Enamel-like Layer of Nanohydroxyapatite Stabilizes Zn Metal Anodes by Ion Exchange Adsorption and Electrolyte pH Regulation

The instability of Zn anode caused by severe dendrite growth and side reactions has restricted the practical applications of aqueous zinc-ion batteries (AZIBs). Herein, an enamel-like layer of nanohydroxyapatite (Ca₅(PO₄)₃(OH), nano-HAP) is constructed on Zn anode to enhance its stability. Benefiting from the ion exchange between Zn²⁺ and Ca²⁺, the adsorption for Zn²⁺ in enamel-like nano-HAP (E-nHAP) layer can effectively guide Zn deposition, ensuring homogeneous Zn²⁺ flux and even nucleation sites to suppress Zn dendrites. Meanwhile, the low pH of acidic electrolyte can be regulated by slightly soluble nano-HAP, restraining electrolyte corrosion and hydrogen evolution. Moreover, the E-nHAP layer features high mechanical flexibility due to its enamel-like organic–inorganic composite nanostructure. Hence, symmetric cells assembled by E-nHAP@Zn show superior stability of long-term cycling at different current densities (0.1, 0.5, 1, 5, and 10 mA cm^–2). The E-nHAP@Zn∥E-nHAP@Cu cell exhibits an outstanding cycling life with high Coulombic efficiency of 99.8% over 1000 cycles. Notably, the reversibility of full cell based on CNT/MnO₂ cathode can be effectively enhanced. This work shows the potential of drawing inspiration from biological nanostructure in nature to develop stable metal electrodes.