Synergistic Sr²⁺ Doping and MXene Engineering of Layered δ-MnO₂ Toward High-Performance Aqueous Zinc-Ion Batteries

Layered δ-MnO₂ is a promising cathode for aqueous zinc-ion batteries (AZIBs) due to its large interlayer spacing and capability for Zn²⁺/H⁺ co-insertion. Here, a strontium-doped δ-MnO₂/Nb₂C MXene (Sr-δ-MnO₂/Nb₂C) composite cathode is rationally engineered through dual regulation of Sr²⁺ doping and MXene content, enabling synergistic enhancement of electrochemical performance. Optimal Sr²⁺ incorporation stabilizes the layered framework and modulates the local electronic environment, while Nb₂C 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 (S₇Mn/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 Zn²⁺/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.