P(VDF-co-HFP)/tetramethylurea sodium salt polymer gel electrolytes. Electrical and electrochemical characterization

Abstract

Rechargeable batteries had become indispensable, as they supply the energy needed for our cars, phones, cameras or even our books. Lithium-ion batteries (LIBs) completely surpassed previous secondary batteries in terms of power density (maximum energy-to-weight ratios). However, problems have arisen related to the low abundance of lithium and its localization in conflictive areas of the planet [1,2], limiting the use of these batteries in long-size stationary electric energy storage systems. Since sodium is the sixth most abundant element in Earth and worldwide distributed, sodium-ion batteries (SIBs) have emerged as a promising alternative to LIBs. Our group is working on the three main components of batteries, both electrodes and the electrolyte. In this communication, we will focus on this last component. The electrolyte plays a crucial role regarding the safety of the system. Carbonate solvents, commonly used in electrolytes, are associated with high flammability. [3]. In order to circumvent this risk, alternative electrolyte components have been proposed. In particular, DESs (deep eutectic solvents) are a good alternative for common organic solvents as the manipulation of the melting temperature, ionic conductivity and the reduction stability of the system can be carried out by selecting a suitable HBD and HBA and their ratios. On the other hand, the use of solid electrolytes, such as gel polymer electrolytes (GPEs), formed by incorporating liquid electrolytes to a polymer matrix, offers many advantages regarding liquid electrolytes, such as immunity from leakage, good ionic conductivity (>10¿3 S cm¿1 at room temperature), wide electrochemical stability window, high specific capacity, long cycle life, rapid charge and discharge, small size and mechanical flexibility [4]. In this work sodium ion P(VDF-co-HFP)/tetramethylurea-Na salt gel polymer electrolytes have been synthesized by solvent casting technique with varying sodium salt (NaTFSI, NaFSI) and tetramethylurea/Na salt ratio.