Boosted sodium ion storage performance in MnO2: Understanding the bond angle-mediated orbital overlap in MnO6 units for fast charge transfer

Symmetric six oxygen-coordinated Mn structural units (MnO6) in MnO2 with small Mn–O orbital overlap hamper electron transfer rates during energy storage. Herein, we report a novel bond angle modulation strategy to manipulate Mn–O orbital overlap in MnO2 through the construction of Mn vacancies (MnO2-VMn), aiming at expediting electron transfer, and thus enhancing energy storage performance. Both experimental and theoretical results disclose that the amplification of Mn–O–Mn bond angles exclusively augments the Mn (dx2-y2)-O (py) orbital overlap and triggers the electron redistribution in MnO2-VMn, inducing an augmented Mn dx2-y2 electron occupation. This heightened presence of active electrons in the Mn dx2-y2 orbital paves the way for accelerating electron transfer and ion transfer in MnO2-VMn. Notably, MnO2-VMn delivers an improved specific capacitance of 425 F g−1 at 1 A g−1 and a superior rate capacity of 265 F g−1 at 20 A g−1. Furthermore, an asymmetric supercapacitor (MnO2-VMn//AC ASC) was fabricated, exhibiting a high energy density of 64.3 Wh kg−1 at a power density of 1000 W kg−1. Furthermore, theoretical insights uncover the profound implications of metal–oxygen–metal bond angle regulation on interatomic orbital overlap modulation. These revelations illuminate pathways for the design of advanced energy storage materials.