Suppressed P2–P2′ phase transition of Fe/Mn-based layered oxide cathode for high-performance sodium-ion batteries

P2-type Fe/Mn-based layered oxide cathode has attracted enormous interest as a prospective candidate for sodium-ion batteries (SIBs) used in large-scale energy storage owing to its low cost, low toxicity, earth abundance and high theoretical specific capacity. However, the P2–P2′ phase transition associated with the Jahn–Teller effect of Mn 3+ at the end of the discharge process often appears, leading to severe capacity fading during charge-discharge process. Herein, we propose an effective strategy of using Al-doping to completely inhibit the harmful P2–P2′ phase transition by suppressing the Jahn–Teller effect of Mn 3+ , which has been verified by ex-situ and in-situ X-ray powder diffraction (XRD), high-resolution transmission electron microscope (HR-TEM) and ex-situ X-ray photoelectron spectroscopy (XPS). Consequently, Al-doped P2-Na 0.67 Fe 0.5 Mn 0.45 Al 0.05 O 2 electrode exhibits excellent cycling stability (92.8% capacity retention after 200 cycles at 1 C within 2.0–4.0 V). Meanwhile, Al-doping could widen interspacing of Na + layer to accelerate the migration of Na + , further improve rate capability (from 32 to 48% under the ratio of 5 to 0.1 C). This work provides a new sight to effectively suppress P2–P2′ phase transition for Fe/Mn-based layered oxide cathode.