Correlation of the crystal structure and ion storage behavior of MoO electrode materials for aluminum-ion energy storage studied using X-ray spectroscopy.

To characterize the correlation of the crystal structure and Al-ion storage behavior, we prepared various crystal structures of MoO (α-MoO, β-MoO and -MoO) electrode materials and studied them X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The α-MoO electrode material possesses a specific capacitance of 575.4 F g and a gravimetric capacity of 207.8 mA h g at a current density of 1 A g. From the XRD results, the crystal structures of α-MoO and β-MoO show a significant distortion, whereas that of -MoO is minorly affected during the insertion or extraction of Al ions. Based on the XAS results, the MoO octahedral structure and Mo ion valence of α-MoO and β-MoO also exhibit a strong variation, whereas those of -MoO are nearly unchanged during the insertion or extraction of Al ions. Notably, XRD and XAS also clearly show a possible phase of AlMoO during the Al insertion and extraction cycles in the α-MoO and β-MoO electrode materials, which may play a crucial role in the behavior of the residue of Al ions and poor cycling stability. We provide clear evidence that the Al-ion energy storage performance of various MoO electrode materials is strongly associated with the corresponding tunnel space and the stability of their crystal structures. This work also provides new insight into a strong correlation between ion-storage efficiency and the corresponding crystal structure, which is greatly helpful for the development and improvement of new electrode materials for Al-ion energy storage.