Synergistic Sr ²⁺ Doping and MXene Engineering of Layered δ‐MnO ₂ Toward High‐Performance Aqueous Zinc‐Ion Batteries

ABSTRACT Layered δ‐MnO ₂ is a promising cathode for aqueous zinc‐ion batteries (AZIBs) due to its large interlayer spacing and capability for Zn ²⁺ /H ⁺ co‐insertion. Here, a strontium‐doped δ‐MnO ₂ /Nb ₂ C MXene (Sr‐δ‐MnO ₂ /Nb ₂ C) composite cathode is rationally engineered through dual regulation of Sr ²⁺ doping and MXene content, enabling synergistic enhancement of electrochemical performance. Optimal Sr ²⁺ incorporation stabilizes the layered framework and modulates the local electronic environment, while Nb ₂ C MXene constructs a continuous conductive network that facilitates charge transfer and ion transport. Structural and spectroscopic analyses reveal expanded interlayer spacing and enhanced interfacial coupling. As a result, the optimized cathode (S ₇ Mn/MX‐3) delivers a high reversible capacity of 407.3 mAh g ⁻¹ at 0.1 A g ⁻¹ , outstanding rate capability (211.3 mAh g ⁻¹ at 1 A g ⁻¹ ), and long‐term cycling stability with 113 mAh g ⁻¹ retained after 2500 cycles at 1.5 A g ⁻¹ . Combined in situ and ex situ characterizations suggest a highly reversible Zn ²⁺ /H ⁺ co‐insertion mechanism, demonstrating that the double‐optimization strategy effectively suppresses Mn dissolution and improves structural stability. Furthermore, flexible AZIB based on this cathode exhibits stable electrochemical performance under mechanical deformation and successfully powers light‐emitting diodes, highlighting its potential for flexible and durable aqueous energy‐storage applications.