The organic electrolyte is an important component of the lithium ion battery. It plays the role of transferring charge between the positive and negative electrodes in the battery, and has an important influence on the main performances such as the operating temperature, specific energy, cycle efficiency and safety of the battery. The particularity of the electrolyte composition also brings some special characteristics to the performance of the battery. In a sense, the organic electrolyte plays a decisive role in the performance of the lithium ion battery. The organic electrolyte generally consists of an electrolyte lithium salt and an organic solvent. The organic solvent is generally a mixed solvent composed of two or more organic solvents. The electrolyte lithium salt used in a commercial lithium ion battery is generally lithium hexafluorophosphate. The organic solvent mainly includes chain and cyclic carbonates such as EC (ethylene carbonate), PC (propylene carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), and EMC (ethyl methyl carbonate). . There are two main factors affecting the advantages and disadvantages of organic electrolytes as electrolytes for lithium-ion batteries: 1 composition of organic electrolyte; 2 substances containing active hydrogen in organic electrolyte molecules and metal ions such as iron, sodium, aluminum and nickel The content of impurities. This paper mainly describes the effects of impurities in organic electrolytes on the performance of organic electrolytes, as well as the control of impurities in laboratory research and industrial production. 
Effect of impurities on the performance of the organic electrolyte organic electrolyte source of impurities in three main aspects:
Effect of impurities on the performance of the organic electrolyte organic electrolyte source of impurities in three main aspects:
- A certain amount of impurities contained in the lithium salt of the electrolyte, in the preparation of lithium hexafluorophosphate, inevitably, the product contains quantitative HF, H2O and other metal impurity ions, and at the same time, due to the hydrogen in the HF, H2O molecules and the oxygen in the lithium hexafluorophosphate Hydrogen bonds are formed, thereby forming a strong chemisorption, which makes it difficult to remove them;
- Organic solvents are mostly prepared from alcohols. It is inevitable to contain trace amounts of water, organic acids, alcohols, aldehydes, ketones, amines, amides and some metal ion impurities. 3 Air contains a certain amount of water (up to 1%) ) and traces of dust, uncontrolled or unreasonable operating environment, may introduce the aforementioned impurities, which can be avoided by establishing a proper clean dry air system. It can be seen from the above analysis that the impurities in the organic electrolyte mainly include three types of substances: 1 water and hydrofluoric acid; 2 organic acids, alcohols, aldehydes, ketones, amines, amides containing active hydrogen atoms;
- Metal impurity ions such as iron, nickel, sodium, and aluminum. The effect of various impurities on the performance of the organic electrolyte will be introduced separately.
1. 1 Effect of water and hydrogen fluoride content on the performance of organic electrolytes <br> The content of water and hydrogen fluoride is the most important factor affecting the performance of organic electrolytes. The effect of water and hydrogen fluoride on the performance of lithium-ion batteries can be divided into The effect of the SEI film on the electrode surface (solid electrolyte phase interface film) and the effect on the stability of the electrolyte itself are two aspects. Trace water and hydrogen fluoride in the first charge and discharge process of the battery will be the reduction product of the surface of the electrode, alkyl lithium carbonate, to form lithium carbonate, lithium fluoride, etc. or react with lithium metal to form lithium oxide, lithium carbonate and lithium fluoride. The components of the SEI film are overlaid on the surface of the electrode. Lithium carbonate is insoluble in organic solvents and has good lithium ion traceability, and is an important component for forming SEI films with excellent properties. Lithium oxide and lithium fluoride are thermodynamically stable SEI membrane components and are important for stabilizing other SEI membrane components such as lithium carbonate. Some research work has shown that the appearance of trace moisture in the DMC-based electrolyte not only does not cause any damage to the performance of the graphite electrode, but will be greatly improved. Therefore, in this respect, the presence of trace amounts of water and hydrogen fluoride in the organic electrolyte has a certain effect. When the content of water and hydrogen fluoride in the organic electrolyte is high, water and hydrogen fluoride react with lithium, which consumes limited lithium ions in the battery, thereby increasing the irreversible capacity of the battery, and on the other hand, a large amount of reaction products appear. Lithium oxide and lithium fluoride are detrimental to the improvement of the electrochemical performance of the electrode, and at the same time, gas products are generated in the aforementioned reaction to cause an increase in the pressure inside the battery. As the content of water and hydrogen fluoride in the organic electrolyte increases, the performance of charge and discharge, cycle efficiency, etc. of the lithium ion battery will decrease significantly. When the content exceeds 0.1%, the lithium ion battery will be completely destroyed. The water contained in the organic electrolyte reacts with the organic solvent to form the corresponding alcohol and acid. Taking PC as an example: PC + H2O propylene glycol + CO2 propylene glycol reacts with lithium hexafluorophosphate to form the corresponding lithium salt and hydrogen fluoride. At the same time, a trace amount of water in the electrolyte will also react with lithium hexafluorophosphate. The hydrolysis reaction generally includes the following processes. (1) LiPF6 is decomposed into LiF and PF5LiPF6 LiF + PF5(2) PF5 reacts with trace water in the electrolyte to form HF and POF3PF5 + H2O 2HF + POF3. The hydrogen fluoride produced in the process in turn catalyzes the acceleration of the above reaction. The electrolyte is not subjected to strict water removal and acid removal. After a certain period of time, the color will become darker and the solution becomes thicker. The content of water will become smaller, and the corresponding content of hydrogen fluoride will increase. When an organic electrolyte containing hydrogen fluoride is used for a lithium ion battery, hydrogen fluoride reacts with the positive electrode material and the SEI film to generate water or the like. Aurbach et al. believe that in the EC-based organic electrolyte, hydrogen fluoride reacts with the SEI membrane mainly in the following reactions: (1) HF reacts with carbonate or carbonate salts on the surface of the electrode to form LiF and CO2. Li2CO3 + 2HF 2LiF + H2O + CO2(2) POF3 firstly undergoes a reduction reaction on the surface of the electrode, and then reacts with LiF to form a LiXPFYOX type compound such as LiOPF2. The water and ethylene glycol produced in the reaction react with lithium hexafluorophosphate to form hydrogen fluoride, and the process continues to circulate, resulting in a decrease in the specific capacity of the battery, cycle efficiency, etc., until the entire battery is destroyed. 006 %。 In a practical lithium-ion battery, the content of water and hydrogen fluoride in the organic electrolyte should be at least less than 0. 006%.
1. 2 The influence of active hydrogen atoms and organic substances on the performance of organic electrolytes in the molecule<br> The organic acid, alcohol, aldehyde, ketone and other substances containing active hydrogen atoms in the molecule are generated during the first charge and discharge of the battery. a compound such as lithium carboxylate or lithium alkoxide, which has a certain solubility in an organic solvent, which on the one hand causes instability of the SEI film, lowers the conductivity of lithium ions, and reduces the cycle efficiency of the battery; In contrast, their reaction with metallic lithium increases the irreversible capacity of the battery. Amines and amides undergo polymerization during charge and discharge, which lowers the conductivity of the electrolyte. At the same time, these substances will also react with the electrolyte lithium salt lithium hexafluorophosphate to form HF. 008 %。 The above analysis, the organic electrolyte containing active hydrogen atoms, the smaller the amount of impurities, the more favorable the improvement of the performance of the battery, the content of these impurities should be at least less than 008%.
1. 3 Effect of metal impurity ions such as iron, nickel, sodium and aluminum salts on the performance of organic electrolytes <br> Metal impurity ions have a lower reduction potential than lithium ions, so during the charging process, metal impurity ions will first be embedded in carbon. In the negative electrode, the position at which lithium ions are embedded is reduced, thus reducing the reversible capacity of the lithium ion battery. The high concentration of metal impurity ions not only causes the reversible specific capacity of the lithium ion battery to decrease, but also the precipitation of metal impurity ions may cause the surface of the graphite electrode to fail to form an effective passivation layer, which may damage the entire battery. However, the lithium ion radius is small, and the migration rate of lithium ions between the graphite layers is greater than that of other metal ions. Therefore, the low concentration of metal impurity ions has little effect on the performance of the battery. Therefore, the content of each metal impurity ion in the organic electrolyte is generally required to be less than 0. . 007 %.
1. 2 The influence of active hydrogen atoms and organic substances on the performance of organic electrolytes in the molecule<br> The organic acid, alcohol, aldehyde, ketone and other substances containing active hydrogen atoms in the molecule are generated during the first charge and discharge of the battery. a compound such as lithium carboxylate or lithium alkoxide, which has a certain solubility in an organic solvent, which on the one hand causes instability of the SEI film, lowers the conductivity of lithium ions, and reduces the cycle efficiency of the battery; In contrast, their reaction with metallic lithium increases the irreversible capacity of the battery. Amines and amides undergo polymerization during charge and discharge, which lowers the conductivity of the electrolyte. At the same time, these substances will also react with the electrolyte lithium salt lithium hexafluorophosphate to form HF. 008 %。 The above analysis, the organic electrolyte containing active hydrogen atoms, the smaller the amount of impurities, the more favorable the improvement of the performance of the battery, the content of these impurities should be at least less than 008%.
1. 3 Effect of metal impurity ions such as iron, nickel, sodium and aluminum salts on the performance of organic electrolytes <br> Metal impurity ions have a lower reduction potential than lithium ions, so during the charging process, metal impurity ions will first be embedded in carbon. In the negative electrode, the position at which lithium ions are embedded is reduced, thus reducing the reversible capacity of the lithium ion battery. The high concentration of metal impurity ions not only causes the reversible specific capacity of the lithium ion battery to decrease, but also the precipitation of metal impurity ions may cause the surface of the graphite electrode to fail to form an effective passivation layer, which may damage the entire battery. However, the lithium ion radius is small, and the migration rate of lithium ions between the graphite layers is greater than that of other metal ions. Therefore, the low concentration of metal impurity ions has little effect on the performance of the battery. Therefore, the content of each metal impurity ion in the organic electrolyte is generally required to be less than 0. . 007 %.
Handmade Apron Kitchen Sink,Kitchen Sink,Stainless Handmade Apron Farmhouse,Handmade Apron Farmhouse Kitchen Sink
Guangdong Huayou Hardware Products Co.,Ltd , https://www.gdhuayouhardware.com