The mechanism of battery capacity degradation has been widely studied and reported. Yunsheng Electronic Technology Co., Ltd. will share with you the factors affecting battery capacity degradation. The main reasons are: the main factor is the reduction of recyclable lithium caused by electrode surface side reactions; The secondary factor is the reduction of active substances (such as metal leaching, structural damage, material phase transition, etc.); The increase in battery impedance. The negative electrode is related to many influencing factors in the attenuation mechanism mentioned above.
Research progress on negative electrode attenuation mechanism: Carbon materials, especially graphite materials, are the most widely used negative electrode materials in lithium-ion batteries. Although other negative electrode materials, such as alloy materials, hard carbon materials, etc., are also widely studied, the research focus is mainly on the morphology control and performance improvement of active materials, and there is relatively little analysis of the mechanism of capacity degradation. Therefore, research on the negative electrode attenuation mechanism is mostly focused on the attenuation mechanism of graphite materials. The attenuation of battery capacity includes the attenuation during storage and use. The attenuation during storage is usually related to changes in electrochemical performance parameters (impedance, etc.). In addition to changes in electrochemical performance during use, there are also changes in mechanical stress such as structure, lithium evolution, and other phenomena.
The change in the electrode/electrolyte interface is widely recognized as one of the main causes of negative electrode attenuation in lithium-ion batteries. During the initial charging process of lithium batteries, the electrolyte is reduced on the negative electrode surface, forming a stable and protective passivation film (referred to as SEI film). During the subsequent storage and use of lithium-ion batteries, the negative electrode/electrolyte interface may undergo changes, leading to performance degradation.
1.1 Thickening/Composition Changes of SEI Films The gradual decrease in power performance of batteries 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 changes in its composition and structure.
Due to differences and limitations in characterization methods and testing conditions, the results of different research institutions may vary, making it difficult to determine the specific composition of SEI membranes. According to existing reports, the components of SEI membranes mainly include inorganic (Li2CO3, LiF) and organic [(CH2OCO2Li) 2, ROCO2Li, ROLi] compounds. The composition and thickness of the SEI film are not constant during use or storage.
Due to the fact that SEI membranes do not have the true function of solid-state electrolytes, solvated lithium ions can still migrate through SEI membranes through other cations, anions, impurities, electrolyte solvents, etc. Therefore, during the long-term cycling or storage process in the later stage, the electrolyte will still decompose and react on the negative electrode surface, leading to the thickening of the SEI film. Meanwhile, due to the continuous expansion and contraction of the negative electrode during the cycling process, the surface SEI film will rupture, creating a new interface that will continue to react with solvent molecules and lithium ions to form an SEI film. As the above surface reactions proceed, an electrochemical inert surface layer is formed on the negative electrode surface, causing some negative electrode materials to isolate and deactivate from the entire electrode, resulting in a loss of capacity
The composition of SEI film is thermodynamically unstable and undergoes dynamic changes of dissolution and redeposition in the battery system. SEI membrane can accelerate the dissolution and regeneration of the membrane under certain conditions (high temperature, HF, metal impurities inside the membrane, etc.), causing a loss of battery capacity. Especially under high temperature conditions, the organic components (such as alkyl lithium carbonate) in the SEI membrane are converted into more stable inorganic components (Li2CO3, LiF), resulting in a decrease in the ion conductivity of the SEI membrane. The metal ions dissolved from the positive electrode diffuse to the negative electrode through the electrolyte, reducing and depositing on the surface of the negative electrode. The elemental metal deposits catalyze the decomposition of the electrolyte, resulting in a significant increase in the negative electrode impedance and ultimately leading to a decrease in battery capacity. By adding high-temperature additives or new lithium salts to improve the stability of SEI films, the service life of negative electrode materials can be extended, thereby achieving performance improvement.
Research has found that the storage performance of different types of graphite materials varies, and artificial graphite has better storage performance than natural graphite at high temperatures. 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 downward trend. Through the analysis of scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) test results, it was found that during high-temperature storage, the content of Li2CO3 and LiOCOOR on the surface of natural graphite increased significantly with the prolongation of storage time. The increase in SEI film thickness is mainly caused by the side reaction of the electrolyte on the negative electrode surface. The surface structure and SEI film morphology of artificial graphite remain basically unchanged.
In addition, when stored at full charge for a certain period of time under conditions below 40 ℃, although negative electrode materials with high specific surface area have higher self discharge rates, the growth rate of SEI films per unit area is similar for different types of negative electrode materials. Its attenuation trend is similar. But at higher temperatures (60 ℃), the thickening rate of natural graphite SEI films with similar specific surface areas is significantly higher than that of artificial graphite.
1.2 Electrolyte decomposition deposition: Electrolyte reduction includes solvent reduction, electrolyte reduction, impurity reduction, etc. The impurities in the electrolyte usually include oxygen, water, and carbon dioxide. During the charging and discharging process of the battery, the electrolyte decomposes and reacts on the negative electrode surface, and its main products include lithium carbonate, fluoride, etc. As the number of cycles increases, the decomposition products gradually increase, which cover the surface of the negative electrode, hindering the deintercalation of lithium ions and causing an increase in the negative electrode impedance.
1.3 Lithium evolution: Due to the embedding potential of graphite materials being 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 between lithium and electrolyte will accelerate the degradation of battery performance. Large area lithium evolution will cause internal short circuits and thermal runaway of the battery. Low temperature charging, less excess of negative electrode relative to positive electrode, mismatched electrode size (positive electrode edge covering negative electrode), and potential effects (different local polarization degrees, electrode thickness and porosity effects) all increase the risk of lithium precipitation.
At present, research mainly focuses on improving the performance of the negative electrode by improving the negative electrode system and optimizing the electrolyte system containing additives that inhibit lithium evolution from the negative electrode. Coating Sn and carbon on the surface of graphite to improve the electrochemical cycling performance of the negative electrode. The Sn on the surface of graphite can reduce the internal resistance of SEI film and electrode polarization at low temperatures. Additionally, performance can be improved by improving the surface of the negative electrode material. Oxidizing graphite in air can increase surface area and edge active sites, resulting in larger pores and smaller particle sizes, thereby reducing the occurrence of lithium evolution caused by uneven charge distribution. AsF6 can improve the stability of the negative electrode at high temperatures, suppress the production of metallic lithium, and decompose LiPF6. In addition, mechanical rolling during the preparation stage of the negative electrode can reduce the pore size, reduce the unevenness of charge distribution, and improve the reversible capacity of the battery.
As the performance of the battery gradually deteriorates, the ordered structure of graphite is gradually disrupted due to changes in the active material of the negative electrode. Lithium batteries are cycled at high rates, and due to the gradient difference in lithium ion concentration, a mechanical stress field is generated inside the material, causing a change in the negative electrode lattice. The initial layer structure of the negative electrode also gradually becomes disordered, but this structural change is not the main reason for the deterioration of battery performance. Deterioration can manifest as lithium evolution or changes in the SEI film, but during this process, the particle size and lattice constant of the negative electrode do not undergo significant changes.
The reversible capacity of graphite particles is related to their orientation and category. For example, due to the presence of new interfaces between disordered particles, lithium ion/electrolyte reactions can occur, making lithium ion insertion more difficult, and disordered graphite particles have lower reversible capacity. Compared with spherical particles, flake graphite has a higher specific capacity at high magnification. Although the negative electrode structure does not change during the attenuation process, the ratio of diamond structure to hexagonal structure will change. The increase of hexagonal structure will reduce the Faraday efficiency of lithium ion insertion in the first and third stages, thereby reducing the reversible capacity of the negative electrode. Therefore, the reversible capacity can be improved by increasing the ratio of diamond/hexagonal structure.
The porosity of graphite electrodes is related to the reversible capacity of the negative electrode. As the porosity increases, the contact area between graphite and electrolyte increases, and the interface reaction increases, resulting in a decrease in reversible capacity. During the long-term charging and discharging process of batteries, the compaction density of graphite electrodes affects the degradation of battery performance. High pressure solid density can reduce the porosity of electrodes, reduce the contact area between graphite and electrolyte, and thereby improve reversible capacity. Moreover, at temperatures above 120 ℃, due to the thermal decomposition of the SEI film, the high-pressure solid negative electrode material will generate more heat.
