility

Sodium layered P2-stacking Na_0.67 MnO₂ 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 Na_0.67 Al_x Mn_1-x O₂ (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 .