Organic electrolyte is an important part of lithium-ion battery. It plays the role of transmitting charge between positive and negative electrodes in the battery, and has an important impact on the main performance of the battery, such as working temperature, specific energy, cycle efficiency, safety and so on. The particularity of electrolyte composition also brings some particularity to the battery performance. In a sense, organic electrolyte plays a decisive role in the performance of lithium-ion battery. Organic electrolyte is generally composed of electrolyte lithium salt and organic solvent. The organic solvent is generally a mixed solvent composed of more than two organic solvents. The electrolyte lithium salt applied in commercial lithium-ion batteries is generally lithium hexafluorophosphate. The organic solvents mainly include EC (vinyl carbonate), PC (propylene carbonate), DMC (dimethyl carbonate), Dec (diethyl carbonate) Chain and cyclic carbonates such as EMC (methyl ethyl carbonate). There are two main factors affecting the advantages and disadvantages of organic electrolyte as the electrolyte of lithium-ion battery: ① the composition of organic electrolyte; ② The content of substances containing active hydrogen and metal ion impurities such as iron, sodium, aluminum and nickel in organic electrolyte molecules. This paper mainly expounds 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 there are three main sources of impurities in organic electrolyte:
A certain amount of impurities contained in electrolyte lithium salt will inevitably make the product contain quantitative HF, H2O and other metal impurity ions in the preparation of lithium hexafluorophosphate. At the same time, hydrogen in HF and H2O molecules will form hydrogen bonds with oxygen in lithium hexafluorophosphate, forming strong chemisorption, which makes it difficult to remove them.
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 a small amount of dust. The above impurities may be introduced if the operation environment control is not ideal or the operation is improper. 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 types of substances: ① water and hydrofluoric acid; ② Organic acids, alcohols, aldehydes, ketones, amines and amides containing active hydrogen atoms in molecules.
Iron, nickel, sodium, aluminum and other metal impurity ions. The effects of various impurities on the performance of organic electrolyte will be introduced respectively.
1.1 effect of water and hydrogen fluoride content on performance of organic electrolyte
The content of water and hydrogen fluoride is the most important factor affecting the performance of organic electrolyte. The effect of water and hydrogen fluoride content on the performance of lithium ion battery can be divided into two aspects: the influence of SEI film on the electrode surface (solid electrolyte phase boundary film) and the stability of electrolyte itself. During the first charge and discharge of the battery, trace water and hydrogen fluoride will be the reduction product on the electrode surface, which will react with alkyl lithium carbonate to generate lithium carbonate and lithium fluoride, or react with metal lithium to generate lithium oxide, lithium carbonate and lithium fluoride, which will be covered on the electrode surface as components of SEI film. Lithium carbonate is insoluble in organic solvents and has good lithium ion searchability. It is an important component for the formation of SEI films with excellent properties. Lithium oxide and lithium fluoride are thermodynamically stable SEI membrane components, which are of great significance to stabilize other SEI membrane components such as lithium carbonate. Some studies have shown that the presence of trace water in DMC based electrolyte will not damage the performance of graphite electrode, but will greatly improve it. Therefore, the existence of trace water and hydrogen fluoride in organic electrolyte plays a certain role. When the content of water and hydrogen fluoride in organic electrolyte is high, water and hydrogen fluoride will react with lithium. On the one hand, limited lithium ions in the battery will be consumed, so as to increase the irreversible capacity of the battery. On the other hand, a large number of lithium oxide and lithium fluoride in the reaction products are unfavorable to the improvement of electrode electrochemical performance. At the same time, gas products will be produced in the above reaction, resulting in the increase of pressure in the battery. With the increase of water and hydrogen fluoride content in organic electrolyte, the charge discharge and cycle efficiency of 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 will react with the organic solvent to produce corresponding alcohols and acids. Take PC as an example: PC + H2O propylene glycol + CO2 propylene glycol will react with lithium hexafluorophosphate to produce corresponding lithium salts 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. In turn, the hydrogen fluoride generated in the process will accelerate the above reaction. Without strict water and acid removal of the electrolyte, the color will become darker and the solution will become viscous after a certain time. The content of water will decrease and the corresponding content of hydrogen fluoride will increase. When organic electrolyte containing hydrogen fluoride is used in lithium ion battery, hydrogen fluoride will react with cathode material and SEI film to produce water, etc. Aurbach et al. Believe that in EC based organic electrolyte, hydrogen fluoride and SEI film mainly react as follows: (1) HF reacts with carbonate or carbonate on the electrode surface to produce LIF and CO2. Li2CO3 + 2hf2lif + H2O + CO2 (2) pof3 first reacts on the electrode surface, and then reacts with LIF to form lixpfyox compounds, such as liopf2. The water and ethylene glycol produced in the reaction will react with lithium hexafluorophosphate to generate hydrogen fluoride. This process continues to cycle, resulting in the continuous reduction of the specific capacity and cycle efficiency of the battery until the whole battery is damaged. Therefore, in practical lithium-ion batteries, it is generally required that the content of water and hydrogen fluoride in organic electrolyte should be at least less than 0.006%.
1.2 effect of active hydrogen atoms and organic substances in molecules on the performance of organic electrolyte
Organic acids, alcohols, aldehydes, ketones and other substances containing active hydrogen atoms in the molecules generate lithium carboxylate or lithium alkoxy compounds during the first charge and discharge of the battery. These substances have certain solubility in organic solvents. On the one hand, they will lead to the instability of SEI membrane, reduce the conductivity of lithium ions and reduce the cycle efficiency of the battery; On the other hand, their reaction with metal lithium increases the irreversible capacity of the battery. Amines and amides will polymerize during charge and discharge, which will reduce the conductivity of electrolyte. At the same time, these substances will also react with the electrolyte lithium salt lithium hexafluorophosphate to form HF. It can be seen from the above analysis that the smaller the impurity mass containing active hydrogen atoms in the organic electrolyte, the more conducive to the improvement of battery performance. Generally, the content of these impurities shall be at least less than 0.008%.
1.3 effect of metal impurity ions such as iron, nickel, sodium and aluminum salts on the performance of organic electrolyte
Metal impurity ions have a lower reduction potential than lithium ions. Therefore, during charging, metal impurity ions will first be embedded in the carbon negative electrode, reducing the insertion position of lithium ions, thus reducing the reversible capacity of lithium-ion batteries. The content of high concentration metal impurity ions will not only lead to the decrease of reversible specific capacity of lithium-ion battery, but also the precipitation of metal impurity ions may lead to the failure to form an effective passivation layer on the surface of graphite electrode and destroy the whole battery. However, the radius of lithium ion is small, and the migration rate of lithium ion between graphite layers is greater than that of other metal ions. Therefore, the low concentration of metal impurity ions has little impact on the performance of the battery. Therefore, it is generally required that the content of each metal impurity ion in the organic electrolyte should be less than 0.007%.