【Science Popularization】Applications of Electrolyte Additives in Lithium Battery Industry
**I. Basic Concepts of Electrolyte Additives**
Electrolyte additives refer to a small amount of additives added to the electrolyte to improve its electrochemical properties and cathode deposition quality. Electrolyte additives are some natural or artificially synthesized organic or inorganic compounds. They generally do not participate in the electrode reactions of the electrolytic process, but can alter the electrochemical properties of the electrolyte system and influence the discharge conditions of ions to make the electrolytic process in a better state. By adding a small amount of additives to the electrolyte of lithium-ion batteries, some properties of the batteries can be improved in a targeted way, such as reversible capacity, electrode/electrolyte compatibility, cycle performance, rate capability and safety, etc. They play a very critical role in lithium-ion batteries. The amount of electrolyte additives is usually very small, but they are indispensable parts of the electrolyte system.
**II. Classification of Electrolyte Additives**
According to their different functions, electrolyte additives can be divided into **film-forming additives, flame retardant additives, overcharge protection additives, high/low temperature additives**, etc.
**1. Film-forming additives**
The role of film-forming additives is to promote the formation of a stable and effective SEI film on the surface of the electrode material. The SEI film is a passivation film formed during the initial cycle of lithium-ion batteries, separating the electrolyte from the carbon material/lithium negative electrode. At a certain potential, the organic solvent molecules, anions of lithium salts, impurities and additives on the negative electrode/electrolyte will undergo reductive decomposition to form insoluble substances deposited on the electrode surface. The properties of the SEI film greatly affect the electrochemical properties of lithium-ion batteries such as the initial irreversible capacity loss, rate capability and cycle life. The ideal SEI film is electronically insulating while allowing free ingress and egress of lithium ions from the electrode, can prevent further reaction between the electrode material and the electrolyte and has structural stability without dissolving in organic solvents.
**2. Flame retardant additives**
The role of flame retardant additives is to increase the flash point of the electrolyte or terminate the free radical chain reactions to prevent combustion. Adding flame retardants is one of the important ways to reduce the flammability of the electrolyte, expand the operating temperature range of lithium batteries, and improve their performance.
The mechanism of flame retardant additives mainly has two types: one is to generate an isolation layer between the vapor phase and the condensed phase to prevent combustion between the condensed phase and the vapor phase; the other is to capture free radicals in the combustion reaction process to terminate the free radical chain reaction and prevent combustion reactions between the vapor phases. Substances mainly used for flame retardation in lithium-ion batteries currently include organophosphates, organic halides and phosphonitriles.
**3. Overcharge protection additives**
The main function of overcharge protection additives is to block the current in a certain way when the battery is overcharged, thereby improving battery safety.
Overcharge additives have good solubility and fast diffusion rate, and can provide protection over a wide range of high current. They have good stability within the operating temperature range of the battery, and have an appropriate oxidation potential between the charging cutoff voltage and the oxidation potential of the electrolyte. In addition, their oxidation products have no other side reactions during the reduction process, are not consumed during charging, and have no side effects on battery performance.
**4. High/Low temperature additives**
Electrolyte high/low temperature additives are chemicals used to improve electrolyte performance under high and low temperature conditions. Their role is to enhance battery performance, stability and lifetime. At high temperatures, the electrolyte may evaporate, decompose and accelerate internal reactions. Therefore, high temperature additives can reduce the evaporation rate of the electrolyte, increase thermal stability, and reduce adverse reactions. While at low temperatures, the ionic conductivity of the electrolyte may decrease, affecting battery performance. Therefore, low temperature additives can improve the low temperature ionic conductivity of the electrolyte and enhance power output and cycling performance.
**5. Other additives**
There are additives with specific requirements such as water removal or HF additives, wetting promoters, conductive additives, etc.
**III. Future Development Directions of Electrolyte Additives**
The development of electrolyte additives in battery technology has always been an active research direction. With the continuous increasing requirements for battery performance, electrolyte additives in the future may develop in the following directions:
**1. High temperature stability**: With the rapid development of electric vehicles and renewable energy, the requirements for battery stability under high temperature conditions are increasingly higher. Future electrolyte additives may focus on improving the thermal stability of the electrolyte, inhibiting the decomposition and evaporation reactions of the electrolyte, thereby extending the service life of batteries.
**2. Safety and durability**: Battery safety has always been an important concern. Future electrolyte additives may focus on improving battery safety, for example by adding additives that can absorb and solidify liquids to reduce the risk of battery leakage or short circuits. In addition, durability is also a critical factor, and future additives may extend battery cycle life and capacity retention.
**3. Low temperature performance**: Under extremely cold environments, battery performance is usually limited. Future electrolyte additives may focus on improving battery power output and cycling performance under low temperature environments to meet the needs of electric vehicles and portable electronic devices in frigid regions.
**4. Environmental friendliness**: With the pursuit of sustainable development, future electrolyte additives may trend towards more environmentally friendly choices. This includes the use of renewable materials, reducing or eliminating reliance on toxic substances, and improving the degradability of additives to reduce their environmental impact.
**5. Multifunctionality**: Future electrolyte additives may have multiple functions to meet the needs of different applications. For example, additives with self-healing capabilities can repair minor damage inside batteries to prolong battery life; additives with self-sensing capabilities can monitor and adjust electrolyte performance internally.
**IV. Conclusion**
Electrolyte additives are chemicals added to the electrolyte to improve battery performance under different temperatures and application conditions. They can enhance battery stability and performance under high and low temperature environments and extend battery life. Future development directions include improving high temperature stability, safety and durability, enhancing low temperature performance, pursuing environmental friendliness and multifunctionality. The specific additives used and their types and amounts depend on the battery type, application scenario and design requirements. In addition, research and development of electrolyte additives is a complex field involving interdisciplinary research and practice across multiple disciplines.
