Development of all-solid-state mediator-enhanced supercapacitors with polyvinylidene fluoride/lithium trifluoromethanesulfonate separators

All-solid-state supercapacitors (SCs) were fabricated using a polyvinylidene fluoride (PVDF)/lithium trifluoromethanesulfonate (LiTFS) membrane as the separator and poly(ethylene oxide) (PEO)/lithium perchlorate (LiClO₄) as the polymer electrolyte in the porous carbon electrodes. Two types of mediators, NaI/I₂ and K₃Fe(CN) ₆/K₄Fe(CN)₆, were added into the PEO/LiClO ₄ polymer electrolyte that was used to fabricate the electrodes. The voltage window in which the SCs operated was 2.5-3 V. The results of electrochemical measurements, including cyclic voltammetry and galvanostatic charge/discharge, indicated that NaI/I₂-containing and K ₃Fe(CN)₆/K₄Fe(CN)₆-containing SCs yielded high specific capacitances of 209.0 and 138.8 F g^-1, respectively. In addition to high specific capacitances for the two mediator-containing SCs, both SCs delivered high specific energies (49.1 Wh kg^-1 at 1.6 kW kg^-1 for the NaI/I₂-containing SC and 33.6 Wh kg^-1 at 1.3 kW kg^-1 for the K ₃Fe(CN)₆/K₄Fe(CN)₆-containing SC) due to the wide voltage window and fast redox reactions between mediators.