Influence of Internal Interfaces on the Structure and Dynamics of IL-Based Electrolytes Confined in a Metal-Organic Framework.

Abstract

Hybrid solid-state electrolytes, which combine ionic liquids with metal-organic frameworks, offer a promising approach to enhancing the safety and energy density of next-generation batteries. A thorough understanding of the interplay between the solid and liquid phases in hybrid solid-state electrolytes is crucial for optimizing their performance as battery electrolytes. This study investigates how interactions between different ionic liquid-based electrolytes and the metal-organic framework ZIF-8 influence the coordination and dynamics of Li⁺ in confinement. To this end, we examine five different ionic liquids, varying the chemical nature of the cation. Raman spectroscopy, supported by 2D solid-state NMR and simulations, are used to elucidate Li⁺ coordination and ion-wall interactions. The impact of these interactions on local Li⁺ dynamics and charge transport in the ionic liquid-ZIF-8 hybrid system is investigated using ⁷Li spin relaxation, impedance spectroscopy, and simulations. The results reveal a competitive interaction between Li⁺ and the ionic liquid cation with the ZIF-8 framework, which can be fine-tuned by modifying the molecular structure of the ionic liquid cation. As a consequence, local Li⁺ dynamics is enhanced, depending on the ionic liquid cation. The beneficial interactions in the confined system can even make Li⁺ the fastest diffusing species, in contrast to bulk electrolyte, where Li⁺ transport is limited by strong Li-anion clusters. Thus, blocking Li⁺-framework interactions through other competitive interactions might be an effective strategy to enhance Li⁺ dynamics and increase Li conductivity in a hybrid solid-state electrolyte. Although confinement within the ZIF-8 model system leads to an overall decrease in conductivity, this study provides valuable insights into the design of hybrid electrolytes for next-generation battery applications.