What is electrolyte?
The main materials of lithium-ion batteries are: positive electrode, negative electrode, electrolyte, and separator . As one of the four key materials of lithium-ion batteries, electrolyte is called the "blood" of lithium-ion batteries.
From the working principle of lithium ions, lithium-ion batteries are secondary batteries (rechargeable batteries) that mainly rely on the movement of lithium ions between the positive and negative electrodes to work. During the charging and discharging process, Li+ is embedded and de-embedded between the two electrodes: when charging, Li+ is de-embedded from the positive electrode and embedded in the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite is true during discharge.
In this process, the electrolyte is the carrier of ion transmission in the battery, playing the role of conducting ions between the positive and negative electrodes of the lithium battery, and is the guarantee for the lithium-ion battery to obtain the advantages of high voltage and high specific energy.
As an important component of lithium-ion batteries, electrolyte plays the role of transporting ions and conducting current between the positive and negative electrodes , and has a great influence on the performance of the current. As the transfer medium of lithium ions in lithium-ion batteries, electrolyte directly contacts the positive and negative electrode materials. Selecting a suitable electrolyte is also the key to obtaining lithium-ion secondary batteries with high energy density and power density, long cycle life and good safety.
Therefore, the basic performance of the electrolyte needs to meet three aspects: first, to provide enough lithium ions to help achieve ion transfer; second, to form a passivation film on the positive electrode surface to inhibit the corrosion of the positive electrode and protect the positive electrode material; third, to form a SEI film on the negative electrode surface to increase the service life of the negative electrode material.
Types of lithium battery electrolyte
The lithium-ion battery electrolytes currently used are divided into three categories: liquid, all-solid and gel-type polymer electrolytes.
1. Liquid electrolyte
Liquid electrolyte is the earliest type of electrolyte used in lithium batteries. Its main components include lithium salt, organic solvent and additives. Among them, lithium salt plays the role of conducting lithium ions, organic solvent is the carrier of lithium ion migration in the battery, and additives can improve the stability and conductivity of the electrolyte.
2. Gel electrolyte
Gel electrolyte is an electrolyte between liquid and solid. It is characterized by high ionic conductivity and low risk of liquid leakage. Gel electrolyte is mainly composed of polymer matrix, lithium salt, organic solvent and additives. By adjusting the ratio of polymer matrix and lithium salt, the gelation of electrolyte can be achieved, thereby improving the safety and cycle life of the battery.
3. Solid electrolyte:
Solid electrolyte refers to lithium battery electrolyte that does not contain organic solvents. Its main components are lithium salt, polymer matrix and additives. Solid electrolyte has higher safety and energy density, but it still faces challenges in ion conductivity and battery cycle life.
What are the components of lithium battery electrolyte?
Electrolyte plays a very important role in lithium batteries. What are its components? Let's take a closer look.
The main components of the electrolyte are high-purity organic solvents, electrolyte lithium salts and necessary additives , which are prepared under certain conditions and in certain proportions.
The electrolyte's solute accounts for the highest cost, approximately 60%, thus significantly influencing the electrolyte's price. Its function is to ensure an ample supply of lithium ions during the battery's charge and discharge cycles. Currently, lithium hexafluorophosphate is the most widely used solute. The solvent constitutes about 30% of the cost and over 80% of the mass, with carbonates being the primary solvents in use. Additives make up about 10% of the cost and are a key factor in differentiating the competitiveness of electrolytes.
1. Solute lithium salt
The solute determines the basic physical and chemical properties of the electrolyte and has an important influence on the characteristics of lithium batteries.
The lithium salt used in the electrolyte provides a large amount of free lithium ions during the charge and discharge process. The type and properties of the lithium salt determine the electrochemical properties of the electrolyte, such as charge and discharge efficiency, stability, and service life . As a provider of lithium ions, lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), and a new lithium salt, lithium bis(fluorosulfonyl)imide (LiFSI), are generally used.
Lithium hexafluorophosphate (LiPF6) has high conductivity and strong electrochemical stability in the electrolyte. It is currently the most mainstream electrolyte variety in the world and is widely used in lithium battery products such as new energy vehicles. Currently, the most widely used and technologically mature routes mostly use lithium phosphate technology.
As the most commonly used electrolyte lithium salt, LiPF6 is the future direction of lithium salt development. Although LiClO4, LiAsF6 and other electrolytes are also used in laboratories, the high temperature performance of batteries using LiC104 is poor. In addition, LiCl04 itself is prone to explosion when hit and is a strong oxidant. It is not safe to use in batteries and is not suitable for large-scale industrial use of lithium-ion batteries.
LiPF6 is stable to the negative electrode, has a large discharge capacity, high conductivity, small internal resistance, and fast charge and discharge speed. However, it is extremely sensitive to moisture and HF acid and is prone to reaction. It can only be operated in a dry atmosphere (such as a glove box with an ambient moisture content of less than 20x10). It is not resistant to high temperatures and decomposes at 80°C to 100°C to generate phosphorus pentafluoride and lithium fluoride, which are difficult to purify. Therefore, the self-decomposition caused by the heat of dissolution of LiPF6 and the thermal decomposition of the solvent should be controlled when preparing the electrolyte.
2. Solvent
The solvent paves a way for lithium ions to be transported, playing a role similar to that of water in a diesel engine.
The commonly used organic solvents in lithium batteries are polar aprotic solvents, mainly carbonates and carboxylates. Solvents account for about 30% of the cost of electrolytes, mainly carbonate solvents, including dimethyl carbonate (DMC), diethyl carbonate, ethyl methyl carbonate, ethylene carbonate and propylene carbonate. Currently, the commonly used electrolyte solvents are a mixed system of 5 carbonate solvents, among which dipropyl dimethyl carbonate (DMC) has the highest permeability.
①Organic carbonate
Organic carbonates are a common component of lithium battery electrolytes. They can expand the operating temperature range of the electrolyte and increase the lithium ion transfer rate. For example, dipropyl dimethyl carbonate (DMC) and ethylene dimethyl carbonate (EMC). The advantages of DMC and EMC are high chemical stability and low volatility. Therefore, they are the components used by some high-end lithium battery manufacturers in making electrolytes.
②Organophosphate
Organic phosphate is another common component of lithium battery electrolyte. It can improve the safety and high temperature resistance of the electrolyte, thereby reducing the risk of explosion under impact or extreme conditions. At the same time, organic phosphate also has high ion conductivity, especially tributyl trimethyl phosphate (TBAP), which has high solid-state conductivity at room temperature and can improve the low-temperature performance of lithium batteries.
3. Additives
Additives, with their low dosage and significant effects, offer an economical and practical approach to improving the performance of lithium-ion batteries. By incorporating small amounts of additives into the electrolyte of lithium-ion batteries, specific properties of the batteries can be selectively enhanced, such as reversible capacity, electrode/electrolyte compatibility, cycling performance, rate capability, and safety. These additives play a crucial role in the functionality of lithium-ion batteries.
Ideal lithium-ion battery electrolyte additives should have the following characteristics:
(1) High solubility in organic solvents;
(2) A small amount of addition can significantly improve one or several properties;
(3) Do not produce harmful side reactions with other components of the battery, affecting battery performance;
(4) Low cost, non-toxic or low toxicity.
Commonly used additives include film-forming additives, conductive additives, flame retardant additives, overcharge protection additives, etc.
In 2023, the global shipment of lithium-ion battery electrolytes reached 1.312 million tons, representing a year-on-year growth of 25.8%. Among these, China's electrolyte shipments amounted to 1.138 million tons, marking a year-on-year increase of 27.7%. China's share of global electrolyte shipments continued to rise, reaching 86.7%.
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