Constructing P2/O3 biphasic structure of Fe/Mn-based layered oxide cathode for high-performance sodium-ion batteries

Fe/Mn-based layered oxide cathode is regarded as a competitive candidate for sodium-ion batteries (SIBs) because of its high theoretical capacity, earth abundance and low cost. However, its poor cycling stability still remains a major bottleneck. Herein, P2/O3 biphasic Na_0.67Fe_0.425Mn_0.425Cu_0.15O₂ layered oxide is successfully synthesized via a sol-gel method. It is observed that Cu substitution can facilitate the conversion of P2 to O3 phase, and the P2/O3 composite structure can be obtained with an appropriate amount of Cu. Meanwhile, in-situ XRD reveals that constructing P2/O3 composite structure can realize the highly reversible phase transition process of P2/O3-P2/P3-OP4/OP2 and decrease the lattice mismatch during Na⁺ insertion/extraction. Consequently, the biphasic P2/O3-Na_0.67Fe_0.425Mn_0.425Cu_0.15O₂ electrode exhibits 87.1 % capacity retention after 100 cycles at 1C, while the single phase P2-Na_0.67Fe_0.5Mn_0.5O₂ electrode has only 36.4 %. Therefore, the constructing biphasic structure is proved to be an effective strategy for designing high-performance Fe/Mn-based layered oxide cathodes.