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1、 Composition of electrolyte in lithium-ion batteries

1. Organic solvents

The commonly used solvents in the electrolyte of lithium-ion batteries include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), etc. Propylene carbonate (PC), ethylene glycol dimethyl ether (DME) and other important solvents used in lithium primary batteries are generally not used. PC is used in secondary batteries and has poor compatibility with the graphite negative electrode of lithium-ion batteries. During the charging and discharging process, PC decomposes on the surface of the graphite negative electrode, causing the graphite layer to peel off, resulting in a decrease in the cycling performance of the battery.

It is generally believed that a mixed solvent of EC and a chain carbonate is an excellent electrolyte for lithium-ion batteries, such as EC+DMC, EC+DEC, etc. But it is not absolute, when PC and related additives are used in lithium-ion batteries, it is beneficial to improve the low-temperature performance of lithium-ion batteries.

The quality of organic solvents must be strictly controlled before use. If the purity is required to be above 99.9%, the moisture content must be below 10 * 10-6. Strictly controlling the moisture content of organic solvents has a decisive impact on the preparation of qualified lithium-ion battery electrolytes. The method of using molecular sieve adsorption, atmospheric or vacuum distillation, and introducing inert gas can achieve the required moisture content.

2. Electrolyte lithium salt

LiPF6 is the most commonly used electrolyte lithium salt and is the direction of future lithium salt development. LiPF6 is stable for the negative electrode, has a large discharge capacity, high conductivity, low internal resistance, and fast charging and discharging speed. However, it is extremely sensitive to moisture and HF acid, prone to reactions, can only operate in a dry atmosphere, and is not resistant to high temperatures. Decomposition reactions occur between 80 ℃ and 100 ℃, producing phosphorus pentafluoride and lithium fluoride, making purification difficult. Therefore, when preparing lithium-ion battery electrolytes, the self decomposition caused by the exothermic dissolution of LiPF6 and the thermal decomposition of solvents should be controlled.

3. Additives

There are various types of additives, and different lithium-ion battery manufacturers have different requirements for the purpose and performance of the battery. The focus of the selected additives also varies. Generally speaking, the additives used have three important uses:

Adding benzyl ether to the electrolyte of lithium-ion batteries can improve the performance of SEI membranes. Adding benzyl ether or its halogenated derivatives to the electrolyte of lithium-ion batteries can improve the cycling performance of the battery and reduce irreversible capacity loss;

Adding metal oxides to the electrolyte of lithium-ion batteries reduces trace amounts of water and HF acid in the electrolyte;

Prevent overcharging and over discharging.

2、 The production of electrolyte for lithium-ion batteries requires the necessary conditions

lithium ion batteries have high conductivity, generally reaching 10-3 to 2 * 10-3 S/cm; The migration number of lithium ions should be close to 1;

Wide potential range for electrochemical stability; There must be an electrochemical stability window of 0-5V;

Good thermal stability, wide temperature range for use;

Stable chemical performance, no chemical reaction with the current collector and active substances inside the battery;

Safe and low toxicity, preferably biodegradable.

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