The electrolyte acts as a catalyst that makes the battery conductive by facilitating the movement of ions from the cathode to the anode during charging and in the reverse direction during discharge. Ions are electrically charged atoms that have lost or gained electrons, and the electrolyte of a battery consists of soluble salts, acids, or other bases in liquid, gelled, and dry forms. Electrolytes also come from polymers, such as those used in solid-state batteries, solid-state ceramics, and molten salts such as sodium-sulfur batteries.
Lead-acid batteries
Lead-acid batteries use sulfuric acid as an electrolyte. When charged, the acid becomes more dense as lead oxide (PbO2) forms on the positive plate, then becomes almost water when fully discharged. Lead-acid batteries are available in flooded and sealed forms, also known as valve-regulated lead acid (VRLA) or maintenance-free.
Sulfuric acid is colorless, slightly yellow-green, soluble in water, and highly corrosive. Corrosion of the anode or water that enters the battery pack may cause rust, which results in a yellowish color.
Lead-acid batteries have different specific gravities (SG). Deep cycle batteries use dense electrolytes with SG up to 1.330 to achieve high specific energy, entry level batteries have an average SG of about 1.265, while stationary batteries have a lower SG of about 1.225 to mitigate corrosion and extend service life.
Nickel-Cadmium (NiCd) Batteries
The electrolyte in NiCd batteries is an alkaline electrolyte (potassium hydroxide). Most NiCd batteries are cylindrical in shape, with several layers of positive and negative electrode materials rolled into a jelly roll. Flooded NiCd batteries are used as marine batteries for commercial aircraft, and in UPS systems operating in hot and cold climates where frequent cycling is required. NiCds are more expensive than lead acid, but have a longer lifespan.
Nickel-Metal Hydride (NiMH) Batteries
NiMH uses the same or similar electrolyte as NiCd, usually potassium hydroxide. NiMH electrodes are unique, consisting of nickel, cobalt, manganese, aluminum, and rare earth metals, which are also used in lithium-ion. NiMH is available only in sealed versions.
Potassium hydroxide is an inorganic compound with the general formula KOH, commonly known as caustic potash. The electrolyte is colorless and has many industrial applications, such as being an ingredient in most soft and liquid soaps.
Lithium-ion (Li-ion) Batteries
Lithium-ion batteries use liquid, gel, or dry polymer electrolytes. The liquid form is a flammable organic form, rather than an aqueous form, and is a solution of lithium salts with an organic solvent similar to ethylene carbonate. Mixing the solution with various carbonates provides higher conductivity and a wider temperature range. Other salts can be added to reduce gassing and improve high temperature cycling.
Lithium-ion with gelled electrolytes accepts many additives to increase conductivity, as do lithium polymer batteries. True dry polymers are only conductive at high temperatures, and this battery is no longer in commercial use. Additives are also added to achieve longevity and unique characteristics. Formulations are classified, and each manufacturer has its own secret recipe.
Electrolytes should be stable, but this is not the case with lithium-ion. A passivating film forms on the anode, called the solid electrolyte interface (SEI). This layer separates the anode from the cathode, but allows ions to pass through like a separator. Essentially, the SEI layer must form for the battery to work properly. The film stabilizes the system and extends the life of the lithium-ion battery, but this results in reduced capacity. Electrolyte oxidation also occurs at the cathode, permanently reducing capacity.
To prevent the film from becoming too localized, additives are mixed into the electrolyte consumed during the SEI layer formation process. Their presence is difficult or even impossible to trace when evaluated for detection. This makes proprietary additives a trade secret, both in terms of their composition and the amount used.
One well-known additive is ethylene carbonate (VC). This chemistry improves the cycle life of lithium-ion batteries, especially at higher temperatures, and maintains low internal resistance with use and aging. VC also maintains a stable SEI film at the anode, with no adverse side effects of electrolyte oxidation at the cathode (Aurbach et al.). It is said that the academic and research communities lag behind battery manufacturers in their understanding and selection of additives, so there is a lot of secrets.
For most commercial lithium-ion batteries, the SEI layer decomposes at cell temperatures of 75–90°C (167–194°F). The type of battery and the state of charge (SoC) affect breakdown at high temperatures. If not properly cooled, self-heating behavior can occur, leading to thermal runaway. Laboratory tests on 18650 cells have shown that such thermal events can take up to two days to develop.
The flammability of lithium-ion electrolytes is a further concern, and experiments have been conducted to produce non-flammable or reduced-flammability electrolytes through additives or development of non-organic ionic liquids, and research has also been conducted on operating lithium-ion batteries at low temperatures.