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 MoO₃ (α-MoO₃, β-MoO₃ and h-MoO₃) electrode materials and studied them via in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The α-MoO₃ 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-MoO₃ and β-MoO₃ show a significant distortion, whereas that of h-MoO₃ is minorly affected during the insertion or extraction of Al³⁺ ions. Based on the in situ XAS results, the MoO₆ octahedral structure and Mo ion valence of α-MoO₃ and β-MoO₃ also exhibit a strong variation, whereas those of h-MoO₃ are nearly unchanged during the insertion or extraction of Al³⁺ ions. Notably, in situ XRD and XAS also clearly show a possible phase of Al_xMoO₃ during the Al³⁺ insertion and extraction cycles in the α-MoO₃ and β-MoO₃ electrode materials, which may play a crucial role in the behavior of the residue of Al³⁺ ions and poor cycling stability. We provide clear evidence that the Al-ion energy storage performance of various MoO₃ 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.