Electrolytes act as catalysts that enable batteries to conduct electricity by facilitating the movement of ions from the cathode to the anode during charging and in the opposite direction during discharging. Ions are charged atoms that lose or gain electrons, and battery electrolytes consist of soluble salts, acids, or other bases in liquid, gel, and dry forms. Electrolytes are also derived 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 the electrolyte. During charging, the acid becomes denser as lead oxide (PbO2) forms on the positive plate, then becomes almost water upon full discharge. Lead-acid batteries are available in overflow and sealed forms, also known as valve-regulated lead-acid (VRLA) or maintenance-free.
Sulfuric acid is colorless, slightly yellowish-green, soluble in water, and highly corrosive. Anodic corrosion or water entering the battery pack can cause rust, resulting in a yellowish tinge.
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 lifespan.
Sulfuric acid has a wide range of applications and can be found in drain cleaners and various cleaning agents. It also serves in mineral processing, fertilizer manufacturing, oil refining, wastewater treatment, and chemical synthesis.
Nickel-cadmium (NiCd) batteries
Nickel-cadmium (NiCd) batteries use an alkaline electrolyte (potassium hydroxide). Most NiCd batteries are cylindrical, with several layers of positive and negative electrode material rolled into a jelly-like roll. Water-immersed NiCd batteries are used as marine batteries in commercial aircraft and in UPS systems that operate in hot and cold climates requiring frequent cycling. NiCd is more expensive than lead-acid batteries but has a longer lifespan.
Nickel-metal hydride (NiMH) batteries
Nickel-metal hydride (NiMH) uses the same or similar electrolyte as nickel-cadmium (NiCd), typically potassium hydroxide. The NiMH electrode is unique, composed of nickel, cobalt, manganese, aluminum, and rare earth metals, which are also used in lithium-ion batteries. NiMH is only available in a sealed version.
Potassium hydroxide is an inorganic compound with the general formula KOH, commonly known as caustic soda. Electrolytes are colorless and have many industrial applications, such as being a component of 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, not an aqueous one, and is a solution of lithium salt with an organic solvent similar to ethylene carbonate. Mixing the solution with various carbonates can provide higher conductivity and a wider temperature range. Other salts can be added to reduce outgassing and improve high-temperature cycling.
Lithium-ion batteries with gelled electrolytes accept numerous additives to increase conductivity, as do lithium polymer batteries. True dry polymers only exhibit conductivity at high temperatures, and such batteries are no longer used commercially. Additives are also added to achieve longevity and unique properties. Formulations are categorized, and each manufacturer has its own secret formula.
Electrolytes should be stable, but this is not the case for lithium-ion batteries. A passivation 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, an SEI layer must form for the battery to function properly. The thin film stabilizes the system and extends the lifespan of a lithium-ion battery, but this leads to a reduction in capacity. Electrolyte oxidation also occurs on the cathode, permanently reducing capacity.
To prevent the film from becoming too confined, additives are mixed into the electrolyte consumed during SEI layer formation. Their presence is difficult, if not impossible, to trace during testing and evaluation. This makes the proprietary additives, both in their composition and dosage, trade secrets.
One well-known additive is ethylene carbonate (VC). This chemical can improve the cycle life of lithium-ion batteries, especially at higher temperatures, and maintain low internal resistance with use and aging. VC also helps maintain a stable SEI film on the anode, and electrolyte oxidation has no adverse effects on the cathode (Aurbach et al.). It is said that academia and research lag behind battery manufacturers in their understanding and selection of additives, thus possessing considerable expertise.
For most commercially available lithium-ion batteries, the SEI layer decomposes at battery temperatures of 75–90°C (167–194°F). Battery type and 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 18,650 batteries showed that such a thermal event can take up to two days to develop.
The flammability of lithium-ion electrolytes is a concern, and experiments have been conducted to produce non-flammable or less flammable electrolytes by using additives or developing non-organic ionic liquids. Research has also been carried out on operating lithium-ion batteries at low temperatures.
