Modulating MnO2 Interface with Flexible and Self-Adhering Alkylphosphonic Layers for High-Performance Zn-MnO2 Batteries.

Metal oxides are essential electrode materials for high-energy-density batteries, but it remains highly challenging to modulate their interfacial charge-transfer process and improve their cycling stability. Here, using MnO2 nanofibers as an example, we describe the application of self-assembled alkylphosphonic modification layers for significantly improved cycling stability and high-rate performance of Zn-MnO2 batteries. Two modifier organic molecules with the same phosphonic functional group but different alkyl tail lengths were employed and systematically compared, including butylphosphonic acid (BPA) and decylphosphonic acid (DPA). The phosphonic groups form strong interfacial covalent bonding and assist the generation of conformal and flexible coatings with few nanometers thickness on a MnO2 surface. The intertwined alkylphosphonic molecules in the modulation layers have interconnected phosphonic groups, which improve interfacial charge transfer of H+ ions for fast conversion of MnO2 to MnOOH without compromising electrolyte wetting. Importantly, the coating layers effectively reduce dissolutive loss of Mn2+ from MnO2 during battery cycling since diffusion of both water molecules and divalent Mn2+ cations was inhibited across the modification layers. The flexible coatings could readily adapt to the morphological changes of MnO2 during battery cycling and provide long-lasting protection. Overall, we identified that BPA has the optimal balance of hydrophobic-hydrophilic components and enabled modified MnO2 cathodes with >30% improved discharge capacity compared with unmodified MnO2 cathodes, together with substantially improved long-term cycling stability with >60% capacity retention for 400 cycles in aqueous ZnSO4 electrolytes without any Mn2+ additive. This work provides new insights into tuning electrochemical pathways that move away from the prevailing rigid, ceramic coating-based surface modifications.