The electrolyte acts as a catalyst that makes the battery conductive by facilitating the movement of ions from the cathode to the anode when charging and the reverse when discharging. Ions are charged atoms that lose or gain electrons, and the electrolyte of a battery consists of soluble salts, acids, or other bases in liquid, gelled, and dry form. Electrolytes also come from polymers used in solid-state batteries, solid-state ceramics, and molten salts such as sodium-sulfur batteries.
Lead Acid Battery
Lead-acid batteries use sulfuric acid as an electrolyte. When charging, the acid becomes denser as lead oxide (PbO2) forms on the positive plates, and then becomes almost water when fully discharged. Lead-acid batteries are available in overflow and sealed form, also known as Valve Regulated Lead Acid (VRLA) or maintenance-free.
Sulfuric acid is colorless, slightly yellowish-green, soluble in water, and highly corrosive. Anode corrosion or water entering the battery pack may rust, resulting in a yellowish color.
Lead-acid batteries have different specific gravity (SG). Deep-cycle batteries use a dense electrolyte with an SG of up to 1.330 to achieve high specific energy, with an average SG of about 1.265 for entry-level batteries and a lower SG of about 1.225 for stationary batteries to moderate corrosion and extend service life.
Sulfuric acid has a wide range of applications and is also found in drain cleaners and various cleaners. It also provides services in mineral processing, mineral processing, fertilizer manufacturing, oil refining, wastewater treatment, and chemical synthesis.
Ni-Cd Battery
The electrolyte of a nickel-cadmium battery is an alkaline electrolyte (potassium hydroxide). Most nickel-cadmium batteries are cylindrical, with several layers of positive and negative electrode material rolled into a jelly roll. Water-immersed nickel-cadmium batteries are used as marine batteries for commercial aircraft, as well as UPS systems that operate in hot and cold climates that require frequent cycling. Nickel-cadmium is more expensive than lead-acid but has a longer lifespan.
Ni-Mh Battery
Nickel-metal hydride uses the same or similar electrolyte as nickel-cadmium, usually potassium hydroxide. NiMH electrodes are unique in that they are composed of nickel, cobalt, manganese, aluminum, and rare earth metals, which are also used in lithium ions. NiMH is only available in a sealed version.
Potassium hydroxide is an inorganic compound with the general formula KOH, commonly known as caustic potassium. Electrolytes are colorless and have many applications in industry, such as the ingredients of most soft and liquid soaps.
Li-ion Battery
Lithium-ion batteries use liquid, gel, or dry polymer electrolytes. The liquid form, which is the flammable organic form, not the aqueous form, is a solution formed by lithium salts with an organic solvent similar to ethylene carbonate. Mixing the solution with a variety of carbonates provides higher conductivity and an extended temperature range, and other salts can also be added to reduce outgassing and improve high-temperature cycling.
Lithium-ion with a gelatinized electrolyte undergoes many additives to increase conductivity, as do lithium-polymer batteries. True dry polymers are conductive only at high temperatures, and the battery is no longer commercially available. Additives are also added to achieve longevity and unique properties. The recipes are classified, and each manufacturer has their own secret formula.
The electrolyte should be stable, but this is not the case with lithium. A passivation film is formed on the anode and is called the Solid Electrolyte Interface (SEI). This layer separates the anode from the cathode but allows ions to pass through like a separator. Essentially, an SEI layer must be formed for the battery to function properly. Thin films stabilize the system and extend the life of lithium-ion batteries, but this results in reduced capacity. Electrolyte oxidation also occurs at the cathode, which permanently reduces capacity.
In order to prevent the film from becoming too restrictive, additives are mixed in the electrolyte consumed during the formation of the SEI layer. It is difficult, if not impossible, to trace their presence when conducting detection assessments. This makes proprietary additives a trade secret, both in terms of their composition and the amount used.
A well-known additive is ethylene carbonate (VC). This chemistry can increase the cycle life of lithium ions, especially at higher temperatures, and maintain a lower internal resistance with use and aging. VC also maintains a stable SEI film on the anode, and electrolyte oxidation has no adverse side effects on the cathode (Aurbach et al.). It is said that both academia and research lag behind battery manufacturers in the understanding and selection of additives, so there is a big recipe.
For most commercial lithium-ion batteries, the SEI layer breaks down at a cell temperature of 75–90°C (167–194°F). The type of battery and state of charge (SoC) affect breakdown at high temperatures. If it is not cooled properly, self-heating behavior can occur, resulting in thermal runaway. Laboratory tests on 18,650 cells have shown that this thermal event can take up to two days to form.
The flammability of lithium-ion electrolytes is a further concern, and experiments have been carried out to produce non-flammable or less flammable electrolytes by additives or the development of non-organic ionic liquids, as well as studies to operate lithium-ion batteries at low temperatures.