Influence of weakly coordinating solvents on electrolyte properties and supercapacitor performance. A combined experimental and computational study

Weakly coordinating solvents can be used as solvents to promote the reduction in insertion energy of alkaline cations into the graphite anode of supercapacitors, thereby speeding up charging at low temperatures. We investigate the thermophysical properties of sodiumbased electrolytes (NaPF6 and NaTFSI) composed of propylene carbonate (PC) and the weakly coordinating solvent cyclopentyl methyl ether (CPME). The solvent characteristics related to the charge-storage process in supercapacitors are also examined through a combination of electrochemical and computational analyses. The addition of CPME to the electrolytes decreases the viscosity, thus increasing the diffusion coefficients of ions, but decreases their ionic conductivities due to an increase in ion-pair formation, particularly in the presence of NaPF6. According to the electrochemical findings, the latter salt exhibits the worst supercapacitor performance among the electrolytes investigated. For the NaTFSIPC:CPME electrolyte, there is evidence of a Faradaic process, probably due to the Na+ solid-state redox insertion/intercalation process occurring in the small number of graphitic domains of the activated carbon electrode. The occurrence of electrolyte degradation was also investigated. The highest specific power and energy were verified for the NaTFSI-PC electrolyte. The Ragone plot calculated from Molecular Dynamics simulations shows excellent agreement with the experimental counterparts. Upon charging the electrodes, CPME leaves the pores, whereas PC follows the flux of Na+. The addition of CPME in the electrolytes increases the contribution of the counter-ion adsorption charging mechanism, which is almost exclusive in NaPF6-PC:CPME. Inside the positive porous electrodes, there is a high degree of ionic pairs or aggregates.