Tunable Zn2+ de-solvation behavior in MnO2 cathodes via self-assembled phytic acid monolayers for stable aqueous Zn-ion batteries

Abstract

Sluggish ion diffusion kinetics at the electrode/electrolyte interface leads to insufficient rate capability and poor structural reversibility, which are mainly attributed to the hydrated Zn²⁺ migration process being inhibited due to its huge de-solvation energy barriers. Herein, a self-assembly method is proposed in which a multifunctional monolayer with phytic acid (PA) is coated on the surface of MnO₂ strongly attaching to the substrate with the formation of chemical bonding to effectively prevent the dissolution of PA in the electrolyte. Due to the negative charge and inherent ultra-hydrophilicity of PA, modified MnO₂ demonstrates stronger adsorption of positive ions and captures reactive water molecules, easily accelerating the de-solvation process of interfacial hydrated Zn²⁺, efficiently achieving reversible Zn²⁺ insertion/extraction. Meanwhile, the coating layers can protect the substrate from attack by active water molecules, thus inhibiting cathode dissolution during battery cycling. Experimental characterization studies reveal that the protected MnO₂ cathode exhibits a remarkable specific capacity of 273 mA h g⁻¹ with a zinc intercalation capacity contribution of more than 60% at a current density of 0.1 A g⁻¹. Additionally, even at a high current density of 1 A g⁻¹, it maintains a capacity of 197 mA h g⁻¹, far exceeding that of pure MnO₂. Furthermore, the Zn-ion pouch cell, serving as a proof of concept, achieves an impressive energy density of 300 W h kg⁻¹ and exhibits remarkable capacity retention of 77% even after 100 cycles. This work offers a universal strategy for expediting the de-solvation process of hydrated Zn²⁺ on the surface of manganese-based cathodes in aqueous zinc-ion batteries.