A Sustainable Redox‐Flow Battery with an Aluminum‐Based, Deep‐Eutectic‐Solvent Anolyte

Physicochemical Properties of Choline Chloride/Acetic Acid as a Deep Eutectic Solvent and Its Binary Solutions with DMSO at 298.15 to 353.15 K

Deep eutectic solvents (DESs) are considered to play an important role in green chemistry and other technological fields as an alternative to organic solvents. The present study reports measurements of density (ρ), speed of sound (u), dynamic viscosity (η), and electrical conductivity (κ) and investigates physicochemical properties of choline chloride/acetic acid (ChCl/AcA DES) and its binary mixtures with dimethyl sulfoxide (DMSO) over the entire composition and temperature (298.15-353.15 K) range. The density data are well fitted by a second-degree polynomial equation in T. DES/DMSO mixtures exhibit negative excess molar volume and isentropic compressibility deviation with a minimum in respective curves at x1 ≈ 0.15 (x1 is the mole fraction of DES in the mixture), which became deeper with increasing temperature. The ChCl/AcA DES and DMSO curves for excess partial molar volume cross each other at x1 ≈ 0.15, showing that the packing effect is dominant over specific interactions. A similar behavior is observed for excess molar viscosity, showing the minima at x1 ≈ 0.62, and substantiates volumetric results. The temperature dependence of viscosity and conductivity is well described by the Vogel-Fulcher-Tammann (VFT) equation.

Enhanced Dynamics and Charge Transport at the Eutectic Point: A New Paradigm for the Use of Deep Eutectic Solvent Systems

Deep eutectic solvents (DESs) are a class of versatile solvents with promise for a wide range of applications, from separation processes to electrochemical energy storage technologies. A fundamental understanding of the correlation among the structure, thermodynamics, and dynamics of these materials necessary for targeted rational design for specific applications is still nascent. Here, we employ differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS), and femtosecond transient absorption spectroscopy (fs-TAS) to investigate the correlation among thermodynamics, dynamics, and charge transport in mixtures comprising a wide range of compositions of choline chloride (ChCl) and ethylene glycol (EG). Detailed analyses reveal that (i) the eutectic composition of this prototypical DES occurs in the 15-20 mol % ChCl in the EG range rather than the previously assumed 33 mol %, and (ii) both rotational dynamics and charge transport at the eutectic composition are enhanced in this composition range. These findings highlight the fundamental interplay between thermodynamics and dynamics in determining the properties of DESs that are relevant to many applications.

Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Tailoring the macroscopic properties of deep eutectic solvents requires knowing how these depend on the local structure and microscopic dynamics. The authors, with computational and experimental tools spanning a wide range of space- and timescales, shed light into the relationship between micro and macroscopic properties in glyceline and ethaline.

Fluorinated Poly-oxalate Electrolytes Stabilizing both Anode and Cathode Interfaces for All-Solid-State Li/NMC811 Batteries

The relatively narrow electrochemical steady window and low ionic conductivity are two critical challenges for Li⁺ -conducting solid polymer electrolytes (SPE). Here, a family of poly-oxalate(POE) structures were prepared as SPE; among them, POEs composed from diols with an odd number of carbons show higher ionic conductivity than those composed from diols with an even number of carbons, and the POE composed from propanediol (C5-POE) has the highest Li⁺ conductivity. The HOMO (highest occupied molecular orbital) electrons of POE were found located on the terminal units. When using trifluoroacetate as the terminating unit (POE-F), not only does the HOMO become more negative, but also the HOMO electrons shift to the middle oxalate units, improving the antioxidative capability. Furthermore, the interfacial compatibility across the Li-metal/POE-F is also improved by the generation of a LiF-based solid-electrolyte-interlayer(SEI). With the trifluoroacetate-terminated C5-POE (C5-POE-F) as the electrolyte and Li⁺ -conducting binder in the cathode, the all-solid-state Li/LiNi_0.8 Mn_0.1 Co_0.1 O₂ (NMC811) cells showed significantly improved stability than the counterpart with poly-ether, providing a promising candidate for the forthcoming all-solid-state high-voltage Li-metal batteries.

A Chemistry and Microstructure Perspective on Ion-Conducting Membranes for Redox Flow Batteries

Redox flow batteries (RFBs) are among the most promising grid-scale energy storage technologies. However, the development of RFBs with high round-trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion-conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.

Organosulfide-Based Deep Eutectic Electrolyte for Lithium Batteries

Deep eutectic electrolytes (DEEs) are a new class of electrolytes with unique properties. However, the intermolecular interactions of DEEs are mostly dominated by Li⋅⋅⋅O interactions, limiting the diversity of chemical space and material constituents. Herein, we report a new class of DEEs induced by Li⋅⋅⋅N interactions between 2,2'-dipyridyl disulfide (DpyDS) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The strong ion-dipole interaction triggers the deep eutectic phenomenon, thus liberating the Li⁺ from LiTFSI and endowing the DEEs with promising ionic conductivity. These DEEs show admirable intrinsic safety, which cannot be ignited by flame. The DEE at the molar ratio of DpyDS:LiTFSI=4:1 (abbreviated as DEE-4:1) is electrochemically stable between 2.1 and 4.0 V vs. Li/Li⁺ , and exhibits an ionic conductivity of 1.5×10^-4  S cm^-1 at 50 °C. The Li/LiFePO₄ half cell with DEE-4:1 can provide a reversible capacity of 130 mAh g^-1 and Coulombic efficiency above 98 % at 50 °C.

Electrochemical Characteristics and Transport Properties of V(II)/V(III) Redox Couple in a Deep Eutectic Solvent: Magnetic Field Effect

Compared with conventional aqueous electrolytes, deep eutectic solvent (DES) has a wider electrochemical stability window, simple preparation, potential biodegradability, and lower cost, leading to its utilization as electrolyte for non-aqueous redox flow batteries (RFB). However, the large viscosity and inferior transport properties hinder the wide spread of DES electrolyte. To circumvent these issues, various additives as well as external fields can be applied separately or synergistically. This work reports a study on the inclusion of a DC magnetic field to the glycol-based DES electrolyte of a RFB. The effects of magnetic field on the physical and electrochemical characteristics of the electrolyte and the active redox couple on mass transfer are studied by cyclic voltammetry and electrochemical impedance spectroscopy. The experimental results show that the viscosity of the vanadium DES electrolyte decreases and the conductivity increases after adding a magnetic field. With the intensity of the added magnetic field increases, the oxidation and reduction peak current densities of the vanadium DES electrolyte keep increasing. Under the magnetic field intensity of 605 mT, the oxidation peak current density and the reduction peak current density increases 41.56 and 30.74%, respectively, compared with those of no added magnetic field. The ohmic resistance and electrochemical reaction resistance of the vanadium DES electrolyte are reduced when adding the magnetic field, reaching to 40.55 and 43.28%, respectively, with a magnetic field intensity of 605 mT. This study shows an effective yet simple way to improve the physical and electrochemical properties of DES electrolyte, which owns the potential to be widely applied in non-aqueous redox flow batteries.

A Critical Evaluation of the Effect of Electrode Thickness and Side Reactions on Electrolytes for Aluminum-Sulfur Batteries

The high abundance and low cost of aluminum and sulfur make the Al-S battery an attractive combination. However, significant improvements in performance are required, and increasing the thickness and sulfur content of the sulfur electrodes is critical for the development of batteries with competitive specific energies. This work concerns the development of sulfur electrodes with the highest sulfur content (60 wt %) reported to date for an Al-S battery system and a systematic study of the effect of the sulfur electrode thickness on battery performance. If low-cost electrolytes made from acetamide or urea are used, slow mass transport of the electrolyte species is identified as the main cause of the poor sulfur utilization when the electrode thickness is decreased, whereas complete sulfur utilization is achieved with a less viscous ionic liquid. In addition, the analysis of very thin electrodes reveals the occurrence of degradation reactions in the low-cost electrolytes. The new analysis method is ideal for evaluating the stability and mass transport limitations of novel electrolytes for Al-S batteries.