Component Regulatory Strategy for Advanced Biphasic Sodium Layered Oxide Cathodes

P2/O3 biphasic sodium layered oxides that combine the advantages of two crystal structures are promising cathodes for Na-ion batteries. However, the compositional optimization and structural regulation of P2/O3 biphasic layered oxides were usually performed by using a "trial and error" mode that adjusts the components and synthetic temperature step by step, which limited the rapid development of P2/O3 biphasic layered oxides. Inspired by the rich compositional diversity and controllable electrochemical performance of sodium layered oxides, we proposed a rational strategy for designing the components of P2/O3 biphasic layered oxides with excellent properties. Specifically, we used the chemical formulas P2-type Na0.7Ni0.25Cu0.05Fe0.1Mn0.35Ti0.25O2 and O3-type Na0.94Ni0.29Cu0.1Fe0.16Mn0.3Ti0.15O2 as the two initial values. Then, by combining two initial values with different mol ratios, we can obtain the components of biphasic layered oxides. As a result, all prepared P2/O3 biphasic layered oxides with different phase ratio showed excellent electrochemical performances including high specific capacity (more than 120 mAh/g), superior cycling stability (more than the capacity retention of 91% after 500 cycles), and excellent rate capability (about the capacity retention of 50% at 8C rate). Ex situ X-ray diffraction measurements indicated P2/O3 biphasic layered oxides showed a reversible phase evolution of P2/O3–P2/P3–P2/O3 upon Na deintercalation/intercalation. This work provided a feasible approach to designing high-performance biphasic layered oxide cathodes for Na-ion batteries.