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.

The advancement of aqueous magnesium ion energy storage devices encounters limitations due to the substantial hydration radius of magnesium ions (Mg²⁺) and their strong electrostatic interaction with the primary material. Consequently, this study successfully developed a MnS/MnO heterostructure through a straightforward hydrothermal and annealing method, marking its initial application in aqueous magnesium ion capacitors (AMICs). The fabricated MnS/MnO heterostructure, characterized by S defects, also generates Mn defects via in-situ initiation of early electrochemical processes. This unique dual ion defects MnS/MnO heterostructure (DID-MnS/MnO) enables the transformation of MnS and MnO, initially not highly active electrochemically for Mg²⁺, into cathode materials exhibiting high electrochemical activity and superior performance. Moreover, DID-MnS/MnO enhances conductivity, improves the kinetics of surface redox reactions, and increases the diffusion rate of Mg²⁺. Furthermore, this study introduces a dual energy storage mechanism for DID-MnS/MnO, which, in conjunction with dual ion defects, offers additional active sites for Mg²⁺ insertion/deinsertion in the host material, mitigating volume expansion and structural degradation during repeated charge-discharge cycles, thereby significantly enhancing cycling reversibility. As anticipated, using a three-electrode system, the developed DID-MnS/MnO demonstrated a discharge specific capacity of 237.9 mAh/g at a current density of 0.1 A/g. Remarkably, the constructed AMIC maintained a capacity retention rate of 94.3% after 10000 cycles at a current density of 1.0 A/g, with a specific capacitance of 165.7 F/g. Hence, DID-MnS/MnO offers insightful perspectives for designing alternative clean energy sources and is expected to contribute significantly to the advancement of the clean energy sector.