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.