How to Choose the Right Battery Electrolyte for Your Application: Available Electrolyte Types – Part 1

The fox seemed perplexed, and very curious.

“Are there hunters on that planet?”

“No.”

“Ah, that is interesting! Are there chickens?”

“No.”

“Nothing is perfect,” sighed the fox.

(Antoine de Saint-Exupéry: The Little Prince)

This is a dialogue from Exupéry’s famous book for children, in fact written for grown-ups. It came to my mind, when I was asked to come up with a short write-up on the finding the right battery electrolyte for an application. I may sigh as the fox, given that there is no perfect solution to this question. There are, however, many trade-offs to consider in the quest for the ideal electrolyte for a custom-designed battery. While there is no recipe for a “magic electrolyte”, you may find it useful to learn, when to choose a battery operated with one electrolyte or another. The discussion below presents a few electrolyte types with advantages and disadvantages. A subsequent blog will discuss how to select the right electrolyte for your application.

What is the role of the battery electrolyte?

Any electrochemical system (either power supply or electrolysis cell) consists of two interfaces, where the mechanism of conductivity changes, namely:  electronic conductor │ ionic conductor │ electronic conductor. The two electronic conductors are the electrodes, while the ionic conductor is the electrolyte. Should the two electrodes get in electrical contact with one another, it would cause a short circuit.

An electrolyte closes the internal electrical circuit of the cell; when the electrodes are placed in an electrolyte and a voltage is applied or generated, the electrolyte will conduct electricity. Hence, it acts as a charge carrier in the internal circuit between the positive and negative electrodes, enabling electrons (i.e., current) to flow through the external circuit.

Ideally, an electrolyte should have a high ionic conductivity, which allows the battery’s internal resistance to be minimized, (this is more important for high power density applications) ; by contrast, its electronic conductivity should be very low (i.e., its electronic resistance high), which minimizes the self-discharge rate of the battery, which translates to long shelf life. It should also not decompose at the voltages of interest, operate across the desired temperature range, and not pose an unacceptable safety risk in the application of interest.

What battery electrolyte types can you choose from?

As an ionic conductor, an electrolyte always contains a salt (positive and negative ions), dissolved in a solvent, or dispersed in a gel or in a solid medium. The solvent may be water or an organic liquid, while the gel can be water-based (hydrogel) or organic (organo-gel). The gel can be regarded as a liquid contained in a flexible lattice framework. Batteries can operate with either aqueous or non-aqueous (organic) electrolytes.

 Some electrolyte types that you can choose from include:

1. Organic electrolyte – An organic electrolyte is a good choice if the battery is required to operate in a wide voltage range (this implies that it can work at higher nominal voltage) and down to very low temperatures (such as -60 oC).

Disadvantages of such electrolytes include their flammability, high vapor pressure (i.e., easy evaporation), toxicity, and higher cost than aqueous electrolytes. They also have a lower conductivity than aqueous electrolytes. Typically this means that cell design has to be relied on, to sufficiently lower the internal resistance, so as to enable use of these electrolytes in high power density applications.

The requirements governing the choice of the salts for organic electrolytes are their compatibility with and solubility in the solvent, cost-effectiveness, and ease of handling. For example, liquid electrolytes in present day lithium-ion batteries (LIBs) consist of a lithium salt, such as LiPF6, LiBF4, or LiClO4, dissolved in an organic solvent, such as ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate, or their mixtures. Typical conductivities of liquid organic electrolytes are in the range of 10 mS cm-1 (at 20 oC), increasing by approximately 30–40% at 40 °C, and decreasing slightly at 0 °C. A mixture of carbonate solvents provides high ionic conductivity and low viscosity, as these two properties are commonly exclusive in a single material.

2. Room temperature ionic liquids – (IL) are a special kind of organic electrolytes, which offer a viable approach to limiting the flammability and volatility of organic electrolytes. They provide a large electrochemical window, of up to 6 V, and a high ionic conductivity (>10–4 S cm-1). For example, IL electrolyte may consist of (trifluoromethyl sulfonylimide) anions and 1-ethyl-3-methyleimidazolium cations. Safety was improved with such electrolyte mixtures; when IL content in the mixture is ≥40%, no flammability is observed. 

3. Aqueous electrolytes – can be acidic, quasi-neutral, or alkaline. . Typically, neutral aqueous electrolytes do not sustain battery operation, as there are no sufficient concentrations of either H+ or OH- ions needed by most cathode reactions.  Classic examples of aqueous electrolytes include potassium hydroxide in alkaline batteries and sulfuric acid in lead acid batteries.

There are several advantages of using water-based electrolytes: they are nonflammable; water can dissolve large concentrations of various ionic compounds, enabling high electrical conductivity; and aqueous electrolytes are lower cost than organic ones. High electrical conductivities are particularly attractive for high power density applications.

Disadvantages of water-based electrolytes include: they can only operate at lower nominal voltages than organic electrolytes, upto 2.1 V (given that water decomposes at high voltage), they are more corrosive than organic solutions (this can be controlled by adding a corrosion inhibitor), and release molecular hydrogen by unwanted chemical reactions (which can be reduced but not eliminated by adding a gassing control agent). Therefore, shelf life is generally lower for batteries operated with aqueous electrolytes as compared with organic electrolytes.

4. Solid electrolytes – are used in solid-state batteries, where both the electrodes and the electrolyte are solid. In most cases, solid-state batteries are low-power density/high-energy power sources. As expected, solid electrolytes are good ion conductors, while being insulating toward electrons. Examples of such materials include Ag4RbI5 for Ag+ ion conduction, LiI/Al2O3 mixtures for Li+ conduction, and β-alumina-type compounds for Na+ or divalent ions.

Advantages of batteries operated with solid electrolytes include the following: they can be produced in thin film form, so they are easy to miniaturize; solids prevent any problems related to electrolyte leakage; they are non-flammable, typically enable a very long shelf life, and can operate at constant parameters in a wide temperature range (no electrolyte freezing or boiling can occur). Owing to their high power-to-weight ratio, solid state batteries show promise for automotive use. A solid state electrolyte lithium-ion battery is expected to double energy density for one fifth of the cost of a traditional lithium-ion battery.

The table below lists some comparative properties of the battery electrolyte types described here.

Comparative Properties of Various Electrolytes

Electrolyte

Electrochemical Window

Conductivity

Temperature Range

Flammability

Cost

[V]

[mS cm-1]

[oC]

Aqueous Electrolytes

2.0-2.1

600-1300

0 to 100

None

Low

Organic Electrolytes

4.2

10-14

-30 to +150

High

Medium

Ionic Liquids

6.6

27

-150 to +260

None

High

Solid Electrolytes

5.1

1.1

-40 to +170

None

High

Clearly, there are a lot of factors to consider when selecting the right electrolyte. In this article, we have summarized the various types of electrolyte choices. In our next article, we will describe the application requirements to consider for selecting the best electrolyte. Subscribe to our blog for updates. You can also check out the battery fundamentals or the battery resources sections of our blog for useful information. As always feel free to contact us for battery development questions. 

About the Author

John

Hi, I'm John, editor-in-chief of an Flexel Battery online magazine!

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