Abstract:

Due to its easy synthesis process and high theoretical capacity, layered $\beta $-NaMnO$_{2}$ is considered as a potential cathode material in sodium ion battery. However, studies on its capacity degradation in the first several cycles are rare. In this article, $\beta $-NaMnO$_{2}$ was prepared by a solid state method, which had a high initial discharge capacity of 184 mA$\cdot$h/g at 10 mA/g with a capacity retention of 73% after 10 cycles. The mechanism of capacity degradation was investigated based on X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscope (XPS). Phase transformation was recorded by XRD in the first three cycles, showing appearance of Na$_{0.91}$MnO$_{2}$ and Na$_{0.7}$MnO$_{2}$ in the discharge process with low crystallinity instead recrystallization of NaMnO$_{2}$. The proportion of disorder structure increased upon Na$^+$-ion extraction, associated with a loss of crystallinity as evidenced by HRTEM observation. XPS further explained the valence variation of Mn in different phases in the cycling.

Key words: sodium ion battery, cathode material, $\beta$-NaMnO$_{2}$, degradation mechanism