The Roles of Mediators In All-Solid-State Supercapacitors at Low-Temperature Application

The present study is focused on increasing ionic conductivity by utilizing quantum tunneling charge transfer between mediators and the percolation phenomenon in mediator-membrane systems at both ambient and low temperatures. Polyvinylidene fluoride (PVDF)/ Lithium trifluoromethanesulfonate LiTFS with mediators {K₄Fe (CN)₆/ K₃Fe(CN)₆) and NaI/ I₂} from 0 to 10 Vol% were fabricated. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDX)  presented surface morphology and mediator particle distribution in the polymer membranes(PEMs). Four-point conductivity measurement device was applied to measure conductivity of membranes. The Newman Ziff geometrical model, Kirkpatrick's mathematical percolation model and Dahms-Ruff method were used to evaluate the conductivities of membranes as a function of the concentration of mediator. All-solid-state SCs fabricated using the PEMs with the optimal concentration of mediator and active carbon-based, polyethylene oxide (PEO)/LiTFS polymer electrolyte containing electrodes were tested by cyclic voltammetry (CV), galvanostatic charge /discharge (GCD) and electrochemical impedance spectroscopy (EIS). Conclusions：1) The electronic and ionic conductivities of PEMs as functions of concentration of mediator reveal characteristics of site-percolation or a geometrical phase transition of a random disordering system. In the pre-percolation regime below 3 Vol% concentration of mediator the ionic conductivity of the PEMs was increased by 50% at 20°C or 100% at -20°C, while in the post-percolation regime above 6.0 Vol % the electronic and ionic conductivity  increased by 10-100 times and 2-6 times respectively. The experimental and modeling results indicate quantum tunneling electron transfer between mediators promoted the ions transport. With the capacitance retention 0.870,charge efficiency 0.932 and specific energy retention 0.980 from 20 to -20°C,the reversibility, power density and energy density of all-solid-state mediator SCs were significantly improved with respect to mediator-free SCs.