Eutectic Electrolytes as a Promising Platform for Next-Generation Electrochemical Energy Storage

ConspectusThe rising global energy demand and environmental challenges have spurred intensive interest in renewable energy and advanced electrochemical energy storage (EES), including redox flow batteries (RFBs), metal-based rechargeable batteries, and supercapacitors. While many researchers focus on the design of new chemistry and structures for high-capacity and stable electrode materials, the electrolyte also plays a significant role in enabling the successful function of these new electrode materials and chemistries. Discovery of new electrolytes is urgently needed to keep up with the rapid growth of EES. Benefiting from the strong intermolecular interaction between different components, eutectic electrolytes possess various specific functionalities that conventional electrolytes do not have, such as highly concentrated systems, non-flammability, high degrees of structural flexibility, and good thermal and chemical stability, thereby leading researchers to consider them as a new class of ionic fluids for EES applications.In this Account, we aim to provide a mechanistic understanding of this energy chemistry and an overview of recent progress in the development of eutectic electrolytes for next-generation EES. First, we describe different mechanisms that guide the formation of eutectic electrolytes and discuss the structure–property relations, electron transfer and ion transport mechanisms, and interfacial chemistry in eutectic electrolytes. Generally, three main intermolecular interactions, namely hydrogen-bond interactions, Lewis acid–base interactions, and van der Waals interactions, control the formation of eutectic electrolytes and determine their unique characters in terms of electrochemical, thermal, ion transport, and interfacial properties. These versatile intermolecular interactions can be further modified by tailoring the functional moieties of organic molecules and/or selecting suitable compositions of mixtures. The solvent-free eutectic electrolyte can maximize the molar ratio of redox-active materials, thus increasing the energy density of RFBs. We discuss the relationships between eutectic parameters (viscosity, polarity, ionic conductivity, surface tension, and coordination environment) and the molar ratio, stability, utilization, and electrochemical reversibility of redox-active materials, RFB power, and energy density. We then introduce the application of both metal- and organic-based eutectic electrolytes in the RFB field, along with the relevant perspective for future study in this field. The highly concentrated eutectic electrolytes show attractive features at electrolyte/electrode interfaces to expand the electrochemical window and meanwhile inhibit metal dendrite formation in metal-based rechargeable batteries, supercapacitors, and hybrids of these. The remaining challenges and potential research directions in these areas are also discussed. Eutectic electrolytes offer enormous opportunities and open appealing prospects as redox reaction and charge transport media for EES. We hope this Account provide guidance for the future design of advanced eutectic electrolytes toward next-generation EES systems.