Tannic Acid/Polyurethane-Laminated Coatings for the Stability of Metal Anodes in Aqueous Zinc-Ion Batteries

Although Zn metal offers low cost, high safety, a high theoretical specific capacity of 820 mAh g⁻¹, and a smaller redox potential (−0.76 V versus SHE), it encounters issues like dendrite rampant generation, corrosion, and passivation reaction when utilized in the anode of aqueous Zn-ion batteries. Together, these issues reduce the cycle life of the battery and the coulombic efficiency. In this chapter, a bifunctional coating is constructed on the surface of the Zn anode by repeated spin-coating twice. The coating is composed of two layers, one layer on the surface of the Zn electrode is a tannic acid (TA) coating, which uses the chelation effect of TA and zinc metal to anchor Zn²⁺ on the cross-linking network of TA molecules, which limits the two-dimensional deposition of Zn²⁺, thereby producing a uniform nucleation site, avoiding the “tip effect”. The polyurethane (PU) coating that touches the electrolyte layer acts as a solid barrier to keep water molecules from reaching the surface of the Zn electrode because of its strong water-repelling properties, preventing both the hydrogen evolution reaction (HER) and severe corrosion reaction from happening. The Zn@PT//Zn@PT symmetrical battery exhibits exceptional cycling stability, enduring over 1200 h at 1 and 0.5 mAh cm⁻², with minimal overpotential. Despite the rise in a current density to 5 mA cm⁻², the cycling performance of the full cell remains stable for over 500 h, surpassing that of Zn//Zn symmetrical cells. The Zn@PT//MnO₂ cells possess a high specific capacity of 271.6 mAh g⁻¹ at a current density of 0.2 A g⁻¹.