Toward Practical Zn-Ion Batteries Through Separator/Electrolyte Co-Design

While substantial research effort has been paid to address short-circuits and parasitic hydrogen evolution reactions (HER) in Zn-ion batteries (ZIBs), a remaining challenge that is often overlooked is ZIB separators. Currently, thick glass fiber (GF) separators are used, which unnecessarily reduce the volumetric cell performance and lead to non-uniform Zn plating/stripping. Unfortunately, separators optimized for Li-ion batteries are intrinsically hydrophobic and therefore incompatible with aqueous ZIB systems. Here, we propose using a hybrid organic-aqueous electrolyte which is compatible with thin polypropylene (PP) separators developed for Li-ion batteries. The hybrid electrolyte, together with the PP separator, enables uniform Zn plating and suppresses HER. Combined, these effects allow for stable cycling for 4360 h at 1 mA cm^-2 and 3 mAh cm^-2 in a Zn symmetrical cell. Further, replacing the currently used 350 µm-thick GF separator with our proposed 23 µm PP separator can increase the energy densities of multi-layer pouch cells by over 2.5-fold. Finally, Zn//NVO full cells using our hybrid electrolyte and separator demonstrate stable cycling performance for over 200 cycles at a high areal capacity of 4.4 mAh cm^-2, demonstrating that electrolyte/separator co-design is an overlooked strategy toward practical ZIBs.