Understanding Bonding Nature of α-Keggin Polyoxometalates [XW12O40]n− (X = Al, Si, P, S): A Generalized Superatomic Perspective

Abstract

α-Keggin polyoxometalates (POMs) [XW₁₂O₄₀]ⁿ⁻ (X = Al, Si, P, S) are widely used in batteries owing to their remarkable redox activity. However, the mechanism underlying the applications appears inconsistent with the widely accepted covalent bonding nature. Here, first-principles calculations show that XW₁₂ are core–shell structures composed of a shell and an XO₄ⁿ⁻ core, both are stabilized by covalent interactions. Interestingly, owing to the presence of a substantial number of electrons in W₁₂O₃₆ shell, the frontier molecular orbitals of XW₁₂ are not only strongly delocalized but also exhibit superatomic properties with high-angular momentum electrons that do not conform to the Jellium model. Detailed analysis indicates that energetically high lying filled molecular orbitals (MOs) have reached unusually high-angular momentum characterized by quantum number K or higher, allowing for the accommodation of numerous electrons. This attribute confers strong electron acceptor ability and redox activity to XW₁₂. Moreover, electrons added to XW₁₂ still occupy the K orbitals and will not cause rearrangement of the MOs, thereby maintaining the stability of these structures. Our findings highlight the structure–activity relationship and provide a direction for tailor-made POMs with specific properties at atomic level.