Spatially expanded built-in electric field via engineering graded junction enables fast charge transfer in bulk MnO@Mn3O4 for Na+ supercapacitors

Built-in electric field (BIEF) has recently emerged as a promising strategy for promoting charge transfer by supplying additional coulomb forces. However, the challenge lies in the intelligent control over the thickness and intensity of BIEF to augment these Coulomb forces, thereby enhancing charge transfer in bulk electrodes. Here we deliberately engineer a graded MnO@Mn3O4 junction characterized by a spatially intensified BIEF to accelerate charge transfer over an extended atom layer thickness. This graded MnO@Mn3O4 junction exhibits a phase transition gradient, characterized by a steady decline in Mn valence and work function from shell to core, giving rise to an intense BIEF across the junction. The enhanced BIEF should be ascribed to the spatially accumulated polarization, resulting in an increase in electron delocalization and a decrease in the energy barrier for ion transfer. As a result, the MnO@Mn3O4 manifests improved capacity and rate performance. This work highlights the significance of modulating the BIEF's thickness and intensity to facilitate charge transfer, thereby pushing the boundaries of electrochemical energy storage.