Lithium polymer supercapacitors with water-processable branched poly (ethylene imine)-based solid-state electrolytes

摘要

Here we report that lithium polymer supercapacitors (LPSCs) with stable charging/discharging characteristics are successfully fabricated by employing polymeric solid-state electrolytes composed of lithium hydroxide (LiOH) and branched-poly(ethyleneimine) (bPEI). The bPEI:LiOH solutions were prepared using deionized water as a solvent by varying the LiOH molar ratio up to 170 mol%. The LPSC devices were fabricated by sandwiching two pieces of bPEI:LiOH film-coated graphite composite/indium-tin oxide (ITO)-glass substrates. The basic cyclic voltammetry tests were performed by varying the sweep rate from 0.1 V/s to 10 V/s, while the ion conductivity of bPEI:LiOH solid-state electrolytes was measured using electrochemical impedance spectroscopy. The inter-action between lithium cations and bPEI chains in the bPEI:LiOH solid-state electrolytes was investigated using X-ray photoelectron spectroscopy (XPS). Results showed that the highest energy density of LPSCs was obtained at LiOH = 90 mol% due to the highest ion conductivity (1 mS/cm) at this molar ratio. The galvanostatic charging/ discharging (GCD) tests showed that the present LPSCs could be effectively charged by a wide range of applied current density (0.02-2.0 mA/g). The repeated GCD test revealed that the optimized LPSCs with the bPEI:LiOH solid-state electrolytes (LiOH = 90 mol%) could still keep their capacity by ca. 85 % even after 700 cycles.