BU-307: How does Electrolyte Work?

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.

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.