BU-307: How does Electrolyte Work?

Learn more about the catalyst that straddles the electrodes of a battery and makes electricity flow

Electrolyte serves as catalyst to make a battery conductive by promoting the movement of ions from the cathode to the anode on charge and in reverse on discharge. Ions are electrically charged atoms that have lost or gained electrons. The electrolyte of a battery consists of soluble salts, acids or other bases in liquid, gelled and dry formats.  Electrolyte also comes in a polymer, as used in the solid-state battery, solid ceramic and molten salts, as in the sodium-sulfur battery.

Lead Acid

Lead acid uses sulfuric acid. When charging, the acid becomes denser as lead oxide (PbO2) forms on the positive plate, and then turns to almost water when fully discharged. The specific gravity of the sulfuric acid is measured with a hydrometer. (See also BU-903: How to Measure State-of-charge). Lead acid batteries come in flooded and sealed formats also known as valve regulated lead acid (VRLA) or maintenance-free. 

Sulfuric acid is colorless with a slight yellow-green tint, soluble in water and is highly corrosive. Discoloration to a brownish tint may be caused by rusting from anodic corrosion or from water entering in the battery pack.

Lead acid batteries come with different specific gravities (SG). Deep-cycle batteries use a dense electrolyte with an SG of up to 1.330 to achieve high specific energy, starter batteries contain an average SG of about 1.265 and stationary batteries come with a low SG of roughly 1.225 to moderate corrosion and promote longevity. (See BU-903: How to Measure State-of-charge.).

Sulfuric acid serves a wide range of applications and is also found in drain cleaners and various cleaning agents. It further serves in mineral processing mineral processing, fertilizer manufacturing, oil refining, wastewater processing and chemical synthesis.

CAUTION:  Sulfuric acid can cause serious damage on skin contact and can lead to permanent blindness if splashed in eyes. Swallowing sulfuric acid causes irreversible damage.


Nickel-cadmium (NiCd)

The electrolyte in NiCd is an alkaline electrolyte (potassium hydroxide). Most NiCd batteries are cylindrical in which several layers of positive and negative materials are wound into a jelly-roll. The flooded version of NiCd is used as the ship-battery in commercial aircrafts and in UPS systems operating in hot and cold climates requiring frequent cycling. NiCd is more expensive than lead acid but lasts longer.

Nickel-metal-hydride (NiMH)

NiMH uses the same or similar electrolyte as NiCd, which is usually potassium hydroxide. The NiMH electrodes are unique and consist of nickel, cobalt, manganese, aluminum and rare earth metals, which are also used in Li-ion. NiMH is available in sealed versions only.

Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash. The electrolyte is colorless and has many industrial applications, such as the ingredient in most soft and liquid soaps. KOH is harmful if indigested.

Lithium-ion (Li-ion)

Li-ion uses liquid, gel or dry polymer electrolyte. The liquid version is a flammable organic rather than aqueous type, a solution of lithium salts with organic solvents similar to ethylene carbonate. Mixing the solutions with diverse carbonates provides higher conductivity and expands the temperature range. Other salts may be added to reduce gassing and improve high temperature cycling.

Li-ion with gelled electrolytes receives many additives to increase conductivity, so does the lithium-polymer battery. The true dry polymer only becomes conductive at elevated temperatures, and this battery is no longer in commercial use. Additives are also administered to achieve longevity and unique characteristics. The recipe is classified and each manufacturer has its own secret sauce. (See also BU-808b: What causes Li-ion to die?

The electrolyte should be stable, but this is not the case with Li-ion. A passivation film forms on the anode that is called solid electrolyte interface (SEI). This layer separates the anode from the cathode but allows ions to pass through much like a separator. In essence, the SEI layer must form to enable the battery to work. The film stabilizes the system and gives the Li-ion a long life but this causes a capacity reduction. Electrolyte oxidation also occurs on the cathode that permanently lowers the capacity. (See also BU-701: How to Prime Batteries)

To prevent the films from becoming too restrictive, additives are mixed with the electrolyte that is consumed during the formation of the SEI layer. It is difficult, if impossible, to trace their presence when doing a forensic evaluation. This keeps proprietary additives a trade secret, both their composition and the amount used. 

A well-known additive is vinylene carbonate (VC). This chemical improves the cycle life of Li-ion, especially at higher temperatures, and keeps the internal resistance low with use and age. VC also maintains a stable SEI film on the anode with no adverse side effects of the electrolyte oxidation on the cathode (Aurbach et al). It is said that academic and research communities are lagging behind cell manufacturers in knowledge and choice of additives, hence the great secret. (See also “Additives and the Effects on Coulombinc Efficiency” as part of  BU-808b: What causes Li-ion to die?

For most commercial Li-ion, the SEI layer will break down at a cell temperature of 75–90°C (167–194°F). The type of cell and state-of-charge (SoC) affects the breakdown at elevated temperature. A self-heating behavior may occur that can lead to a thermal runaway if not properly cooled. Lab tests done on 18650 cells have shown that such a thermal event can take two days to develop.

The flammability of the Li-ion electrolyte is a further concern and experiments are done to produce non-flammable or reduced flammable electrolytes by additives or developing non-organic ionic liquids. Research is also conducted to operate Li-ion at low temperatures. At time of writing, none of these electrolytes are in wide commercial use.

Drying up or slowly turning the liquid electrolyte into a solid form is one more aging event that lowers the performance of Li-ion. “When the liquid is gone, the batteries are dead,” says Jeff Dahn, specialist in Li-ion batteries and Professor of Physics. Liquidity of the electrolyte is one more state-of-health indicator that relates to all battery chemistries.

Last Updated 2017-10-06

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Comments (11)

On December 28, 2015 at 1:33pm
Thomas wrote:

What confuses me is, why doesn’t the anode and cathode short out through the electrolyte if it conducts electricity? Am I missing something here? Is it only able to conduct when the anode and cathode are connected externally?

On February 10, 2016 at 7:25pm
Michael Bates wrote:

Thomas, the electrolyte conducts ions, not electrons. The flow of electrons through the external circuit must be balanced by the flow of ions through the electrolyte (similar to a “salt-bridge” in a typical Gen.Chem. Cu/Zn galvanic cell demo). Therefore, if the ionic conductivity of the electrolyte is too low, that becomes the “bottleneck” limiting cell discharge rates.
For the author(s), it would be very helpful to expand this section somewhat to discuss the LiB solvent compositions. For instance, what effect does the relative composition of EC, DMC, DEC or EMC have on the battery performance?
Thanks for developing this outstanding web resource!

On March 7, 2016 at 8:07pm
Santa Claus wrote:

So basically the electrolyte is the 2 other parts combined? Very helpful

On May 15, 2016 at 9:49pm
Jason wrote:

So the functions of electrolyte are enabling the flow of mobile ions and balancing the charge of the chemical cell. Am I right?

On May 15, 2016 at 10:07pm
Jason wrote:

And I would like to ask why can’t the two electrodes be totally submerged into the electrolyte in a chemical cell?

On October 8, 2016 at 1:43pm
Clair Smyers wrote:

I do not understand the functions fully but what Thomas wrote was answered in the next comment. I did indeed remember that the internal conduction in the cell is ionic. The external conduction is electrons. It’s been a while since I have studied this but it’s worth revisiting. Thank you for what you wrote Micheal.

On January 10, 2017 at 3:27am
mir mehraj wrote:

Can SEI formation be prevented /restricted or completely avoided in Lithium batteries?

On February 16, 2017 at 12:02pm
Mohsin wrote:

How can we prepare Red lead (lead Dioxide) from lead at home in easy way smile

On May 4, 2017 at 4:04am
Nader Vakili wrote:

I understand the main idea in the battery is to use the affinity of one metal (zinc) to lose electron and the affinity of another (copper) to gain electron to produce a current by causing the electrons to flow from one metal to another. What I don’t understand are the following (in non-rechargable batteries):
1) How does the electrolyte (sulphuric acid) cause the release of electrons in the anode (Zn)?
2) What happens to electrons that arrive at the cathode (Cu)?
3) What chemical reaction is taking place at the cathode ? Does this reaction involve the electrons that arrive at the cathode?
4) In which chemical in the battery is the chemical energy stored?
5) What is physically happening to both electrodes during the life of the battery?
6) What does happen to the electrolyte during the course of the life of the battery?
7) Are the zinc and copper electrodes in their pure form or some variation of them (oxide, ion, etc)?
8) If the anode is zinc and the electrolyte is sulphuric acid, would they react as soon as the zinc is inserted or does the zinc anode have to be connected to the copper cathode for it to happen? If the connection to copper cathode is required, does the cathode have to also be inserted in the same sulphuric acid or can it be outside of it?
8) Do the same electrons repeat flowing through the circuit or does an electron’s flow end after only one trip through the circuit?

On November 25, 2017 at 1:12am
maybank bank wrote:

great blog and thanks for thepost

On May 22, 2018 at 3:44pm
Peter wrote:

Why electrolyte conducts ions if it consists of these? So, it conducts itsef? I don’t understand. Sorry for my English.