P2/O3 Biphasic Layered Oxide Heterojunction: A Cathode for High-Capacity Sodium-Ion Batteries

Layer oxides are representative materials for sodium-ion battery (SIB) cathodes. However, the commonly used single-phase layer oxide cathodes have poor solid solubility of Na+ and cycling stability, limiting their practical application. In this work, a unique P2/O3 biphasic cathode material, NaFe1–xCoxO2 (x = 1/3, 1/2, and 2/3), is obtained by Fe/Co bimetallic metal–organic framework (MOF). The introduction of Co enriches the redox reaction and converts the O3 phase to the P2 phase. At the same time, the construction of the biphasic heterojunction is beneficial to increase Na+ diffusion, inhibit the irreversible phase transition, buffer the lattice stress, and enhance structural stability. Hence, NaFe1/3Co2/3O2 exhibits a high specific capacity of 199.6 mAh g–1 at 0.2 C due to the synergistic effect of P2 and O3 phases. Furthermore, the capacity remains 98.7 mAh g–1 after 150 cycles at 1 C, with no significant degradation. This strategy for constructing a composite phase provides insight into the design of cathode materials with long lifespan and high capacity for SIBs.