Lithium battery refers to a battery that contains lithium (including metallic lithium, lithium alloys, lithium ions, and lithium polymers) in an electrochemical system.

The basic components of lithium-ion batteries include: positive electrode, negative electrode, electrolyte, other accessories, and so on. Lithium ion batteries have extremely important applications in electronic products, electric vehicles, aerospace and other fields due to their high energy density, high output voltage, long cycle life, and low environmental pollution.

The most important components in lithium-ion batteries are electrode materials and electrolyte. Among them, the main components of lithium battery electrolyte are introduced as follows:

Transparent colorless liquid (>35 ℃), crystalline solid at room temperature. Boiling point: 248 ℃/760mmHg, 243-244 ℃/740mmHg; Flash point: 160 ℃; Density: 1.3218; Refractive index: 1.4158 (50 ℃); Melting point: 35-38 ℃; This product is a good solvent for polyacrylonitrile and polyvinyl chloride. In the battery industry, it can serve as an excellent solvent for lithium battery electrolytes.

It is an excellent polar solvent. This product is mainly used for polymer operations, gas separation processes, and electrochemistry.

Colorless liquid with a slight odor; Vapor pressure 1.33kPa/23.8 ℃; Flash point 25 ℃ (flammable liquids can evaporate into steam and run into the air. As the temperature rises, evaporation accelerates.) Melting point -43 ℃; Boiling point 125.8 ℃; Solubility: Insoluble in water, miscible in most organic solvents such as alcohols, ketones, esters, etc; Density: Relative density (water=1) 1.0; Relative density (air=1) 4.07; Stability: Stable; Danger label 7 (flammable liquid); Main use: as a solvent and for organic synthesis.

solventIt is the foundation of the electrolyte for homework, as well as the carrier of solutes and additives. The properties of a solvent depend on its composition and molecular structure. The interaction forces between solvent molecules determine the viscosity, vapor pressure, freezing point, boiling point, and other properties of the solvent. The interaction force between solvent molecules and solute molecules also affects the solubility and solvation degree of solutes. Meanwhile, solvent is also one of the key factors determining the temperature scale of the electrolyte used in the operation, which has a great impact on the electrical function of electrolytic capacitors.

There are the following basic requirements for solvents used in the electrolyte for homework:

The main factors affecting the high-temperature long life of electrolytes include saturated vapor pressure value, high-temperature stability performance, flash voltage, oxidation efficiency, and oxide film performance. To ensure the high temperature and long service life of lithium-ion battery products, the electrolyte should have the following properties: extremely low saturation vapor pressure will reduce volatilization;Very goodHigh temperature stability to prevent degradation of high-temperature performance; The higher flashing voltage is to prevent breakdown; Higher oxidation efficiency can timely repair damaged oxide films to reduce leakage current, etc.

Among them, electrolyte is the key to high-temperature and long-life products of lithium-ion batteries. Due to the fact that the full vapor pressure of the electrolyte depends on the full vapor pressure of the solvent, electrolytes with small full vapor pressures can greatly improve the lifespan of electrolytic capacitors. In recent years, there have been frequent reports of fires and even explosions caused by lithium-ion batteries, mainly due to the fact that the higher the vapor pressure of the electrolyte, the greater the volatility. During the high-temperature use of the battery, the volatilization performance of the electrolyte is accelerated, resulting in excessive internal pressure of the battery. Therefore, the safety issues of lithium-ion batteries have attracted widespread attention; At the same time, safety issues are also a bottleneck that restricts the development of lithium-ion batteries towards large-scale and high-energy directions.

In the article "Research Progress on the Safety of Lithium ion Battery Electrolytes", the research status of non flash point solvents and flame retardant electrolytes is introduced from the perspectives of the combustion performance of electrolytes and the thermal stability of battery electrode materials. The influencing factors of the thermal stability between electrode materials and electrolytes, as well as the measures to improve their thermal stability, are also discussed.

The most important components of lithium-ion batteries are electrode materials and electrolyte. Lithium ion batteries use flammable organic solvents as electrolytes, which are prone to volatilization. It is one of the main causes of fire or explosion accidents in lithium-ion batteries. After the battery is damaged, organic solvents and their vapors are prone to catching fire, causing fires and even explosions. In addition, the safety performance of lithium-ion batteries also includes the thermal stability between electrode materials and electrolytes, including the thermal stability performance of the battery itself that is not damaged during normal charging and discharging processes, and even under abnormal abuse conditions.

Under normal circumstances, based on the traditional ammonium adipate/ethylene glycol water system working electrolyte, the boiling point of the electrolyte is increased and the vapor pressure is reduced by adjusting the components, forming an excellent electrolyte that meets the requirements. The applicable range is expanded from -40~+85 ℃ to -40~+105 ℃, and no additional additives need to be added.

In terms of solvents, we believe that using only ethylene glycol can already meet the requirements for use at 135 ℃. However, due to the use of airtight butyl rubber, we chose to mix ethylene glycol with other polyol solvents to reduce the saturated vapor pressure, which can effectively prevent premature bottoming of capacitors during high-temperature operation.

In the article "Electrochemical Behavior of Organic Electrolytes in Flexible Packaging Lithium ion Batteries" published by the School of Metallurgical Science and Engineering at Central South University, a flexible packaging lithium-ion battery was prepared and a lithium-ion battery composed of electrolytes and four mixed solvents was studied. And tested the vapor pressure of the electrolyte and the high-temperature storage performance of the battery.

In the article, the author studied the electrochemical properties of the electrolyte. Among them, the effect of gamma butyrolactone on the vapor pressure of electrolyte. When it comes to working in a temperature environment of 338K for flexible packaging lithium-ion batteries, the thickness change of the battery is small and the battery safety is good. The increase in battery thickness refers to the change in battery thickness under certain conditions based on the thickness of the battery after cremation. Due to the soft texture and low back pressure of the aluminum-plastic composite film used in soft packaging lithium-ion batteries, it can cause severe expansion of the battery. Therefore, it is required that the electrolyte has a lower vapor pressure in a temperature environment above 323K. So as to study how to effectively reduce the vapor pressure characteristics of the electrolyte.

Figure 3 shows the electrolyte 1mol/L LiPF6 EC/DMC (mass ratio 1:1), 1mol/L LiPF₆EC/GBL (mass ratio 4:3) and 1mol/L LiPF₆The vapor pressure of EC/DMC/GBL (mass ratio 4:4:3) in the temperature range of 273-373K. As shown in the figure, the vapor pressure value of EC/DMC electrolyte increases rapidly with the increase of temperature, especially when the temperature is above 313K, the rate of change in vapor pressure is very large. The changes in vapor pressure of EC/GBL and EC/DMC/GBL electrolytes are relatively small, indicating that these two electrolytes are less affected by temperature.

Figure 3 Relationship between electrolyte vapor pressure and temperature variation

a: 1mol/L LiPF6 EC/DMC (mass ratio 1:1)

b: 1mol/L LiPF6 EC/GBL (mass ratio 4:3)

c: 1mol/L LiPF6 EC/DMC/GBL (mass ratio 4:4:3)

Overall, the main purpose of conducting this experimental study is to investigate how to reduce the vapor pressure of the electrolyte. Because the higher the vapor pressure of the electrolyte, the greater the volatility of the electrolyte. For packaged lithium-ion batteries, especially under high temperature conditions, the internal pressure will also increase, which can easily lead to explosions and reduce the battery's service life.

Electrolytes are generally flammable organic solvents. If their vapor pressure is very high, they can easily evaporate, causing evaporation and loss during storage or use. At the same time, the released vapor combines with air and can easily cause fires or explosions when exposed to open flames. Therefore, the State Administration of Work Safety has issued the "Measures for the Identification and Classification of Physical Hazards of Chemicals" and the "Recommendations on the Transport of Dangerous Goods", which clearly stipulate the necessity of vapor pressure testing for the classification, packaging, storage, and transportation of flammable liquids in hazardous chemicals. Therefore, testing the vapor pressure is very necessary.

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