Computational Study of the Structure and Transport in Pyrrolidinium-Li-TFSI-Silica Ionogels

Abstract

Ionogels (IGs) are a unique class of composite materials with attributes that make them promising materials for applications in electrochemical energy storage. Due to the solid porous matrix that confines the ionic liquid (IL) in the IG, they can be used as self-supporting electrolytes. Furthermore, interactions of the IL with the porous matrix can have beneficial effects on transport, such as lowering the freezing/glass transition temperature of the conducting IL. In this work, we employ molecular dynamics simulations to investigate the influence of the porous morphology and solid volume fraction on ionic conductivity and Li+ diffusivity using a representative 0.5 M Li-bis(trifluoromethane)­sulfonimide (TFSI)-pyrrolidinium (Pyr1.3) IL confined in a nanoporous silica matrix. The effect of the morphology of the confining matrix is compared using the pure IL as a baseline. We find that the tracer and collective Li+ diffusion and ionic conductivity of all the model IGs have significantly lower temperature dependence than the corresponding pure IL. In general, low-silica IGs with wide pores displayed the best transport properties at high temperatures, but the trends with the morphology for the nested set of transport coefficients we examined changed as the collective behavior of the Li+ ions and the molecular IL components were considered. Remarkably, some of the model IGs displayed better transport properties on a volume of fluid basis at low temperatures than the constituent IL. These trends were tied to structural changes revealed by the radial distribution functions of the IL components and the silica surface, including a decreasing Li+ adsorption peak of the surface silica indicating a change in the relative contributions of bulk-like and surface-like transport in the confined IL.

Tailoring of the electrochemical stability and dielectric behaviour of inorganic matrix-based ionogels

“…IGs are a special type of quasi-solid electrolytes having properties that make them attractive for different applications in optical devices, ESDs, gas absorption, catalysis, and pharmaceuticals [7,8]. They can be utilized as selfsupporting electrolytes because of the solid porous matrix that holds the IL within the pores of IG [9]. IGs are new materials prepared by reacting a tetraethyl orthosilicate (TEOS) with IL and carboxylic using the sol-gel (hydrolytic or non-hydrolytic) method [10].…”