Rational design of Na0.67Ni0.2Co0.2Mn0.6O2 microsphere cathode material for stable and low temperature sodium ion storage

Layered transition metal oxides with a classic composition of Na0.67Ni0.33Mn0.67O2 have attracted extensive attention due to their high specific capacity and suitable working voltage. However, its inherent structural defects can lead to a prone to phase change upon charging/discharging and cause degradation of electrochemical performance. In this work, a suitable Co-substituted Na0.67Ni0.2Co0.2Mn0.6O2 microsphere is designed and synthesized. The introduction of cobalt can effectively increase the interlayer spacing of transition metal oxides (TMO2), resulting in a faster Na+ diffusion kinetics. Such a structure enables the charge compensation from different redox couples (Ni2+/3+/4+, Co3+/4+ and Mn3+/4+), leading to a high specific capacity. Furthermore, the drawbacks of P2-O2 phase transition and Na+/vacancy ordering can be simultaneously restrained by a suitable content of Co substitution, which is confirmed by both cyclic voltammetry test and in-situ X-ray diffraction (XRD). The optimized Na0.67Ni0.2Co0.2Mn0.6O2 demonstrates a superior long-term cycle stability with a capacity retention rate of 83% over 200 cycles at 2 C. The electrochemical performance under extreme low-temperature (−40 °C) of this Na0.67Ni0.2Co0.2Mn0.6O2 material has also been explored for the first time, and the DNa+ values of this material can be maintained decently within a range from 10−11 to 10−10 cm2s−1. A high reversible discharge capacity of 132.2 mAh g−1 at 0.2 C is achieved at −40 °C, which is 80% of that at room temperature, indicating a satisfactory sodium storage performance at extremely low temperature.