Toward Practical High-Areal-Capacity Aqueous Zinc-Metal Batteries: Quantifying Hydrogen Evolution and a Solid-Ion Conductor for Stable Zinc Anodes

The hydrogen evolution in Zn metal battery is accurately quantified by in situ battery–gas chromatography–mass analysis. The hydrogen fluxes reach 3.76 mmol h⁻¹ cm⁻² in a Zn//Zn symmetric cell in each segment, and 7.70 mmol h⁻¹ cm⁻² in a Zn//MnO₂ full cell. Then, a highly electronically insulating (0.11 mS cm⁻¹) but highly Zn²⁺ ion conductive (80.2 mS cm⁻¹) ZnF₂ solid ion conductor with high Zn²⁺ transfer number (0.65) is constructed to isolate Zn metal from liquid electrolyte, which not only prohibits over 99.2% parasitic hydrogen evolution but also guides uniform Zn electrodeposition. Precisely quantitated, the Zn@ZnF₂//Zn@ZnF₂ cell only produces 0.02 mmol h⁻¹ cm⁻² of hydrogen (0.53% of the Zn//Zn cell). Encouragingly, a high-areal-capacity Zn@ZnF₂//MnO₂ (≈3.2 mAh cm⁻²) full cell only produces maximum hydrogen flux of 0.06 mmol h⁻¹ cm⁻² (0.78% of the Zn//Zn cell) at the fully charging state. Meanwhile, Zn@ZnF₂//Zn@ZnF₂ symmetric cell exhibits excellent stability under ultrahigh current density and areal capacity (10 mA cm⁻², 10 mAh cm⁻²) over 590 h (285 cycles), which far outperforms all reported Zn metal anodes in aqueous systems. In light of the superior Zn@ZnF₂ anode, the high-areal-capacity aqueous Zn@ZnF₂//MnO₂ batteries (≈3.2 mAh cm⁻²) shows remarkable cycling stability over 1000 cycles with 93.63% capacity retained at ≈100% Coulombic efficiency.