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

The vanadium boride (VB2) air battery is currently known as a primary battery with the highest theoretical specific capacity, 4060 mA h g-1, which originates from an extraordinary 11 electrons per VB2 molecule oxidation process. However, the parasitical reaction between VB2 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 VB2 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 VB2 in the open circuit to avoid corrosion but also ensured that the hydroxide ions can enter the VB2 particle surface to participate in the reaction during discharge. According to the corrosion suppression test, the remaining amount of VB2@PDA is 90 wt % stored at 65 °C for 2 weeks, which is 10 wt % more than the uncoated VB2. The assembled pouch cell with the VB2@PDA anode can deliver a high capacity of 325 mA h at 250 mA g-1, retaining an improved Coulombic efficiency of 86.3%, which is 18.7% higher than that of the cell with the raw VB2 anode. Moreover, the 0.05 V higher discharge voltage of the VB2@PDA-based cell further shows that the PDA membrane can effectively conduct hydroxide ions during discharge.