Unveiling the Impact of the Cations and Anions in Ionic Liquid/Glyme Hybrid Electrolytes for Na-O2 Batteries

A series of hybrid electrolytes composed of diglyme and ionic liquids (ILs) have been investigated for Na-O₂ batteries, as a strategy to control the growth and purity of the discharge products during battery operation. The dependence of chemical composition of the ILs on the size, purity, and distribution of the discharge products has been evaluated using a wide range of experimental and spectroscopic techniques. The morphology and composition of the discharge products found in the Na-O₂ cells have a complex dependence on the physicochemical properties of the electrolyte as well as the speciation of the Na⁺ and superoxide radical anion. All of these factors control the nucleation and growth phenomena as well as electrolyte stability. Smaller discharge particle sizes and largely homogeneous (2.7 ± 0.5 μm) sodium superoxide (NaO₂) crystals with only 9% of side products were found in the hybrid electrolyte containing the pyrrolidinium IL with a linear alkyl chain. The long-term cyclability of Na-O₂ batteries with high Coulombic efficiency (>90%) was obtained for this electrolyte with fewer side products (20 cycles at 0.5 mA h cm^-2). In contrast, rapid failure was observed with the use of the phosphonium-based electrolyte, which strongly stabilizes the superoxide anion. A high discharge capacity (4.46 mA h cm^-2) was obtained for the hybrid electrolyte containing the pyrrolidinium-based IL bearing a linear alkyl chain with a slightly lower value (3.11 mA h cm^-2) being obtained when the hybrid electrolyte contained similar pyrrolidinium-based IL bearing an alkoxy chain.

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

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.

Effects of Strain Rate and Temperature on the Mechanical Properties of Simulated Silica Ionogels

Ionogels are hybrid materials formed by impregnating the pore space of a solid matrix with a conducting ionic liquid. By combining the properties of both component materials, ionogels can act as self-supporting electrolytes in Li batteries. In this study, molecular dynamics simulations are used to investigate the dependence of mechanical properties of silica ionogels on solid fraction, temperature, and pore width. Comparisons are made with corresponding aerogels. We find that the solid matrix fraction increases the moduli and strength of the ionogel. This varies nonlinearly with temperature and strain rate, according to the contribution of the viscous ionic liquid to resisting deformation. Owing to the temperature and strain sensitivity of the ionic liquid viscosity, the mechanical properties approach a linear mixing law at high temperature and low strain rates. The median pore width of the solid matrix plays a complex role, with its influence varying qualitatively with deformation mode. Narrower pores increase the relevant elastic modulus under shear and uniaxial compression but reduce the modulus obtained under uniaxial tension. Conversely, shear and tensile strength are increased by narrowing the pore width. All of these pore size effects become more pronounced as the silica fraction increases. Pore size effects, similar to the effects of temperature and strain rate, are linked to the ease of fluid redistribution within the pore space during deformation-induced changes in the geometry of the pores.