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

Abstract

Conspectus The 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...

Deep Eutectic Solvents for Boosting Electrochemical Energy Storage and Conversion: A Review and Perspective

“…[46] Yu et al recently reviewed the use of eutectic electrolytes for energy storage devices with a focus on redox flow batteries. [47] Accordingly, here we will discuss the recent process of DESs as electrolytes for supercapacitors and metalbased batteries. The common physicochemical properties of various previously reported DES-based electrolytes, including melting temperature, viscosity, and ionic conductivity, are summarized in Table 1.…”

Non‐Flammable Liquid and Quasi‐Solid Electrolytes toward Highly‐Safe Alkali Metal‐Based Batteries

“…As a new category of solvents, DESs are eutectic mixtures of at least two components that intimately interact with each other. The charge delocalization due to the intermolecular interactions (i.e., hydrogen bonds, Lewis acid‐base interactions, and van der Waals interactions) [ 118 ] contributes to decreasing the lattice energy of the mixture. Thus, the melting point of a DES is significantly lower than that of each individual component.…”

Bipolar Redox‐Active Molecules in Non‐Aqueous Organic Redox Flow Batteries: Status and Challenges

“…The interactions of the anionic and/or cationic components reduce the lattice energy of the mixture, which has the ability to lower the melting point of the mixture below the individual melting points of the separate species (Figure 11c). [54] This strategy enables the preparation of highly concentrated redox-active materials by physically mixing components, thereby making great energy densities readily accessible. Several eutectic mixtures have employed organic molecules as the electroactive species, such as viologen, TEMPO, and phthalimide-based electrolytes.…”

“…Several eutectic mixtures have employed organic molecules as the electroactive species, such as viologen, TEMPO, and phthalimide-based electrolytes. [54,55] Zhang et al prepared a biredox eutectic electrolyte by physically mixing N-butyl-phthalimide and 1,1-dimethylferrocene (BRM 17) (Figure 11a-c). [56] While BRM 17 contains the same redox-active species as BRMs 10 and 11, the eutectic strategy enables a concentration of 3.5 M, a more than 4-fold improvement upon 11 and an almost 12-fold improvement on 10 (Figure 11a).…”