What are the effects of impurities in lithium ion electrolyte on battery perform
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2022/05/19 15:34:32
Organic electrolyte is an important part of lithium ion battery, which plays the role of charge transfer between anode and cathode in the battery, and has an important impact on the main performance of the battery, such as operating temperature, specific energy, cycle efficiency, safety and so on. The particularity of electrolyte composition also brings some particularity to battery performance. In a sense, organic electrolyte plays a decisive role in the performance of lithium ion battery. Organic electrolyte generally consists of two parts, electrolyte of lithium salt and organic solvents, the organic solvent is usually composed of two or more of the organic solvent mixed solvent, has been applied in the commercialization of the lithium ion battery electrolyte of lithium salt generally is six lithium fluoride phosphate, organic solvents are mainly EC (ethylene carbonate), PC (propylene carbonate), DMC (dimethyl carbonate), DEC(diethyl carbonate), EMC(methyl ethyl carbonate) and other chain and ring carbonate. There are two main factors affecting the advantages and disadvantages of organic electrolyte as lithium ion battery electrolyte: (1) the composition of organic electrolyte; (2) Organic electrolyte molecules containing active hydrogen substances and iron, sodium, aluminum, nickel and other metal ion impurities. This paper mainly describes the influence of impurities in organic electrolyte on the performance of organic electrolyte and the control of impurities in laboratory research and industrial production.
The influence of impurities on the performance of organic electrolyte The sources of impurities in organic electrolyte mainly include three aspects:
Contains a certain amount of impurities in the electrolyte of lithium salt, in six lithium fluoride phosphate preparation, inevitably causes the product contains quantitative HF, H2O and other metal impurity ions, at the same time because of HF, H2O molecules of hydrogen and oxygen in six lithium fluoride phosphate form hydrogen bond, thus forming strong chemical adsorption, led to their removal is difficult;
Organic solvents are mostly prepared from alcohols, which inevitably contain trace amounts of water, organic acids, alcohols, aldehydes, ketones, amines, amides and some metal ion impurities. ③ The air contains a certain amount of moisture (up to 1%) and trace dust, the operating environment control is not ideal or improper operation are likely to introduce the aforementioned impurities, these impurities can be avoided by establishing an appropriate clean and dry air system. According to the above analysis, the impurities in organic electrolyte mainly include three kinds of substances :① water and hydrofluoric acid; ② Organic acids, alcohols, aldehydes, ketones, amines and amides containing active hydrogen atoms;
Iron, nickel, sodium, aluminum and other metal impurities ions. The influence of various impurities on the performance of organic electrolyte will be introduced separately.
1.1 Influence of water and HYDROGEN fluoride content on the performance of organic electrolyte
The content of water and hydrogen fluoride is the most important factor affecting the performance of organic electrolyte. The influence of the content of water and hydrogen fluoride on the performance of lithium ion battery can be divided into two aspects: the influence of the content of water and hydrogen fluoride on the SEI film (solid electrolyte phase interface film) on electrode surface and the influence on the stability of electrolyte itself. In the first charging and discharging process of the battery, trace water and hydrogen fluoride will react with the reduction products of lithium alkyl carbonate on the electrode surface to form lithium carbonate and lithium fluoride or react with metal lithium to form lithium oxide, lithium carbonate and lithium fluoride and cover the electrode surface as components of SEI film. Lithium carbonate is insoluble in organic solvents and has good discoverability of lithium ions. It is an important component of SEI films with excellent properties. Lithium oxide and lithium fluoride are thermodynamically stable SEI film components, which are of great significance for stabilizing other SEI film components such as lithium carbonate. Some studies have shown that the presence of trace water in DMC-based electrolyte not only does not damage the performance of graphite electrode, but also greatly improves it. Thus, in this respect, the presence of trace water and hydrogen fluoride in organic electrolytes is of some importance. When the content of water and hydrogen fluoride in organic electrolyte is high, water and hydrogen fluoride can react with lithium, on the one hand, consume limited in lithium ion batteries, so that increase the irreversible capacity of the battery, on the other hand, the reaction product of lithium bromide and lithium fluoride against electrode electrochemical performance improvement, can have the gas at the same time, the reaction product to produce pressure inside the cell. With the increase of water and hydrogen fluoride content in organic electrolyte, the performance of lithium ion battery such as charge and discharge, cycle efficiency will decrease obviously, when the content is more than 0.1%, lithium ion battery will be completely destroyed. The water contained in the organic electrolyte will react with the organic solvent to generate the corresponding alcohols and acids. Take PC as an example :PC+H2O propylene glycol +CO2 propylene glycol will react with lithium hexafluorophosphate to generate the corresponding lithium salt and hydrogen fluoride. At the same time, trace water in the electrolyte will also react with lithium hexafluorophosphate. The hydrolysis reaction generally includes the following processes. (1)LiPF6 decomposes into LiF and PF5LiPF6LiF+PF5. (2)PF5 reacts with trace water in the electrolyte to generate HF and POF3PF5+H2O2HF+POF3. The hydrogen fluoride produced in this process in turn catalyzes the acceleration of the above reaction After time, the color will become darker and the solution will become thicker. The water content will decrease and the corresponding hydrogen fluoride content will increase. When organic electrolyte containing hydrogen fluoride is used in lithium ion battery, hydrogen fluoride will react with anode material and SEI film to generate water, etc. Aurbach et al. believed that hydrogen fluoride and SEI film in EC based organic electrolyte mainly reacted as follows :(1)HF reacted with carbonate or carbonate salt on electrode surface to generate LiF and CO2, etc. The reduction reaction of Li2CO3+2HF2LiF+H2O+CO2(2)POF3 on the electrode surface first occurs, and then reacts with LiF to generate LiXPFYOX compounds, such as LiOPF2. The water and ethylene glycol produced in the reaction will react with lithium hexafluorophosphate to form hydrogen fluoride. The continuous cycle of this process leads to the reduction of specific capacity and cycle efficiency of the battery, until the entire battery is destroyed. Therefore, in practical lithium ion batteries, the content of water and hydrogen fluoride in organic electrolyte should be at least 0.006%.
1.2 The influence of active hydrogen atoms and organic substances in molecules on the performance of organic electrolyte
Molecules containing active hydrogen substances such as organic acids, alcohols, aldehydes, ketones, for the first time the battery charge and discharge process, generate the carboxylic acid lithium or alkoxy lithium compounds, such as the material has certain solubility in organic solvents, they can lead to the instability of the SEI film on the one hand, reduce the conductivity of the lithium ion, reduced the cell cycle efficiency; On the other hand, their reaction with lithium increases the battery's irreversible capacity. Amines and amides polymerize in the charging and discharging process, reducing the conductivity of the electrolyte. These substances will also react with the electrolyte lithium hexafluorophosphate to produce HF. According to the above analysis, the smaller the impurity mass containing active hydrogen atoms in the organic electrolyte, the better the performance of the battery. Generally, the content of these impurities should be less than 0.008%.
1.3 Influence of metal impurity ions such as iron, nickel, sodium and aluminum salts on the performance of organic electrolyte
Metal impurity ions have lower reduction potential than lithium ions, so in the charging process, metal impurity ions will be embedded in the carbon anode first, reducing the position of lithium ion embedding, thus reducing the reversible capacity of lithium ion battery. The high concentration of metal ions not only reduces the reversible specific capacity of lithium-ion battery, but also leads to the failure of forming effective passivation layer on the surface of graphite electrode, which leads to the destruction of the battery. However, the radius of lithium ions is small, and the migration rate of lithium ions between graphite layers is higher than that of other metal ions. Therefore, the low concentration of metal impurity ions has little influence on the performance of batteries. Therefore, the content of metal impurity ions in organic electrolyte is generally required to be less than 0.007%.
This article is posted on Battery Community by Lankway Electronics Technology
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