Se-dopant Modulated Selective Co-Insertion of H⁺ and Zn²⁺ in MnO₂ for High-Capacity and Durable Aqueous Zn-Ion Batteries

MnO₂ is commonly used as the cathode material for aqueous zinc-ion batteries (AZIBs). The strong Coulombic interaction between Zn ions and the MnO₂ lattice causes significant lattice distortion and, combined with the Jahn-Teller effect, results in Mn²⁺ dissolution and structural collapse. While proton intercalation can reduce lattice distortion, it changes the electrolyte pH, producing chemically inert byproducts. These issues greatly affect the reversibility of Zn²⁺ intercalation/extraction, leading to significant capacity degradation of MnO₂. Herein, we propose a novel method to enhance the cycling stability of δ-MnO₂ through selenium doping (Se-MnO₂). Our work indicates that varying the selenium doping content can regulate the intercalation ratio of H⁺ in MnO₂, thereby suppressing the formation of ZnMn₂O₄ by-products. Se doping mitigates the lattice strain of MnO₂ during Zn²⁺ intercalation/deintercalation by reducing Mn-O octahedral distortion, modifying Mn-O bond length upon Zn²⁺ insertion, and alleviating Mn dissolution caused by the Jahn-Teller effect. The optimized Se-MnO₂ (Se concentration of 0.8 at.%) deposited on carbon nanotube demonstrates a notable capacity of 386 mAh g^-1 at 0.1 A g^-1, with exceptional long-term cycle stability, retaining 102 mAh g^-1 capacity after 5000 cycles at 3.0 A g^-1.