Mitigating the P2–O2 phase transition of Ni–Co–Mn based layered oxide for improved sodium-ion batteries via interlayered structural modulation

Mn-based layered oxides are promising cathode materials for sodium-ion batteries owing to their high reversible capacities. However, P2–O2 phase transition and Jahn–Teller distortion of Mn3+ frequently occur at the end of the discharging process, resulting in low cycling stability and severe capacity degradation during the cycling processes. In this study, we proposed an effective strategy to inhibit the harmful phase transition and suppress the Jahn–Teller distortion using Al substitution. The proposed method was validated via in situ and ex situ X-ray diffraction. Furthermore, the Al-substituted P2-Na0.7Ni0.2Co0.2Mn0.58Al0.02O2(NCMA2) electrode exhibited only a topotactic phase transition from P2 to OP4 and excellent cycling stability (80% retention of the initial capacity at 170 mAh/g after 50 cycles at 0.1 C within 1.50–4.50 V). X-ray photoelectron spectroscopy analysis showed that the Mn4+ content of the Al-substituted electrode was higher than the bare NCM (Na0.7Ni0.2Co0.2Mn0.6O2) electrode in both the charge and discharge states. This confirms that Al substitution can improve the electrochemical performance by mitigating the irreversible P2–O2 phase transition and suppressing Jahn–Teller distortion. This study provides a new perspective for effectively mitigating the P2–O2 phase transition in Mn-based layered oxide cathodes via interlayered structural modulation.