Novel electrolytes evaluated for high voltage electrochemical double layer capacitors

Glyme refers to any of the glycol ethers, a class of solvents, usually dimethoxyethane, if not otherwise stated. These ethers possess several unique properties that make them attractive solvents for electrochemical energy storage. Specifically, their physical properties such as freezing point, boiling point and viscosity can be tuned over a wide range by varying the chain length.

Generally, they are typically chemically and thermally stable with a large electrochemical window. These properties are particularly attractive as electrolyte solvents for electrochemical double layer capacitors which can operate at 4V, as opposed to currently used solvents such as acetonitrile, which are restricted to ~ 3V or less. Because energy storage scales as the square of the voltage, 4V capacitors are particularly attractive.

Previous studies by Professor Steven Greenbaum and his colleagues have been focusing on potential application as solvents or co-solvents in auspicious electrolytes for emerging battery technologies, such as: lithium sulfur, lithium-air, non-aqueous redox flow batteries, magnesium batteries, and sodium-ion chemistries. As of now, published literature has highlighted that glymes contain multiple ether groups which can chelate with metal cations to form complexes like crown ethers. Further, some of the resultant glyme cation complexes have been reported to form a new class of electrolytes, i.e. solvate ionic liquids (SILs), that has increased thermal and electrochemical stability. Currently, much of these novel complexes have not been fully investigated.

To continue their efforts in glyme-based electrolytes research, Hunter College of the City University of New York scientists: Daniel Morales (PhD candidate) and CUNY Distinguished Professor Steven Greenbaum from the Department of Physics & Astronomy in collaboration with Dr. Rose Ruther and Dr. Jagjit Nanda at the Oak Ridge National Laboratory investigated the transport properties of LiPF₆ and NaPF₆ salts in different solvents: Monoglyme (G1), Diglyme (G2), and Tetraglyme (G4). Their objective was to compare the transport properties of LiPF₆ and NaPF₆ salts in glymes to similar electrolytes with LiCF₃SO₃ and NaCF₃SO₃ salts. Their work is currently published in the research journal, Electrochimica Acta.

In brief, the team selected the LiPF₆ and NaPF₆ salts due to the fact that they are popular electrolytes for lithium and sodium-ion batteries and have desirable characteristics. The researchers measured electrolyte conductivity using the impedance response of a dip-style conductivity probe. Moreover, they collected ATR-FTIR spectra of the electrolytes at room temperature. Some of the chemicals used in the study were monoglyme, diglyme, tetraglyme, LiPF₆ and NaPF₆ salts.

The authors reported that a comparison of the conductivity from NMR diffusion and from direct EIS measurement revealed that the ions were highly associated in the monoglyme solvent, and less so with increasing chain length, an observation that was seen to agree well with previous experiments on glyme solvents. Additionally, an increase in ion association with rising temperature for all electrolytes was noted and even correlated with the decreased dielectric constant of the solvent with increasing temperature. Although the longer-chain glymes exhibit less ion association, they are also more viscous, suggesting that there may be an optimal glyme length that yields sufficiently high conductivity for capacitor applications.

In summary, the study presented an in-depth assessment of the ion transport and association study of glyme based electrolytes with lithium and sodium salts. Comparison of conductivities calculated from the measured diffusivities and the Nernst-Einstein equation with conductivity measurements deduced from Electrochemical Impedance Spectroscopy (EIS) determined the ion association degree, i.e. the electrolytes were shown to exhibit stronger ion pairing with increased temperature which was attributed to a decrease in the dielectric constant of the solvents with increasing temperature. Overall, the ion association was shown to decrease with increasing solvent molecular size, an observation that was consistent with FTIR findings.