Component Regulatory Strategy for Advanced Biphasic Sodium Layered Oxide Cathodes
Xufeng Hu, Hao Guo, Jianxiang Gao, Zhiwei Liu, Xiaobai Ma, Zhengyao Li, Yanyan Ge, Zhang Wen, Xuesheng Jiao, Kai Sun, Dongfeng Chen
Abstract
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 Na 0.7 Ni 0.25 Cu 0.05 Fe 0.1 Mn 0.35 Ti 0.25 O 2 and O3-type Na 0.94 Ni 0.29 Cu 0.1 Fe 0.16 Mn 0.3 Ti 0.15 O 2 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.