Viscosity of Battery Mixtures

Written by Eden Reid, RheoSense Senior Sales and Marketing Operations

A battery is a device that stores chemical energy and converts that into electrical energy. Batteries are made of an electrochemical, or galvanic cell, which consists of the anode/ cathode separator and electrolytes. There is a difference in potential between each metal electrode and the electrolyte. The cathode has positive potential (with respect to the solution), and the anode has negative potential (with respect to solution). This potential difference allows electrons to flow from anode to cathode creating a current. While this is occurring, positive ions move through the electrolyte solution and the viscosity of this solution will affect the movement of ions and thus the conductivity and performance of your battery.

Figure 1: Battery element schematic

Viscosity is a fluids resistance to flow. Shear viscosity is sensitive to what is happening at the molecular level, because at the molecular level viscosity of the fluid is the friction between molecules of the fluid, which reflects how molecules interact and how these interactions will influence molecular structure. Size, shape and interactions of molecules can be reflected through viscosity as well as any possible microstructure.

Figure 2: A diagram showing the movement of a fluid from left to right and how shear rate changes perpendicular to flow direction

Viscosity is dependent on the properties of a battery such as size of ions, solvation of ions, nature of electrolyte, and the nature of the solvent. The conductivity, and performance, of your battery will depend on viscosity and on dielectric constant. Mobility is inversely proportional to viscosity and the size of the ion, therefore a higher viscosity will give you a lower conductivity.

Dielectric constant is defined as the reflection of the way molecules of a solution are polarized by local electric fields. This means a higher dielectric constant means a solvent that more readily associates salts into constituent ions, resulting in a higher concentration of ions in solution and a higher conductivity. To achieve high conductivity, you are looking for a formulation with low viscosity and high dielectric constant.

Figure 3: A diagram showing dielectric constant v. viscosity data for different battery solvents

There are many battery types, including both rechargeable (or secondary) and non-rechargeable (or primary). Of the rechargeable battery types, lithium-ion batteries are of a particular interest as they are used in devices such as cell phones, laptop computers, power tools and the rapidly expanding application – electric vehicles (EV). Lithium-ion battery electrolyte solutions are typically composed of a binary mixture of cyclic carbonates, e.g., ethylene carbonate (EC), and linear carbonates, e.g., dimethyl carbonate (DMC) or ethyl methyl carbonate (EMC).

In an experiment to monitor how temperature, salt concentration, and solvent concentration affect viscosity, the RheoSense team measured various electrolyte systems containing varying ratios of DMC, EMC, EC with the VROC^® initium one plus automated viscometer. Our experiment showed that viscosity decreases as temperature increases for the various samples. It also shows that viscosity increases as you add EC to your solution, and a larger viscosity mixture has a steeper decrease in viscosity with increase in temperature.

Figure 4: A diagram showing steady shear viscosity as a function of temperature for mixtures containing varying ratios of DMC, EMC, and EC.

Battery conductivity increases with increasing lithium salt concentration (LiClO4), however after a certain threshold is reached conductivity decreases with increasing salt concentration because the conductivity in this concentration range is primarily affected by the viscosity of the solution which increase with the concentration of the salt.

Figure 5: A diagram showing the effect of salt concentration on conductivity

Our data shows battery solutions exhibit Newtonian behavior (viscosity is constant and therefore independent of shear rate). Based on our data, adding EC increases viscosity of the electrolyte, which correlates adding EC to carbonate based electrolytes can contribute to salt dissociation and a higher dielectric constant. Increasing salt concentration of the solution will also increase the viscosity of the solution. In a solution that does not contain EC a higher salt concentration can be beneficial as it can promote ion dissociation, which will ultimately increase conductivity of the system.

Figure 6: A diagram showing steady shear viscosity as a function of LiClO4 concentration for EC-content electrolytes at 40°C. As expected, with increasing the LiClO4 concentration the viscosity increases.

All of this data shows that conductivity and performance of different electrolyte solutions will be enhanced by lower values of viscosity and higher values of your dielectric constant. Our experimental data and results also show how you can combine different electrolyte concentrations, solvents, and temperatures to achieve the desired properties for your battery formulations. To optimize your performance, you must achieve a balance amongst the concentrations and temperature, and because viscosity is sensitive to all these factors, you are able to accurately and reliably use viscosity measurements to optimize your battery performance.

For More Resources:

C-Therm and RheoSense joint webinar: https://ctherm.com/resources/webinars/ev_battery_fluid_characterization/

Detailed paper on the effect of salt on battery viscosity: https://www.rheosense.com/application-salt-concentration-of-battery

Detailed paper on battery viscosity as a function of temperature: https://www.rheosense.com/application-viscosity-temperature-dependence-of-battery-solutions

References:

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Logan, E. R., et al. “A study of the transport properties of ethylene carbonate-free Li electrolytes.” Journal of The Electrochemical Society 165.3 (2018): A705.

Bezabh, Hailemariam Kassa, et al. “Roles of film-forming additives in diluted and concentrated electrolytes for lithium metal batteries: A density functional theory-based approach.” Electrochemistry Communications 113 (2020): 106685.

Aktekin, Burak, et al. “Concentrated LiFSI–Ethylene Carbonate Electrolytes and Their Compatibility with High-Capacity and High-Voltage Electrodes.” ACS Applied Energy Materials (2022).

Sheng, Yangping. Investigation of electrolyte wetting in lithium ion batteries: Effects of electrode pore structures and solution. Diss. The University of Wisconsin-Milwaukee, 2015