Mastering electrolyte properties and compatibility: organically modified ionogels for sodium-ion batteries

Abstract

Silica-based ionogels based on sol-gel synthesis are widely investigated to serve as solid electrolytes for sodium-ion batteries (SIBs). SIBs are more sustainable than lithium-ion batteries (LIBs) due to the high abundance of sodium. Ionogels, in which an ionic liquid electrolyte (ILE) is confined within a silica matrix, are attractive potential solid electrolytes because of their high ionic conductivity, thermal stability, and broad electrochemical window. 1 However, brittleness of such ionogels may adversely affect the performance of the final battery. Increasing the content of ionic liquid has been reported to improve the mechanical properties. 2-3 However, as the ionic liquid is an important cost factor of ionogels, due to the lack of a scalable synthesis, the content of ionic liquid should preferentially remain low. 4 Additionally, increasing the ILE content in silica-based ionogels synthesized through a non-aqueous process only results in a limited mechanical improvement. Therefore, it remains crucial to make further improvements based on organic modifications. It was hypothesized that this would allow lowering of the rigidity of the silica matrix by preventing full condensation due to the presence of an unreactive organic group attached to Si, whilst maintaining the electrochemical properties. A non-aqueous sol-gel route with formic acid (FA) is used to obtain monolithic organically modified ionogels which are homogeneous, transparent, and depending on the phenyl content compliant. The hardness and storage modulus (nano-indentation) were improved upon organic modification whilst a slight reduction of ionic conductivity (electrochemical impedance spectroscopy, EIS) could be noticed as well. Besides, thermal stability (thermogravimetric analysis, TGA), chemical interactions (ATR-FTIR), and anodic stability (linear scanning voltammetry, LSV) were studied as well. These organically modified ionogels are finally researched in SIB full cells to demonstrate their performance as solid electrolyte and the cell's stability. Figure 1: Graphical abstract of organically modified ionogels Acknowledgments