Suppressing Self-Discharge of Vanadium Diboride by Zwitterionicity of the Polydopamine Coating Layer.

The vanadium boride (VB) air battery is currently known as a primary battery with the highest theoretical specific capacity, 4060 mA h g, which originates from an extraordinary 11 electrons per VB molecule oxidation process. However, the parasitical reaction between VB and hydroxide ions in the alkaline electrolyte leads to obvious self-discharge, which results in severe capacity loss during discharge. In this work, we applied the polydopamine (PDA) membrane to modify the surface of VB particles, which contains amine groups and phenolic hydroxyl groups exhibiting fully reversible, pH-switchable permselectivity. The "smart" membrane with pH-switching characteristics successfully coordinated the conflict between the electrolyte and VB in the open circuit to avoid corrosion but also ensured that the hydroxide ions can enter the VB particle surface to participate in the reaction during discharge. According to the corrosion suppression test, the remaining amount of VB@PDA is 90 wt % stored at 65 °C for 2 weeks, which is 10 wt % more than the uncoated VB. The assembled pouch cell with the VB@PDA anode can deliver a high capacity of 325 mA h at 250 mA g, retaining an improved Coulombic efficiency of 86.3%, which is 18.7% higher than that of the cell with the raw VB anode. Moreover, the 0.05 V higher discharge voltage of the VB@PDA-based cell further shows that the PDA membrane can effectively conduct hydroxide ions during discharge.