Correlation of the crystal structure and ion storage behavior of MoO 3 electrode materials for aluminum-ion energy storage studied using in situ X-ray spectroscopy.

To characterize the correlation of the crystal structure and Al-ion storage behavior, we prepared various crystal structures of MoO 3 (α-MoO 3 , β-MoO 3 and h -MoO 3 ) electrode materials and studied them via in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The α-MoO 3 electrode material possesses a specific capacitance of 575.4 F g -1 and a gravimetric capacity of 207.8 mA h g -1 at a current density of 1 A g -1 . From the in situ XRD results, the crystal structures of α-MoO 3 and β-MoO 3 show a significant distortion, whereas that of h -MoO 3 is minorly affected during the insertion or extraction of Al 3+ ions. Based on the in situ XAS results, the MoO 6 octahedral structure and Mo ion valence of α-MoO 3 and β-MoO 3 also exhibit a strong variation, whereas those of h -MoO 3 are nearly unchanged during the insertion or extraction of Al 3+ ions. Notably, in situ XRD and XAS also clearly show a possible phase of Al x MoO 3 during the Al 3+ insertion and extraction cycles in the α-MoO 3 and β-MoO 3 electrode materials, which may play a crucial role in the behavior of the residue of Al 3+ ions and poor cycling stability. We provide clear evidence that the Al-ion energy storage performance of various MoO 3 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.