Orchestrating a Dual-Cation Kinetic Relay in α-MnO2 Cathodes via Mg–K Site Competition for High-Rate Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) hold great promise for grid-scale energy storage, yet their performance is limited by sluggish Zn2+ diffusion and irreversible Mn dissolution in MnO2 cathodes. Here, we report a Mg-K dual-doping strategy that establishes a site-competition mechanism in α-MnO2 nanoneedles to enable a synergistic dual-cation kinetic relay. K+ preferentially occupies the 2 × 2 tunnels, enlarging the tunnel spacing and lowering the interfacial desolvation barrier, while Mg2+ stabilizes the MnO6 framework and facilitates bulk Zn2+ migration. This cooperative behavior tunes the pseudocapacitive contribution from ∼29% to ∼70%, indicating that charge-storage kinetics are governed by cation-site occupancy rather than fixed intrinsic properties. The optimized K0.147Mn0.965Mg0.035O2 cathode delivers 322 mAh g-1 at 0.5 A g-1,retains 63.7% capacity after 500 cycles, and exhibits excellent rate recovery (195.07 → 310 mAh g-1 upon returning to 0.1 A g-1). Mechanistic studies reveal a stepwise H+/Zn2+ co-intercalation process accompanied by the formation of ZnMn2O4 and Zn4SO4(OH)6·xH2O, while a self-limiting interfacial passivation layer gradually forms during cycling, effectively suppressing parasitic side reactions and maintaining structural robustness. This work provides insight into multi-ion site competition, offering a generalizable strategy for durable, high-rate manganese-based AZIB cathodes.