Lithiated Nafion plasticised by a mixture of ethylene carbonate and sulfolane

Abstract

A Nafion-115 membrane in its lithiated form (Li-Nafion) was plasticised using a binary mixture of ethylene carbonate (EC) and sulfolane (SL) to obtain a polymer electrolyte with single-ion lithium conductivity and enhanced stability. The weight fraction of SL in the binary mixture varied from pure EC to pure SL with a step of 5-10 wt. %. Quantum chemical calculations at the M05-2X/TZVP + SMD theory level predicted that an EC/SL binary mixture can be a more effective plasticiser for the Li-Nafion membrane than its single components. A significant influence of mixing temperature (at which the membranes were saturated with a plasticiser) on the swelling degree of the membranes was observed; when the temperature was below the melting point of pure EC, membrane saturation was substantially less effective despite the liquid state of the binary plasticiser EC/SL. The temperature dependences of ionic conductivity of the electrolytes obey the Arrhenius law with activation energies 21 to 33 kJ mol−1. The best sample demonstrated single-ion lithium conductivity of 6.3 × 10−6 to 6.9 × 10−4 S cm−1 within the temperature range of -40 to +80 °C and a negligible contribution of electronic conductivity (~10−9 S cm−1 at 25 °C) together with electrochemical stability window 0 to 6 V vs. Li/Li+. Electrochemical properties of Li-Nafion swollen with the non-volatile EC/SL binary plasticiser appear to be suitable for practical applications in lithium batteries.