Organic Eutectic Electrolytes for Future Flow Batteries

Abstract

In this issue of Chem, Yu and coworkers report phthalimide-based eutectic anolytes, which achieved a high concentration and enhanced redox reversibility. The organic-molecule-based eutectic electrolytes take advantages of both the superior tunability of the organic molecule and the high molar concentration of the redox-active molecules of the eutectic solvents. With combined computational and experimental analysis, this work demonstrates that forming eutectic electrolytes with self-containing redox-active organics is a promising strategy for the future development of high-energy-density redox-flow batteries. In this issue of Chem, Yu and coworkers report phthalimide-based eutectic anolytes, which achieved a high concentration and enhanced redox reversibility. The organic-molecule-based eutectic electrolytes take advantages of both the superior tunability of the organic molecule and the high molar concentration of the redox-active molecules of the eutectic solvents. With combined computational and experimental analysis, this work demonstrates that forming eutectic electrolytes with self-containing redox-active organics is a promising strategy for the future development of high-energy-density redox-flow batteries. The rising concerns of limited fossil fuels and energy security, as well as the ever-increasing global CO2 emissions, promote intensive research on the utilization of abundant, clean renewable energy sources. Among all energy storage systems, redox-flow batteries (RFBs) have been widely recognized as one of the most promising grid-scale energy storage techniques because of their unique feature of decoupled power and energy components, which allows easy scalability and high flexibility in modular design.1Dunn B. Kamath H. Tarascon J.-M. Electrical energy storage for the grid: a battery of choices.Science. 2011; 334: 928-935Crossref PubMed Scopus (10070) Google Scholar Although significant efforts and progress have been made toward increasing the energy storage capacity and reducing the material cost, existing aqueous RFBs based on inorganic transition metals (e.g., V, Zn, and Fe) and halogens (e.g., Br2 and I2) are still plagued by (1) low energy density due to the limited cell voltage and solubility of the inorganic salts, (2) the high cost of electrolytes, and (3) material toxicity and environmental concerns.2Winsberg J. Hagemann T. Janoschka T. Hager M.D. Schubert U.S. Redox-flow batteries: from metals to organic redox-active materials.Angew. Chem. Int. Ed. 2017; 56: 686-711Crossref PubMed Scopus (599) Google Scholar, 3Chen H. Cong G. Lu Y.-C. Recent progress in organic redox flow batteries: active materials, electrolytes and membranes.J. Energy Chem. 2018; 27: 1304-1325Crossref Scopus (147) Google Scholar Alternatively, redox-active organics have been receiving intense attention as active materials for RFBs because of their potential low cost, vast abundance, and environmental benignity. Nonetheless, the solubility of most organic redox species in both aqueous and non-aqueous solvents is very limited, leading to energy density that is comparable to or even lower than that of conventional inorganic RFBs. Unlike transitional-metal salts in which redox processes are realized by the valence-state change of the transition-metal centers, redox reactions of organic molecules are based on changing the charge state of the organic moieties with a conjugated structure or atoms with lone-pair electrons (such as O, S, and N), which permits flexible tunability of the properties, such as redox potential, solubility, diffusion behavior, and kinetics.3Chen H. Cong G. Lu Y.-C. Recent progress in organic redox flow batteries: active materials, electrolytes and membranes.J. Energy Chem. 2018; 27: 1304-1325Crossref Scopus (147) Google Scholar Chemical modification of the organic structures has been proven to be an effective strategy for improving the solubility and reversibility of organic redox species.4Cong G. Zhou Y. Li Z. Lu Y.-C. A highly concentrated catholyte enabled by a low-melting-point ferrocene derivative.ACS Energy Lett. 2017; 2: 869-875Crossref Scopus (68) Google Scholar Traditionally, this process is carried out through trial and error, which is time consuming and inefficient. Powered by computational chemistry and advances in computation techniques and synthetic chemistry, functionalization of organic redox species is expected to be more effective and efficient. Another strategy that bypasses the solubility limitation is dispersing the redox-active organics and electrical conductive carbon network into electrolyte to make a semisolid suspension.5Chen H. Zhou Y. Lu Y.-C. Lithium–organic nanocomposite suspension for high-energy-density redox flow batteries.ACS Energy Lett. 2018; 3: 1991-1997Crossref Scopus (37) Google Scholar However, the low density of the organic materials and the addition of redox-inactive carbon into electrolyte set a moderate limitation on the effective concentration (less than 4 M). Besides, inherent problems including high viscosity of semisolid suspension and possible phase separation need to be carefully evaluated for practical applications. An emerging solution for maximizing the concentration of active materials in RFBs is to form a eutectic electrolyte with redox species and organic and inorganic additives.6Takechi K. Kato Y. Hase Y. A highly concentrated catholyte based on a solvate ionic liquid for rechargeable flow batteries.Adv. Mater. 2015; 27: 2501-2506Crossref PubMed Scopus (116) Google Scholar, 7Wang Y. Zhou H. A green and cost-effective rechargeable battery with high energy density based on a deep eutectic catholyte.Energy Environ. Sci. 2016; 9: 2267-2272Crossref Google Scholar, 8Zhang C. Ding Y. Zhang L. Wang X. Zhao Y. Zhang X. Yu G. A sustainable redox-flow battery with an aluminum-based, deep-eutectic-solvent anolyte.Angew. Chem. Int. Ed. 2017; 56: 7454-7459Crossref PubMed Scopus (99) Google Scholar, 9Zhang L. Zhang C. Ding Y. Ramirez-Meyers K. Yu G. A low-cost and high-energy hybrid iron-aluminum liquid battery achieved by deep eutectic solvents.Joule. 2017; 1: 623-633Abstract Full Text Full Text PDF Scopus (92) Google Scholar, 10Zhang C. Niu Z. Ding Y. Zhang L. Zhou Y. Guo X. Zhang X. Zhao Y. Yu G. Highly concentrated phthalimide-based anolytes for organic redox flow batteries with enhanced reversibility.Chem. 2018; 4: 2814-2825Abstract Full Text Full Text PDF Scopus (81) Google Scholar The interactions between redox species and the additives reduce the lattice energy and lower the melting point to room temperature or even lower. In this way, the amount of redox-inactive solvent used to dissolve the active material and supporting electrolyte in conventional electrolytes for RFBs could be greatly reduced, thus enhancing the effective concentration of the redox species. In this issue of Chem, Yu and coworkers demonstrate highly concentrated phthalimide-based eutectic RFB anolytes composed of phthalimide derivatives (Phds), urea, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (Figure 1).10Zhang C. Niu Z. Ding Y. Zhang L. Zhou Y. Guo X. Zhang X. Zhao Y. Yu G. Highly concentrated phthalimide-based anolytes for organic redox flow batteries with enhanced reversibility.Chem. 2018; 4: 2814-2825Abstract Full Text Full Text PDF Scopus (81) Google Scholar The coordination and interactions between the Li+ and O atoms of both TFSI− and the Phds weaken the ionic bonds of Li+ and TFSI−, promoting the formation of eutectic solvents. Urea was added into the Phds/LiTFSI binary eutectic system to reduce the viscosity and meanwhile improve the stability of the Phd radical. The optimized LiTFSI/NMePh/urea ternary system, which has a molar ratio of 2:2:1.6, achieved a ∼4 M NMePh concentration, nearly six times greater than that of conventional NMePh-based electrolytes. In addition, the redox potential of NMePh was found to be as low as –1.77 V versus Ag/AgNO3 in DMF/DCE-LiTFSI. Both features make the phthalimide-based eutectic electrolytes desirable anolytes for developing high-voltage, high-energy RFBs. To probe the mechanism behind the formation and stability of eutectic electrolyte and understand the roles played by urea, Yu and colleagues performed density functional theory (DFT) calculations on the LiTFSI/NMePh/urea eutectic system with different molar ratios. DFT results revealed that the tendency of coordination between the electrophilic C=O group of NMePh and the Li+ of LiTFSI reduces both the intermolecular forces of NMePh and the ionic binding of LiTFSI and hence lowers the melting point of the eutectic system. Li+ was triple coordinated by the O atoms of NMePh and TFSI− in the NMePh/LiTFSI binary system, which is only metastable. Urea was found to be able to decrease the mean interaction energy, leading to the reduction in both melting point and viscosity. In addition, the N and O atoms in urea tended to coordinate with Li+ in LiTFSI, which led to a more stable tetra-coordinated geometry (Figure 1B). The interaction between the C=O group of NMePh and Li+ was also confirmed by the blue shift of the C=O stretching peak and the red shift of the S–O–S bending peak on the Raman spectrum of the LiTFSI/NMePh/urea electrolyte. Urea was also found to play an important role in stabilizing the NMePh anion radical and enhancing the reversibility of the NMePh molecule by decreasing the interaction energies between Li+ and the O atom of NMePh. The authors employed both the Li|NMePh half cell and the NMePh|Fc full cell to evaluate the electrochemical performance of the LiTFSI/NMePh/urea eutectic electrolyte. Moderate Coulombic efficiencies (CEs) of around 95% were demonstrated for the Li|NMePh half cell, and NMePh concentrations varied from 0.1 to 1.0 M. Even lower CEs between 80%–90% were delivered by the NMePh|Fc full cell. The relative low CEs for both the half-cell and full-cell systems could be ascribed to (1) the degradation of the NMePh anion radical, which was proved by the gradual decay of the peak at 240 nm on the UV-visible spectra, and (2) the inefficiency of the separator (the instability of LAGP against the NMePh radical for the Li|NMePh half cell and the inefficient ion selectivity of the porous Celgard for the NMePh|Fc full cell). By taking advantage of the high molar concentration of redox-active species of the eutectic electrolytes and the high tunability of the organic molecule (Figure 1D), Yu and coworkers have demonstrated a promising strategy for increasing the energy densities of RFBs by forming organic-molecule-based eutectic anolytes. Compared with that of their metal-based counterparts,7Wang Y. Zhou H. A green and cost-effective rechargeable battery with high energy density based on a deep eutectic catholyte.Energy Environ. Sci. 2016; 9: 2267-2272Crossref Google Scholar, 8Zhang C. Ding Y. Zhang L. Wang X. Zhao Y. Zhang X. Yu G. A sustainable redox-flow battery with an aluminum-based, deep-eutectic-solvent anolyte.Angew. Chem. Int. Ed. 2017; 56: 7454-7459Crossref PubMed Scopus (99) Google Scholar, 9Zhang L. Zhang C. Ding Y. Ramirez-Meyers K. Yu G. A low-cost and high-energy hybrid iron-aluminum liquid battery achieved by deep eutectic solvents.Joule. 2017; 1: 623-633Abstract Full Text Full Text PDF Scopus (92) Google Scholar the redox potential of the eutectic electrolytes with electroactive organic molecules can be easily tuned by chemical functionalization; for example, the addition of an electron-donating group, such as amine (–NR2), that is stronger than the methyl group to the N atom of phthalimide could further lower its potential. Future work, as suggested by the authors, should focus on understanding the degradation mechanisms of the organic radicals and developing more efficient separators. Highly Concentrated Phthalimide-Based Anolytes for Organic Redox Flow Batteries with Enhanced ReversibilityZhang et al.ChemSeptember 20, 2018In BriefFacile and effective eutectic-based anolytes are developed to achieve high concentration and enhanced reversibility of phthalimide-derived redox organic molecules. A 6-fold increase in solubility is obtained with eutectic-based electrolytes. A redox flow battery using N-methylphthalimide as anolyte shows high capacity and stable cycle life. This promising strategy that takes advantage of forming eutectic electrolyte is effective for further development of concentrated redox-active organic molecules for organic RFBs as grid-scale energy storage systems. Full-Text PDF Open Archive