Structural disorder Triggered by small organic molecules Boosting δ-MnO2 cathode for Ultra-Stable aqueous zinc ion batteries

The inferior cycling stability of MnO2-based cathodes, resulting from Mn dissolution and crystal structure transition during Zn-ion insertion/extraction, poses a significant challenge hindering their widespread adoption in aqueous zinc-ion batteries (AZIBs). To mitigate these issues, we propose an approach involving the incorporation of small organic molecules (methylformamide, dimethylformamide, and formamide) to modify the crystalline structure of MnO2. These molecules, characterized by their low orbital energy levels, facilitate the creation of energetically equivalent electron transfer pathways with the unoccupied eg orbitals in Mn4+, as evidenced by the DFT calculations. Furthermore, they modulate the electron cloud density of Mn-O and introduce structural disorder in the MnO2 crystalline lattice, thereby greatly enhancing the structural stability of the Mn-based cathode. In AZIBs tests, the MnO2/methylformamide (MO/NMF) cathode demonstrates a remarkable reversible specific capacity exceeding 270 mAh g-1 at 1 A g-1 and exceptional durability, exhibiting negligible capacity degradation after 2000 cycles at 3 A g-1. In-situ X-ray diffraction, neutron powder diffraction and highangle annular dark-field scanning transmission electron microscopy analyses confirm the structure stability of MO/NMF during Zn-ion insertion/extraction, attributed to the NMF modification. Additionally, the electrochemical data indicate that modifications with dimethyl formamide and formamide also positively impact the electrochemical performance of the MnO2 cathode. This study offers valuable insights for the development of high-performance Mn-based cathodes.