The Main Mechanism And Countermeasures Of Lithium Ion Battery Negative Electrode Attenuation

Research progress of negative electrode attenuation mechanism:

Carbon materials, especially graphite materials, are the most widely used anode materials in lithium-ion batteries. Although other negative electrode materials, such as alloy materials, hard carbon materials, etc., are also being studied extensively, the research focuses mainly on the morphology control and performance improvement of active materials, and there is little analysis of the mechanism of its capacity decay. Therefore, most of the research on the attenuation mechanism of the negative electrode is about the attenuation mechanism of graphite materials. The attenuation of battery capacity includes attenuation during storage and use. Attenuation during storage is usually related to changes in electrochemical performance parameters (impedance, etc.). In addition to changes in electrochemical performance, it is also accompanied by changes in mechanical stress such as structure and lithium evolution. And other phenomena.

1.1 Change of negative electrode/electrolyte interface

For lithium-ion batteries, the change of the electrode/electrolyte interface is recognized as one of the main reasons for the attenuation of the negative electrode. During the initial charging of lithium batteries, the electrolyte is reduced on the surface of the negative electrode to form a stable protective passivation film (SEI film for short). During the subsequent storage and use of lithium-ion batteries, the negative electrode/electrolyte interface may change, leading to the degradation of its performance.

1.1.1 Thickening of SEI film/change in composition

The gradual decrease in power performance of the battery during use is mainly related to the increase in electrode impedance. The increase in electrode impedance is mainly caused by the thickening of the SEI film and the changes in composition and structure.

Due to differences and limitations in characterization methods and test conditions, the results of different research institutions are not the same, so it is difficult to determine the specific composition of the SEI film. According to previous reports, the composition of SEI film mainly includes inorganic (Li2CO3, LiF) and organic [(CH2OCO2Li)2, ROCO2Li, ROLi] two types of compounds. During use or storage, the composition and thickness of the SEI film are not static.

Since the SEI membrane does not have the function of a real solid electrolyte, the solvated lithium ions can still migrate through the SEI membrane through other cations, anions, impurities, and electrolyte solvents. Therefore, in the later period of long-term cycling or storage, the electrolyte will still decompose and react on the surface of the negative electrode, resulting in thickening of the SEI film. At the same time, because the negative electrode has been in a state of expansion and contraction during the cycle, the surface SEI film will be broken, creating a new interface, and the new interface will continue to react with solvent molecules and lithium ions to form an SEI film. With the progress of the above-mentioned surface reaction, an electrochemically inert surface layer is formed on the surface of the negative electrode, so that part of the negative electrode material is isolated and deactivated from the entire electrode. Cause a loss of capacity. As shown in Figure 1, after long-term cycling, the SEI film on the surface of the negative electrode is significantly thicker.

Figure 1. Scanning electron micrograph of negative electrode surface after long-term cycling

The composition of SEI film is thermodynamically unstable, and dynamic changes of dissolution and redeposition will occur continuously in the battery system. SEI film will accelerate the dissolution and regeneration of the film under certain conditions (high temperature, HF, metal impurities in the film, etc.), causing loss of battery capacity. Especially under high temperature conditions, the organic components (lithium alkyl carbonate, etc.) in the SEI film are converted into more stable inorganic components (Li2CO3, LiF), resulting in a decrease in the ionic conductivity of the SEI film. The metal ions eluted from the positive electrode diffuse to the negative electrode through the electrolyte, and are reduced and deposited on the surface of the negative electrode. The elemental metal deposits catalyze the decomposition of the electrolyte, which significantly increases the resistance of the negative electrode and ultimately leads to the attenuation of battery capacity. By adding high-temperature additives or new lithium salts to improve the stability of the SEI film, the service life of the negative electrode material can be prolonged, and the performance can be improved.

Studies have found that different types of graphite materials have different storage performance, and the storage performance of artificial graphite at high temperatures is better than that of natural graphite. With the increase of storage time. The lithium content in artificial graphite is basically stable, but the lithium content in natural graphite shows a linear decline. Through scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) test results analysis, during high temperature storage, the content of Li2CO3 and LiOCOOR on the surface of natural graphite increases significantly with the extension of storage time. The increase in the thickness of the SEI film is mainly caused by the side reaction of the electrolyte on the surface of the negative electrode. The surface structure of artificial graphite and the morphology of SEI film are basically unchanged.

In addition, when fully charged and stored for a certain period of time under the condition of lower than 40℃, although the negative electrode material with high specific surface area has a higher self-discharge rate, the growth rate of the SEI film per unit area of different types of negative electrode materials is similar. The decay trend is similar. However, at a higher temperature (60°C), the thickening rate of natural graphite SEI film with similar specific surface area is significantly higher than that of artificial graphite.

1.1.2 Decomposition and deposition of electrolyte

Electrolyte reduction includes solvent reduction, electrolyte reduction, and impurity reduction. Impurities in the electrolyte usually include oxygen, water and carbon dioxide. During the charging and discharging process of the battery, the electrolyte decomposes on the surface of the negative electrode, and its main products include lithium carbonate and fluoride. As the number of cycles increases, the decomposition products gradually increase. These products cover the surface of the negative electrode and hinder the deintercalation of lithium ions, resulting in an increase in the impedance of the negative electrode.

1.1.3 Lithium analysis

Since the intercalation potential of graphite materials is close to the lithium potential, once the deposition of metallic lithium or the growth of lithium dendrites occurs during the charging process, the subsequent reaction of lithium with the electrolyte will accelerate the degradation of battery performance, and the large-area lithium evolution will Causes the internal short circuit of the battery and the occurrence of thermal runaway. Low-temperature charging, low excess of the negative electrode of the battery relative to the positive electrode, mismatched electrode size (the edge of the positive electrode covers the negative electrode), and potential effects (different local polarization degree, electrode thickness and porosity effects) all increase the risk of lithium evolution.

The degree of disorder within the graphite material and the unevenness of the current distribution will affect the lithium evolution on the surface of the negative electrode. In the third and fourth stages of graphite lithium insertion, the disorder of the material causes uneven distribution of charges in the electrode, resulting in the production of dendritic deposits. The growth of the deposit between the separator and the negative electrode is closely related to temperature and current density. As the temperature increases, the charging rate increases and the reaction rate accelerates, and metallic lithium is deposited on the surface of the negative electrode. The voltage plateau in the battery discharge curve and the decrease in Coulomb efficiency can be used to determine whether the battery has lithium evolution.

The current research is mainly to improve the performance of the negative electrode from the aspects of improving the negative electrode system and optimizing the electrolyte system containing additives to inhibit lithium evolution in the negative electrode. Coating Sn and carbon on the graphite surface improves the electrochemical cycling performance of the negative electrode. Sn on the graphite surface can reduce the internal resistance of the SEI film and the electrode polarization at low temperatures. In addition, the performance can also be improved by improving the surface of the negative electrode material. Oxidizing graphite in the air can increase the surface area and edge active sites, increase the pores and reduce the particle size, thereby reducing the occurrence of lithium evolution caused by uneven charge distribution. AsF6 can improve the stability of the negative electrode at high temperatures, inhibit the production of metallic lithium and the decomposition of LiPF6. In addition, the mechanical rolling in the preparation stage of the negative pole piece can reduce the pore size, reduce the unevenness of the charge distribution, and increase the reversible capacity of the battery.

1.2 Changes in negative electrode active material

In the process of gradual deterioration of battery performance, the ordered structure of graphite is gradually destroyed. Lithium batteries are cycled at high rates. Due to the gradient of lithium ion concentration, a mechanical stress field is generated inside the material, which changes the negative electrode lattice, and the initial sheet structure of the negative electrode gradually becomes disordered. Structural changes are not the main reason for the deterioration of battery performance. Deterioration can be expressed as changes in lithium evolution or SEI film, but during this process, the particle size and lattice constant of the negative electrode will not change significantly.

The reversible capacity of graphite particles is related to their orientation and type. For example, the lithium ion/electrolyte reaction can occur due to the presence of a new interface between disordered particles, the insertion of lithium ions is more difficult, and the reversible capacity of disordered graphite particles is lower. Compared with spherical particles, flake graphite has a higher specific capacity at high magnification. Although the structure of the negative electrode does not change during the decay process, the ratio of the rhomboid structure/hexagonal structure will change. The increase of the hexagonal structure will reduce the Faraday efficiency of the first and third stages of lithium ion insertion, thereby reducing the reversible capacity of the negative electrode. Therefore, the reversible capacity can be increased by increasing the ratio of the rhombic structure/hexagonal structure.

1.3 Changes in the negative electrode

The particle size of the graphite material has a greater impact on the performance of the negative electrode. Small particle materials can shorten the diffusion path between graphite materials, which is conducive to high-rate charge and discharge. However, the small particle size material has a larger specific surface area, and will consume more lithium ions at high temperatures, resulting in an increase in the irreversible capacity of the negative electrode. Therefore, the thermal stability of the graphite anode is mainly related to the particle size of the graphite material.

The porosity of the graphite pole piece has a certain relationship with the reversible capacity of the negative electrode. As the porosity increases, the contact area between graphite and the electrolyte increases, and the interface reaction increases, resulting in a decrease in the reversible capacity. During the long-term charge and discharge of the battery, the compaction density of the graphite electrode affects the degradation of the battery performance. High compaction density can reduce the porosity of the electrode, reduce the contact area of graphite and electrolyte, and then increase the reversible capacity. Moreover, at a temperature higher than 120°C, due to the thermal decomposition of the SEI film to produce gas, the compacted negative electrode material will generate more heat.

in conclusion:

The negative electrode decay of lithium ion batteries includes several degradation mechanisms. Among them, lithium is the main factor leading to the rapid degradation of battery life. The decomposition of the electrolyte and subsequent film formation on the surface of the negative electrode lead to an increase in the internal resistance of the battery and a decrease in the amount of recyclable lithium. The above mechanism has little effect on the crystal structure of the negative electrode. Measures such as optimizing the electrolyte system, adding stabilizers and temperature treatment can reduce the occurrence of these reactions and improve the performance of the negative electrode material.