Lithium supercapacitors with environmentally-friend water-processable solid-state hybrid electrolytes of zinc oxide/polymer/lithium hydroxide

Abstract

Environment-friendly and safe solid-state electrolytes are of utmost importance for energy storage devices, but no study has been reported on water-processable hybrid electrolytes that take advantage of both organic and inorganic components. Here, for the first time, we demonstrate that novel water-processable hybrid films, consisting of zinc oxide (ZnO) nanoparticle, lithium hydroxide (LiOH), and branched-poly(ethylene imine) (bPEI), act as efficient solid-state electrolytes for lithium supercapacitors. The ZnO:bPEI:LiOH (ZPL) hybrid solutions were prepared using water by varying ZnO molar ratios up to 60 mol% to the repeating unit of bPEI. The enhanced ionic conductivity (∼1.36 mS/cm), which is higher than that of bPEI:LiOH electrolytes, was achieved at ZnO = 30 mol%. The best ZPL supercapacitors, exhibiting high performances of peak potential (2.0 V) and energy density (3.38 mWh/kg) upon charging at 0.4 mA/g only, could be stably operated over 1000 charging/discharging cycles. Series connection of multiple ZPL supercapacitors delivered increased output potentials (∼4.5 V from three devices in series) with reproducible charging/discharging performances. The experimental result supports that the present ZPL hybrid electrolyte technology paves the way for eco-friendly processes of next-generation solid-state electrolytes that can be a key in safe energy storage devices for applications to electrical vehicles etc.