The evolution of lithium, the thickening of the passivation film on the electrode surface, the loss of recoverable lithium amount and the destruction of the active material structure may all lead to the decay of lithium battery life. Among them, the negative electrode is the main factor causing the capacity decay of lithium batteries. This paper summarizes the main principles of negative electrode decay during battery use and proposes several methods to reduce capacity decay.
The mechanism of battery capacity decay has been widely studied and reported. The main factors affecting the battery capacity decay are, the reduction of recoverable lithium caused by the electrode surface side reactions, and the secondary factors are the reduction of active materials (such as dissolution of metals, destruction of structure, phase change of materials, etc.). Increase in cell impedance. Among the above decay mechanisms, the negative electrode is related to many factors.
Research progress of negative electrode attenuation mechanism
Carbon materials, especially graphite materials, are the most widely used cathode materials in lithium-ion batteries. Although other cathode materials such as alloy materials and hard carbon materials have also been extensively studied, the research focuses mainly on the morphological control and performance improvement of the active materials, and less analysis has been done on their capacity decay mechanisms. Therefore, the research on the decay mechanism of negative electrode is mainly on the decay mechanism of graphite materials. The decay of battery capacity includes the decay during storage and use, which is usually related to the changes of electrochemical performance parameters (impedance, etc.). In addition to the changes in electrochemical properties, they are accompanied by changes in mechanical stresses such as structure and lithium separation.
1) Changes in the anode/electrolyte interface
For Li-ion batteries, changes in the electrode/electrolyte interface are considered to be one of the main causes of anode decay. During the initial charging of Li-ion batteries, the electrolyte is reduced on the surface of the negative electrode to form a stable protective passivation film (SEI film). During subsequent storage and use of Li-ion batteries, the cathode/electrolyte interface may change, resulting in degradation of its performance.
2) Thickening/composition change of SEI film
The decrease in battery power performance 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 its composition and structure.
The composition of the SEI film is thermodynamically unstable and dynamic changes in dissolution and redeposition occur continuously in the cell system. the SEI film under certain conditions (high temperature, HF, metal impurities in the film, etc.) will accelerate dissolution and regeneration, resulting in loss of cell capacity. In particular, at high temperatures, the organic components (lithium alkyl carbonate, etc.) in the SEI membrane are converted to more stable inorganic components (Li2CO3, LiF), resulting in a decrease in the ionic conductivity of the SEI membrane. The dissolved metal ions from the positive electrode diffuse through the electrolyte to the negative electrode and are deposited reductively on the negative surface. The metal element deposits catalyze the decomposition of the electrolyte, which significantly increases the cathode impedance and ultimately leads to the decay of the battery capacity. Improving the stability of SEI films by adding high-temperature additives or new lithium salts can extend the service life of cathode materials and improve performance.
It is found that the storage performance of different types of graphite materials varies, and the high-temperature storage performance of artificial graphite is better than that of natural graphite. With the increase of storage time. The lithium content in artificial graphite was basically stable, while the lithium content in natural graphite showed a linear decreasing trend. The results of scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) showed that the Li2CO3 and LiOCOOR contents on the surface of natural graphite increased with the increase of storage time. the increase of SEI film thickness was mainly caused by the side reaction of electrolyte on the negative surface. The surface structure and the morphology of the SEI film of artificial graphite were basically unchanged.
It is difficult to determine the specific composition of the SEI film because of the differences and limitations of the characterization methods and testing conditions, and the different results from different research institutions. According to previous reports, the composition of SEI membranes mainly consists of inorganic (Li2CO3, LiF) and organic [(CH2OCO2Li)2, ROCO2Li and ROLi] compounds. The composition and thickness of SEI membranes are not constant during use or storage.
Since the SEI membrane does not function as a true solid electrolyte, the dissolved lithium ions can still migrate through the SEI membrane via other cations, anions, impurities, solvents, etc. of the electrolyte. Therefore, the electrolyte will still decompose on the surface of the negative electrode during long-term cycling or storage at a later stage, leading to the thickening of the SEI film. At the same time, because the negative electrode is always expanding and contracting during cycling, the surface SEI film will break and generate a new interface, which will continue to react with solvent molecules and lithium ions to generate the SEI film. Through the above surface reaction, an electrochemically inert surface layer is formed on the surface of the negative electrode, which isolates and inactivates part of the negative electrode material from the whole electrode. This leads to a loss of capacity. As shown in Fig. 1, the SEI film on the negative electrode surface is significantly thickened after long-term cycling.
In addition, the growth rate of SEI film per unit area is similar for different types of negative electrode materials, although the negative electrode material with high specific surface area has a higher self-discharge rate when full charge storage is performed for a certain time at less than 40°C. Their decreasing trends are similar. However, the thickening rate of SEI film of natural graphite with similar specific surface area was significantly higher than that of artificial graphite at higher temperature (60°C).
3) Electrolyte decomposition deposition
Electrolyte reduction includes solvent reduction, electrolyte reduction, impurity reduction, etc. Impurities in the electrolyte usually include oxygen, water and carbon dioxide, etc. In the process of battery charging and discharging, electrolyte decomposition reaction occurs on the surface of negative electrode, and its main products are lithium carbonate, fluoride, etc. As the number of cycles increases, the decomposition products gradually increase. These products cover the surface of the negative electrode and hinder the de-embedding of lithium ions, resulting in an increase in the impedance of the negative electrode.
4) Analysis of lithium
The degree of disorder within the graphite material and the uneven degree of current distribution will affect the evolution of lithium on the anode surface. In the third and fourth stages of lithium graphite, the disorder of the material leads to uneven charge distribution in the electrode, resulting in the generation of dendrite deposition. The growth of deposits between the film and the anode is closely related to the temperature and current density. As the temperature, charging rate and reaction rate increase, lithium metal is deposited on the surface of the cathode. It can be judged by the voltage plateau in the cell discharge curve and the degree of reduction in coulombic efficiency.
As the graphite material is close to the lithium embedding potential potential, once the deposited lithium metal or lithium dendrite growth occurs during the charging process, the reaction between lithium and electrolyte will accelerate the decay performance of the battery, and the large precipitation of lithium will cause short circuit and the occurrence of thermal control of the battery. Low temperature charging, negative electrode is larger than positive electrode, electrode size mismatch (positive electrode edge covering negative electrode), potential effect (different local polarization degree, electrode thickness and pore effect) all increase the risk of lithium evolution.
Current research focuses on improving the performance of the cathode in terms of improving the anode system and optimizing the electrolyte system containing lithium corrosion inhibitor. Cladding tin and carbon on the graphite surface improves the electrochemical cycling performance of the anode. Tin on the graphite surface reduces the internal resistance of the SEI film and low-temperature polarization of the electrode. In addition, the performance can also be improved by improving the surface of the cathode material. Oxidation of graphite in air can increase the surface area and edge active sites, increase the pore size, reduce the particle size, and further reduce the generation of lithium evolution caused by uneven charge distribution. asF6 can improve the stability of the cathode at high temperature and inhibit the production of lithium metal and the decomposition of LiPF6. In addition, the mechanical roller pressure applied at the preparation stage can reduce the pore size, reduce the unevenness of charge distribution, and improve the reversible capacity of the cell.
5) Changes in the negative active material
During the degradation of battery performance, the ordered structure of graphite is gradually destroyed. Due to the poor lithium ion concentration gradient, a mechanical stress field is generated inside the material during the high rate cycle of lithium batteries. The negative lattice changes, and the initial layer of negative electrode lamellar structure gradually becomes disordered. However, this structural change is not the main reason for the deterioration of the battery performance. The degradation is mainly manifested as a change in the lithium or SEI film; however, the particle size and lattice constant of the negative electrode do not change significantly during this process.
The reversible capacity of graphite particles is related to their orientation and type. For example, new interfaces exist between disordered particles, which can undergo lithium ion/electrolyte reactions, making it more difficult to embed lithium ions, and the reversible capacity of disordered graphite particles is lower. Compared to spherical particles, flake graphite has a higher specific capacity at high power. Although the negative structure does not change during the decay process, the ratio of rhombic/hexagonal structure changes. The increase of hexagonal structure decreases the Faraday efficiency in the first and third stages of Li-ion injection, thus decreasing the reversible capacity of the negative electrode. Therefore, the reversible capacity can be increased by increasing the ratio of rhombus-hexagonal structure.
6) Negative electrode variation
There is a relationship between the porosity of graphite electrode and the reversible capacity of negative electrode. With the increase of porosity, the contact area between graphite and electrolyte increases, and the interfacial reaction increases, leading to the decrease of reversible capacity. During the long-term charging and discharging of the battery, the compaction density of the graphite electrode affects the decay of the battery performance. High pressure solid density can reduce the porosity of the electrode, decrease the contact area between graphite and electrolyte, and increase the reversible capacity. In addition, when the temperature is higher than 120°C, the SEI film will generate gas through thermal decomposition, and the high-pressure solid negative electrode material will generate more heat.
The particle size of graphite material has a great influence on its negative performance. The small particle size material can shorten the diffusion path between graphite materials, which is beneficial to the high rate of charge and discharge. However, due to the large specific surface area of the small particle size material, more lithium ions will be consumed at high temperatures, resulting in an irreversible capacity increase of the negative electrode. Therefore, the thermal stability of graphite cathode is mainly related to the particle size of graphite material.
Final conclusion
The negative decay of lithium-ion batteries includes several degradation mechanisms. Among them, the separation of lithium ions is the main factor leading to the rapid decay of the battery life. The decomposition of the electrolyte and the formation of thin films on the anode surface lead to an increase in internal resistance and a decrease in the amount of recoverable lithium. The above mechanism has little effect on the crystal structure of the negative electrode. Optimization of the electrolyte system, addition of stabilizers and temperature treatment can reduce the occurrence of these reactions and improve the performance of the anode material.
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