Breaking diffusion constraints through fluorine-regulated MnO_x/KMnF₃ heterostructures for enhanced aqueous Mg²⁺ storage

With the continuing demand for clean and sustainable energy storage devices, aqueous magnesium-ion capacitors have gained prominence as a viable electrochemical solution. However, high-performance aqueous magnesium-ion storage devices for energy need to satisfy rigorous requirements due to the large hydrated ionic radius of Mg²⁺ cations and the structural collapse of host materials during insertion/extraction. Herein, we propose a fluorine-mediated structural regulation strategy to design fluorine-mediated multivalent manganese oxide (F-m-MnO_x) as cathode materials. By partially substituting oxygen sites with fluorine atoms, high-strength Mn–F bonds are formed within the MnO₂ lattice, which locally enhance the framework stability by reinforcing the tunnel structure and effectively suppressing structural degradation during cycling. Furthermore, the robust Mn–F bond energy enables a unique “pinning effect” anchoring hydrothermally synthesized KMnF₃ nanoparticles onto the MnO₂ matrix. These KMnF₃ nanoparticles act as dynamic bridges during Mg²⁺ insertion/extraction processes, with their surface-exposed chemically active sites facilitating transient yet reversible interactions with migrating Mg²⁺ ions. This innovative design significantly enhances Mg²⁺ diffusion kinetics through the bulk phase, offering a groundbreaking mechanism to overcome the inherent sluggish ion transport in multivalent cation systems. The F-m-MnO_x cathode delivers exceptional performance metrics: a high specific capacity of 142 mAh/g at 0.1 A/g, outstanding cycling stability (89.6% retention after 1800 cycles), and rapid kinetics. This research not only establishes an innovative design concept for advanced electrode materials through halogen-mediated structural engineering but also elucidates the dual magnesium-ion storage mechanism involving both KMnF₃ and MnO₂ in F-m-MnO_x through ex-situ characterization, enabling new possibilities for future clean energy storage.