A hydrophilic Janus-faced separator with functionalized nanocarbon for stable cycling of aqueous Zn-metal batteries

Aqueous zinc (Zn) metal batteries (AZBs) are one of the promising candidates for post-Li-ion batteries for cost-effective and safe large-scale energy storage systems. However, the irreversibility of the Zn metal anode (ZMA) due to Zn dendrite growth and "dead Zn" formation, which is an electrically isolated Zn particle, is a crucial issue for the practical use of AZBs. In response to these drawbacks, a Janus-faced separator composed of an acid-treated carbon supportive layer (aCSL) is proposed, which can perform various functions, rendering additional reaction sites for Zn deposition/dissolution and uniform Zn2+ ion flux owing to excellent wetting by endowed hydrophilicity. By manipulating current density and Zn plating capacity, hydrogen evolution reaction (HER)-dominant and HER-minimized protocols for Zn coulombic efficiency (CE) are proposed for assessing the Zn reversibility. Remarkably, the aCSL exhibits the highest initial CE of each newly suggested protocol, which indicates that the aCSL can suppress the water-induced side reactions and achieved an excellent CE of 99.6% (@ 1 mA cm-2) during 900 cycles and stable symmetric cell performance over 1200 hours (@ 1 mA cm-2). Due to these interesting features, the Zn||alpha-MnO2/CNT full cell shows stable cycle performance with capacity retention of 61.3% during 2000 cycles. A Janus separator incorporating an aCSL, an acid-enhanced carbon nanopowder layer, enhances Zn plating uniformity and boosts reversibility, curtailing "dead" Zn and minimizing H2O-related side reactions.