Zwitterionic Polymer Gradient Interphases for Reversible Zinc Electrochemistry in Aqueous Alkaline Electrolytes

Aqueous alkaline zinc batteries are of scientific and technological interest because of the potential they offer for cost-effective and safe storage of electrical energy. Poor electrochemical reversibility and shape change of the Zn anode, propensity of Zn to become passivated by surface oxides and hydroxide films upon prolonged exposure to the electrolyte, and electroreduction of water are well-studied but remain unsolved challenges. Here, we create and study electrochemical and transport properties of precise, spatially tunable zwitterionic polymer interphases grown directly on Zn using an initiated-chemical vapor deposition polymerization methodology. In aqueous alkaline media, spatial gradients in composition─from the polymer-electrolyte interface to the solid-polymer interface─promote highly reversible redox reactions at high current density (20 mA cm-2) and high areal capacity (10 mAh cm-2). Via molecular dynamics and experimental analyses, we conclude that the interphases function by regulating the distribution and activity of interfacial water molecules, which simultaneously enables fast ion transport and suppression of surface passivation and the hydrogen evolution reaction. To illustrate the practical relevance of our findings, we study aqueous Zn||NiOOH and Zn||air batteries and observe that zwitterionic polymer interphases produce extended life at high currents and high areal capacity.