Analysis of capacity attenuation of lithium-ion batteries
Positive and negative electrodes, electrolyte,s and diaphragm are important components of lithium-ion batteries. The positive and negative electrodes of the lithium-ion battery have the embedded release reaction of lithium respectively, and the amount of lithium embedded in the positive and negative electrodes has become the main factor affecting the capacity of the lithium-ion battery. Therefore, it is necessary to maintain the balance of the positive and negative capacity of lithium-ion batteries to ensure the best performance of the battery.
Generally speaking, lithium-ion batteries are commonly used as an electrolyte solution composed of organic solvents and electrolytes (lithium salts). The electrolyte solution should have sufficient conductivity, stability, and be compatible with electrodes. For the diaphragm, its performance is the main factor that determines the internal resistance and interface structure of the battery, which has a direct impact on the attenuation change of battery capacity. If the quality and performance of the diaphragm are superior, the capacity and comprehensive performance of lithium-ion batteries will be significantly improved. In general, the diaphragm mainly plays the role of separating the positive electrode of the battery from the negative electrode of the battery to avoid the short circuit of the battery due to the contact of the positive and negative electrode. At the same time, it can also release electrolyte ions to give full play to the utility of the battery.
The chemical reactions in lithium-ion batteries include not only redox reactions in the process of lithium ion embedding and detachment, but also side reactions such as the production and destruction of the negative surface SEI film, the decomposition of electrolyte, and structural change and dissolution of active materials. These side reactions are the original causes of lithium-ion battery capacity attenuation. Because. Capacity attenuation and loss in the process of battery cycle is an inevitable phenomenon. Therefore, in order to improve battery capacity and performance, scholars in various fields at home and abroad have fully studied the mechanism of lithium battery capacity loss. At present, it can be known that the main factors causing the capacity attenuation of lithium-ion batteries include the formation of SEI passivation film on the positive and negative surface, lithium metal deposition, dissolution of electrode active materials, cathode-and-a-anode reduction reaction or side reaction, structural change and phase change. At present, the attenuation change of lithium-ion battery capacity and its reasons are still under continuous research.
II. Overcharge
2.1 Negative overcharge reaction
There are many kinds of active materials that can be used as the anode of lithium-ion batteries, with carbon-based anode materials, silicon-based and tin-based anode materials, lithium titanate anode materials, etc. as the main materials. Different types carbon materials have different electrochemical properties. Among them, graphite has the advantages of high electrical conductivity, excellent layer structure and high crystallinity, which is more suitable for the embedding and release of lithium. At the same time, graphite materials are affordable and have a large stock, so they are widely used.
When the lithium-ion battery is charged and discharged for the first time, the solvent molecules will decompose on the surface of graphite and form a passivation film called SEI, which will cause the loss of battery capacity and is an irreversible process. During the overcharging of lithium-ion batteries, lithium metal deposition will occur on the negative electrode surface, which is easy to occur when the positive active material is excessive compared to the negative active material. At the same time, lithium metal deposition may also occur under high magnification conditions.
Generally speaking, the causes of the attenuation of lithium battery capacity caused by the formation of lithium metal mainly include the following aspects: first, the amount of circulating lithium in the battery is reduced; second, lithium metal reacts with electrolytes or solvents to form other by-products; third, lithium metal is mainly deposited between the anode and the diaphragm, resulting in separation. The membrane pores are blocked, resulting in an increase in the internal resistance of the battery. There are also some differences in the influence mechanism of the attenuation of lithium-ion battery capacity due to different graphite materials. The specific surface area of natural graphite is relatively high. Therefore, the self- discharge reaction will lead to the loss of lithium battery capacity, and natural graphite, as the negative electrode of the battery, has a higher electrochemical reaction impedance than that of artificial graphite. In addition, the dissociation of the negative layered structure during the cycle process, the dispersion of the conductive agent in the production process of the electrode, the increase of electrochemical reaction impedance in the storage process and other factors are all important factors leading to the loss of lithium battery capacity.
2.2 Positive overcharge reaction
Positive overcharging mainly occurs when the proportion of positive materials is too low, resulting in an imbalance in the capacity between electrodes, resulting in an irreversible loss of lithium battery capacity. The coexistence and continuous accumulation of oxygen and combustible gases decomposed from cathode materials and electrolytes may bring potential safety hazards to the use of lithium batteries
2.3 Electrolyte reacts at high voltage
If the charging voltage of the lithium battery is too high, it will lead to the oxidation reaction of the electrolyte and generate some by-products, which will block the micropores of the electrode and hinder the migration of lithium ions, resulting in the attenuation of the circulating capacity. The change trend of electrolyte concentration and electrolyte stability is inversely proportional. The higher the electrolyte concentration, the lower the stability of the electrolyte, which affects the capacity of lithiumion batteries. During the charging process, the electrolyte will be consumed to a certain extent, so it needs to be replenished during assembly, resulting in the reduction of battery active materials and affecting the initial capacity of the battery.
III. Electrolyte decomposition
Electrolytes include electrolytes, solvents and additives, whose properties will have an impact on the service life, specific capacity, multiplier charge and discharge performance and safety performance of the battery. The decomposition of electrolytes and solvents in the electrolyte will cause the loss of battery capacity. When charging and discharging for the first time, solvents and other substances form a SEI film on the negative surface, which will form an irreversible capacity loss, but this is inevitable. If there are impurities such as water or hydrogen fluoride in the electrolyte, the electrolyte LiPF6 may decompose at a high temperature, and the resulting product reacts with the cathode material, which will affect the battery capacity. At the same time, some products will also react with the solvent and affect the stability of the SEI film on the negative electrode surface, which will cause the performance attenuation of lithium-ion batteries. In addition, if the product decomposed by the electrolyte is not compatible with the electrolyte, the positive pores will be blocked during the migration process, resulting in the attenuation of battery capacity. In general, the occurrence of side reactions between the positive and negative electrodes of the electrolyte and the battery, as well as the by-products, are the main factors causing the attenuation of battery capacity.
IV. Self-recharge
In general, lithium-ion batteries will have capacity loss. This process is called self-recharge, which is divided into reversible capacity loss and irreversible capacity loss. The solvent oxidation rate has a direct impact on the self-discharge rate. The positive and negative active material may react with the solute during charging process, resulting in the capacity imbalance and irreversible attenuation of lithium-ion migration. Therefore, it can be seen that reducing the surface area of the active material can reduce the capacity loss rate, and the decomposition of the solvent will affect the battery. Storage life. In addition, diaphragm leakage can also lead to capacity loss, but this possibility is low. If the self-discharge phenomenon exists for a long time, it will lead to the deposition of lithium metal and further lead to the attenuation change of the positive and negative electrode capacity.
V. Electrode instability
During charging, the active material at the positive electrode of the battery is unstable, which will cause it to react with the electrolyte and affect the battery capacity. Among them, the structural defects of cathode materials, high charging potential and carbon black content are the main factors affecting the battery capacity.
5.1 Structural phase change
5.1.1 LiMn2O4
LiMn2O4 has rich resources in China, low price and good thermal stability. It is the main material of battery cathode. The LiMn2O4 cathode in the storage and battery charge and discharge cycle in a high temperature environment will lead to the attenuation of the battery capacity, which is mainly caused by the following factors: first, under high voltage conditions, the electrochemical reaction of the electrolyte is generally higher than 4.0V; secondly, LiMn2O4 The Mn contained in the material dissolves in the electrolyte, resulting in a disproportionation reaction, which destroys the crystalline phase structure of the cathode material. For lithium-ion batteries with LiMn2O4 as the positive electrode and C as the negative electrode, it will cause the solvent to decompose under high pressure, and with the oxidation reaction of the C negative electrode, the resulting oxidation product will migrate to the positive electrode and dissolve with the positive electrode. The divalent manganese ions formed after dissolution will be reduced at the negative electrode and deposited together with other impurities. The oxide of Mn will only be deposited in the direction of the negative pole near the diaphragm, not in the direction of the collecting fluid, that is, the oxide of Mn is only deposited on the surface of the SEI film, which is why the battery capacity will be attenuated. Adding inhibitors to the electrolyte can effectively inhibit the dissolution of metal ions and improve the circulation performance of the battery
In addition, in the process of charging and discharging, lithium-ion batteries with LiMn2O4 as the positive electrode and C as the negative electrode may change the lattice constant of LiMn2O4 with the embedding and release of lithium ions, and phase transition between the cubic and the tetragonal crystal system. The diffusion rate of lithium ions in the cathode material is lower than that of lithium ions on the cathode surface. When the potential is about 4V, lithium ions gather on the surface of LiMn2O4, and the Jahn-Teller effect occurs, causing the structure to be distorted and transformed, resulting in battery capacity attenuation.
5.1.2 LiCoO2
The application of LiCoO2 in the cathode material of lithium-ion batteries has great advantages, mainly in the ability to reversible embed and detach lithium ions, and has a large lithium-ion diffusion coefficient, reversible insertion amount and degree of structural change. Therefore, it plays an important role in improving the charge and discharge current of lithium batteries. At the same time, the structure of the material is stable, and the de-embedding reversibility of lithium ions is good, which can effectively ensure the Cullen efficiency of charging and discharging and the service life of the battery. Through the research of relevant scholars at home and abroad on the capacity attenuation mechanism of the LiCoO2 system, it is found that the factors affecting the change of capacity attenuation in the cycle of lithium batteries are mainly due to the increase of positive interface impedance and the loss of negative capacity. At the same time, relevant scholars also found that the higher the number of cycles, the lower the capacity loss of the positive and negative electrodes compared with the loss of the whole battery capacity, and the decrease in the transfer capacity of active lithium ion will have a greater impact on the overall capacity attenuation of the battery. And it can be seen from Figure 1 that after the battery cycle > 200 times, the cathode material has not undergone phase change, while the regularity of the LiCoO2 layer structure is reduced, and the mixing of lithium ions and chromium ions increases, making it difficult for lithium ions to effectively delay, resulting in battery capacity attenuation. In addition, increasing the discharge magnification will promote the mixing of lithium and chromium atoms, which will lead to the transformation of the original hexagonal crystal type of LiCoO2 into a cubic crystal type, thus causing the attenuation change of the capacity of lithium-ion batteries.
In addition, in the LiCoO2 system, through the study of the attenuation law of the battery cycle capacity in 25℃ (i.e. at room temperature) and 60℃, it can be found that before 150 cycles, the discharge capacity of the battery below 60℃ is higher than that of the battery capacity and rated capacity at room temperature, because the electrolyte is viscous at high temperature. The degree is reduced, which increases the lithium-ion migration rate, thus improving the utilization rate of activated lithium, and the battery shows a high charge and discharge capacity. After 300 cycles, the polarization capacity loss of the battery at 60℃ is much higher than that at room temperature. It can be seen that the temperature increase exacerbates the electrochemical polarization of the electrode during the charging and discharging of lithium-ion batteries, making the capacity loss of lithium batteries more serious during the charging and discharging.
5.1.3 LiFePO4
LiFePO4 has a wide range of sources, cheap price, and has good stability and safety performance. It can reach the theoretical specific capacity of 170mAh/g, and its specific power and specific energy are similar to LiCoO2, which can achieve good compatibility with electrolyte solutions. Therefore, it is widely used in lithium batteries. Extremely. Using this material, the factors affecting the battery capacity mainly include the following two points: first, due to the side reaction between the positive and negative electrodes, resulting in the reduction of recyclable lithium, which seriously destroys the balance between the positive and negative electrodes; second, due to structural deterioration, electrode layer separation, material dissolution, particle separation and other factors, the production of active materials Loss, thus affecting the battery capacity.
5.2 Carbon black content of cathode materials
Because carbon black itself is a non-active substance, it does not participate in the discharge reaction. If the amount of carbon black contained in the cathode material is too high, it will affect the strength and capacity of the cathode material, so it needs to be added as appropriate. In addition, the transmission carrier produces substances with catalytic properties on the surface of carbon black, which can improve the decomposition rate of metal ions and effectively promote the dissolution of active substances.
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