What materials make up the electrolyte of a lithium-ion battery?

Electrolyte is an important part of lithium-ion battery, not only in the positive and negative electrodes to transport and conduct current, but also to a large extent to determine the working mechanism of the battery, affecting the specific energy, safety performance, multiplier charge and discharge performance, cycle life and production cost of the battery.

The electrolyte plays the purpose of conducting electrons between the positive and negative electrodes in lithium-ion batteries, and is the guarantee for lithium-ion batteries to obtain advantages such as high voltage and high specific energy. Electrolyte generally consists of high-purity organic solvent, electrolyte lithium salt (lithium hexafluorophosphate, LiFL6), the necessary additives and other raw materials, under certain conditions, in a certain proportion of the preparation.

1、Organic solvent

Organic solvent is the main part of the electrolyte, and the performance of the electrolyte is closely related to the performance of the solvent. The solvents commonly used in lithium-ion battery electrolyte are ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), methyl carbonate (EMC), etc. Generally, propylene carbonate (PC), ethylene glycol dimethyl ether (DME) and other important solvents used in lithium primary batteries are not used. During charging and discharging, PC decomposes on the surface of the graphite cathode and causes the flaking of the graphite layer, resulting in the degradation of the cycle performance of the battery. However, a stable SEI film can be established in EC or EC+DMC composite electrolyte. It is generally considered that the mixture of EC and a kind of chain carbonate solvent is an excellent electrolyte for lithium-ion batteries, such as EC+DMC, EC+DEC, etc. The same electrolyte lithium salt, such as LiPF6 or LiC104, PC+DME system about the intermediate phase carbon microsphere C-MCMB material always shows the worst charge/discharge performance (related to EC+DEC, EC+DMC system). However, it is not absolute, when PC with related additives is used for Li-ion batteries, it is beneficial to improve the low-temperature performance of the batteries.

Organic solvents must be strictly controlled in quality before use, such as the requirement of purity above 99.9% and the moisture content must be below 10*106. There is a close connection between the purity of the solvent and the stabilization voltage The oxidation potential of organic solvents of the purity standard is around 5V, and the oxidation potential of organic solvents is of great significance regarding the study of preventing battery overcharge and safety. Strictly control the moisture of organic solvent has decisive influence on the preparation of qualified electrolyte.

The water content is reduced to below 10*106, which can reduce the decomposition of LiPF6, slow down the decomposition of SEI film, and prevent gas rise, etc.

By using molecular sieve adsorption, atmospheric or reduced pressure distillation, and inert gas passage, the moisture content can be made to meet the requirements.

2、Lithium salt of electrolyte

LiPF6 is the most commonly used electrolyte lithium salt, which is the future direction of lithium salt development. Although the laboratory is also useful for LiClO4, LiAsF6 and other electrolytes, but because of the use of LiC104 battery high temperature performance is not good, coupled with LiC10: itself easy to explode by impact, and is a strong oxidizing agent, used in the battery safety is not good, not suitable for industrial large-scale use of lithium-ion batteries.

LiPF6 is stable to the negative electrode, large discharge capacity, high conductivity, small internal resistance, fast charging and discharging, but extremely sensitive to moisture and HF acid, easy to react, can only be operated in a dry atmosphere (such as environmental moisture less than 20 × 10 glove box), and is not resistant to high temperature, 80 ℃ ~ IO0 ℃ decomposition reaction, the generation of phosphorus pentafluoride and lithium fluoride, purification difficulties, so the preparation of electrolyte should be Therefore, when preparing the electrolyte, we should control the autolysis and thermal decomposition of the solvent caused by the exothermic dissolution of LiPF6.

3、Additives

There are a wide variety of additives, different lithium-ion battery manufacturers for different battery applications, performance requirements, the focus of the selected additives also differ. In general, the additives used are important for three purposes.

(1) The addition of anisole to the electrolyte to improve the performance of SEI films The addition of anisole or its halogenated derivatives to the electrolyte of lithium-ion batteries can improve the cycling performance of the battery and reduce the irreversible capacity loss of the battery. Huang Wenhuang has studied the mechanism and found that anisole reacts with the reduction products of the solvent to produce LiOCH, which facilitates the formation of an efficient and stable SEI film on the electrode surface, thus improving the cycling performance of the battery. The discharge plateau of the battery can measure the energy that the battery can release above 3.6V, which to some extent reflects the high current discharge characteristics of the battery. In practice, we found that adding anisole to the electrolyte can prolong the discharge plateau of the battery and improve the discharge capacity of the battery.

(2) Adding metal oxides reduces the trace water and HF acid in the electrolyte As mentioned earlier, lithium-ion batteries have very strict requirements for water and acid in the electrolyte. In addition, some metal oxides such as Al2O3, MgO, BaO, Li2Co3, CaCO3, etc. are used to remove HF, but the rate of acid removal is too slow compared with the hydrolysis of LiPFs, and it is difficult to filter out cleanly. The total content of Li, P and F in Li-ion battery electrolyte is 96.3%, and the total content of other important impurity elements such as Fe, K, Na, CI and A1 is 0.055%.

(3) Prevent overcharging and over discharging

Battery manufacturers have very urgent requirements for battery overcharge and discharge resistance. Traditional anti-overcharge through the battery internal protection circuit, now hope to add additives to the electrolyte, such as imidazole sodium circle, biphenyl class, carbazole class and other compounds, the class is in the research stage.