1 Lithium analysis and SEI film
This article comprehensively analyzes the mechanism of capacity degradation in lithium-ion batteries, classifies and organizes the factors that affect the aging and lifespan of lithium-ion batteries, and elaborates on various mechanisms such as overcharging, SEI film growth and electrolyte, self discharge, active material loss, and current collector corrosion. It summarizes the research progress of scholars in various fields in battery aging mechanisms in recent years, analyzes in detail the influencing factors and modes of action of lithium-ion battery aging, and elaborates on the modeling methods of aging side reactions.
Classification and Effects of Aging Causes of Lithium ion Batteries
1. Classification of Aging Causes of Lithium ion Batteries
The aging process of lithium-ion batteries is influenced by various factors such as their grouping method in electric vehicles, environmental temperature, charge discharge rate, and discharge depth. The degradation of capacity and performance is usually the result of multiple side reaction processes, which are related to numerous physical and chemical mechanisms. The degradation mechanism and aging form are very complex. It shows the comprehensive mechanism analysis of lithium-ion battery aging. In the actual aging process of lithium-ion batteries, different side reactions or phase transition processes occur in each component of the lithium-ion battery, and each process has different effects on capacity degradation.
Based on recent research progress both domestically and internationally, the main factors affecting the capacity degradation mechanism of lithium-ion batteries include SEI film growth, electrolyte decomposition, self discharge of lithium-ion batteries, loss of electrode active materials, and corrosion of current collectors. In the actual aging process of lithium-ion batteries, various side reactions occur simultaneously with electrode reactions, and various aging mechanisms work together and couple with each other, increasing the difficulty of studying aging mechanisms.
2. Aging effects of lithium-ion batteries
The aging of lithium-ion batteries has a profound impact on their overall performance, mainly manifested in the decline of charge and discharge performance, available capacity degradation, and thermal stability.
The main external characteristics of lithium-ion batteries after aging are a decrease in available capacity and an increase in internal resistance, which in turn leads to a decrease in the actual charge and discharge capacity and maximum available charge and discharge power of lithium-ion batteries; At the same time, due to the increase in internal resistance of lithium-ion batteries, there are problems such as increased heat generation, temperature rise inside the module, and increased temperature inconsistency during use, which require higher requirements for the thermal management system of lithium-ion batteries; However, the internal side reactions of lithium-ion batteries vary due to differences in battery grouping and connection structures, leading to differences in individual usage conditions. As the battery is used, the aging rate of each individual cell within the battery varies, exacerbating the inconsistency of lithium-ion battery packs.
The open circuit voltage curve of lithium-ion batteries characterizes the current internal electromotive force of lithium-ion batteries. As lithium-ion batteries age, the open circuit voltage curve will shift or deform to a certain extent relative to the original state, resulting in changes in the actual charge and discharge voltage curve of lithium-ion batteries, which affects the accuracy of battery state estimation in the battery management system during actual use. With the aging of lithium-ion batteries, the maximum available charge and discharge rate of lithium-ion batteries will also decrease. If the battery management system does not make adaptive adjustments, it is easy to cause overcharging, overdischarging, and high-power use of lithium-ion batteries, which increases the safety risks of lithium-ion battery use.
MECC Lithium Battery Manufacturer In China
We have high quality LiFePO4 battery with A grade battery cell and long life circle in our procucts
Mechanism of Capacity Decline in Lithium ion Batteries
1. Analysis of Capacity Decline Impact Caused by Lithium Precipitation
The figure shows the loss of active lithium ions caused by lithium deposition from the negative electrode, which refers to the process of lithium deposition from the electrolyte to the electrode surface. The lithium deposition on the negative electrode surface is an important cause of aging in lithium-ion batteries and a significant factor affecting battery safety. When the negative electrode potential exceeds the threshold of 0V (relative to Li/Li+), lithium deposition occurs on the negative electrode surface.
Lithium precipitation can lead to irreversible loss of lithium ion inventory, resulting in a decrease in available capacity. The growth of lithium dendrites leads to the loss of active lithium ions, as shown in the figure. There are many factors that affect lithium deposition in batteries. Some scholars believe that the slow insertion rate of lithium ions into graphite negative electrodes or the fast transfer rate of lithium ions to the negative electrodes may cause lithium deposition. There are also studies showing that the diffusion rate of lithium ions slows down when working under low temperature conditions, and the negative electrode working potential is very close to the lithium deposition potential, making it easier to cause lithium deposition. In addition, a too small N/P (ratio of negative electrode capacity to positive electrode capacity) can lead to lithium deposition, and local electrode polarization and geometric mismatch may also cause lithium deposition.
Lithium evolution is closely related to the aging process. M ü hlbauer et al. believe that electrode lithium deposition is more likely to occur in batteries with existing internal defects. Kabir and Demirocak found that the lithium deposition phenomenon in batteries accelerates in the later stages of aging, becoming one of the main reasons for the occurrence of battery capacity inflection points. The reason is that as the battery ages, SEI generation leads to a decrease in the porosity of the negative electrode, and the gradient of electrolyte potential at the negative electrode increases. Therefore, during the charging process, the negative electrode potential decreases and is more likely to drop below 0V, resulting in lithium deposition; The lithium precipitation process can lead to a decrease in negative electrode porosity and an increase in electrolyte potential gradient, resulting in accelerated battery aging. When the battery is in a discharge state, lithium on the dendrites may dissolve, but this material cannot obtain electrons due to the lack of contact with the current collector, and cannot participate in electrode reactions during charging and discharging, forming dead lithium. Lithium deposition leads to the loss of active lithium ions as shown in the figure.
2. The effect of SEI film growth on capacity degradation
SEI film is a passive film formed on the negative electrode surface of lithium-ion batteries, which has ion conductivity and prevents electrons from passing through, separating the electrolyte from the negative electrode. SEI film growth is the main side reaction of lithium-ion batteries at the negative electrode/electrolyte interface, which can lead to irreversible capacity loss. The battery rate, lifespan, and safety characteristics are closely related to the SEI film; Under normal usage conditions, SEI film is the main factor causing the loss of active lithium in batteries.
The SEI film is mainly composed of inorganic substances such as Li2CO3, LiF, Li2O, as well as organic substances such as ROCO2Li, ROLi, RCOO2Li (where R is an organic group). For some batteries, the thickness of the SEI film can reach over 100nm. The charging and discharging process of lithium-ion batteries is accompanied by the repeated extraction and insertion of lithium ions between the positive and negative electrodes. During charging, the active lithium ions in the positive electrode material will pass through the separator to reach the negative electrode surface, undergo a half cell reaction, and then be embedded in the negative electrode material. Due to the fact that the working potential of the negative electrode surface of lithium-ion batteries is generally lower than the thermodynamically stable potential window of the electrolyte, once the lithium ions, electrolyte, and electrons on the negative electrode surface come into contact, there is a possibility of reduction of the electrolyte. In addition, there are various complex reactions between substances near the negative electrode, resulting in the formation of SE film on the negative electrode surface, causing the loss of active materials in lithium-ion batteries, leading to a decrease in maximum available capacity and an increase in impedance.
The formation of SEI film is also one of the main reasons for calendar aging under high temperature and high state of charge (SOC) conditions. Compared with new batteries and SEI films generated under normal temperature cycling, SEI films generated at higher temperatures have better thermal stability and higher density than those generated at lower temperatures, which can slow down the aging rate of batteries. Although the growth of negative SEI film may have a negative impact on the capacity and internal resistance of lithium-ion batteries, a stable SEI film can improve the interface characteristics of electrode materials and enhance battery cycling performance. Some scholars also believe that the double-layer structure formed by the dense inner layer (initial SEI film) and porous outer layer (long-term growth layer) of SEI film can better explain the influence of SEI film on battery characteristics.
Although the composition of SEI film is still difficult to accurately analyze, the growth, rupture, and regeneration process of SEI film is considered to be closely related to the battery capacity degradation process. The SEI film is formed during the initial formation, and at this time, the SEI film is loose and porous. The electrolyte infiltrates through the pores on the surface of the film and undergoes a decomposition reaction when in contact with the electrode. The products fill the pores, causing the SEI film to become dense. However, during the long-term use cycle of the battery, the electrode material itself also experiences phenomena such as expansion and rupture, causing the SEI mode on the surface to bear stress and become thinner, resulting in the continuous growth of the SEI film during the cycle. However, the SEI film can also be damaged during rapid discharge, during which the electrode volume shrinks rapidly, causing the SEI film to rupture under high stress, resulting in the failure of the SEI film. The SEI film that has ruptured gradually repairs itself during the subsequent cycling process. However, local rupture will cause the overall structure of the SEI film to be irregular, and the current density near the growing part will be high, forming a positive feedback to accelerate the growth, rupture, and regrowth of the SEI film in that part, leading to abnormal aging in the local area and gradually causing the overall capacity decline of the battery.
Reasonable formation technology can improve the density of SEI film, thereby slowing down the aging process. At the same time, low-temperature environments are also conducive to the generation of dense SEI film, thereby improving the service life of batteries.
2 Corrosion of current collectors and loss of active materials
This article comprehensively analyzes the mechanism of capacity degradation in lithium-ion batteries, classifies and organizes the factors that affect the aging and lifespan of lithium-ion batteries, and elaborates on various mechanisms such as overcharging, SEI film growth and electrolyte, self discharge, active material loss, and current collector corrosion. It summarizes the research progress of scholars in various fields in battery aging mechanisms in recent years, analyzes in detail the influencing factors and modes of action of lithium-ion battery aging, and elaborates on the modeling methods of aging side reactions.
Capacity loss caused by corrosion of current collectors
The current collector is a key component in lithium-ion batteries, responsible for carrying active materials, collecting and outputting them. The currently widely used current collectors are copper and aluminum: copper is prone to oxidation at high potentials and is suitable as a current collector for negative electrode materials such as graphite and silicon; Due to its advantages in cost, mechanical strength, conductivity, and thermal conductivity, aluminum is generally considered one of the most suitable materials for battery positive electrode current collectors.
Corrosion of the current collector will reduce the lifespan of the battery and affect its stability and safety. Under extreme operating conditions such as over discharge, such as when the voltage drops to 1.5V, copper will be oxidized into copper ions in the electrolyte, resulting in the dissolution of copper current collectors. The copper ions oxidized by overdischarging will precipitate and deposit on the surface of the negative electrode material in the form of metallic copper during subsequent charging. The copper deposited on the negative electrode surface will hinder the insertion and removal of lithium and cause thickening of the SEI film, resulting in capacity degradation of lithium-ion batteries.
The aging of batteries caused by corrosion of current collectors is mainly manifested by an increase in internal resistance. The research results of Xu Zhiyou et al. indicate that batteries with aluminum foil as the current collector have a higher AC impedance, and their capacity decays to 10% of the initial value after 350 cycles at 10 ° C; Corroded aluminum foil has shown significant improvement compared to aluminum foil, but its stability is still poor. After 350 cycles at 10 ° C, the capacity decays to 22% of the initial value. Research by Song Wenji and others has shown that in electrolytes with lithium hexafluorophosphate as the electrolyte, a small amount of water can promote electrolyte decomposition and produce stable inorganic salts, thereby inhibiting the corrosion of aluminum current collectors. But with the generation of moisture, the oxidation decomposition products of the electrolyte undergo electrochemical reactions on the surface of the aluminum foil, leading to and accelerating the corrosion of the aluminum foil. Liu Xiao et al. analyzed the changes in the thickness of copper current collectors during the cycling process using scanning electron microscopy. The results showed that the thickness of the porous layer gradually increased/the thickness of the current collector decreased. During the electrochemical cycling process, the dissolution and formation of the porous layer caused by the corrosion of the copper current collector resulted in a continuous decrease in the thickness of the copper current collector, leading to an increase in internal resistance.
Capacity degradation caused by loss of electrode active materials
During the charging and discharging process, lithium ions will be embedded and deintercalated in the positive and negative electrodes, causing changes in the volume of the electrode material and forming mechanical stress. During the discharge process, the negative electrode material undergoes volume shrinkage due to lithium removal, while the positive electrode material undergoes volume expansion due to lithium insertion. When the volume shrinkage of the negative electrode is greater than the volume expansion of the positive electrode, the external performance of the battery will be a total volume shrinkage, otherwise the battery will exhibit volume expansion; During high rate charging, the battery will continue to expand, while during low rate charging, the battery will expand in volume during the early stages of charging, contract during the middle stages of charging, and expand again in the later stages of charging. The volume change of graphite negative electrode under charge and discharge conditions does not exceed 10%, but the stress generated by the volume change during this process still has the possibility of damaging the negative electrode material.
The positive electrode material also undergoes deformation during charging and discharging, such as the presence of LiFePO4 and FePO4 phases in lithium iron phosphate material, with a volume change of approximately 6.81% during the charging and discharging process; The deformation of LiMn2O4 and Mn2O4 during charge and discharge is about 6.5%. Compared to negative electrode materials, positive electrode materials are more affected by stress. Research has found that the diffusion process increases the concentration gradient of lithium ions in electrode materials, leading to local volume expansion. This uneven expansion generates diffusion induced stress (DIS). When the diffusion induced stress exceeds a certain threshold, particle breakage may occur, and the schematic diagram of positive electrode material loss is shown in Figure 5. This phenomenon is more pronounced during rapid charge and discharge processes.
The thermal stress of batteries is mainly caused by internal temperature differences and temperature changes. Shi Qitong indirectly characterized the effect of temperature changes on internal stress by changes in battery thickness direction, but did not analyze battery damage caused by thermal stress. Lu Shigang et al. used simulation modeling methods to quantitatively analyze the factors affecting thermal stress based on the distribution information of the internal temperature field and thermal stress field of square batteries. They found that the temperature was highest at the geometric center, and the central area of the battery was subjected to stress compression due to high-temperature expansion, while the lateral area was subjected to tensile stress; At the same time, there is a phenomenon of concentrated thermal stress at the center of the side. Carlstedt and Asp analyzed the effects of volume and temperature changes on internal stress during the charging and discharging process of cylindrical batteries based on diffusion induced stress caused by differences in lithium ion concentration in electrode materials and thermal stress generated by electrochemical cycling. They believed that stress is related to parameters such as charging and discharging rates and stacking dimensions. Ge et al. believe that electrodes made of materials with negative thermal expansion coefficients can effectively eliminate severe expansion and contraction caused by lithium ion insertion and extraction.
3 Electrolyte and diaphragm decomposition
This article comprehensively analyzes the mechanism of capacity degradation in lithium-ion batteries, classifies and organizes the factors that affect the aging and lifespan of lithium-ion batteries, and elaborates on various mechanisms such as overcharging, SEI film growth and electrolyte, self discharge, active material loss, and current collector corrosion. It summarizes the research progress of scholars in various fields in battery aging mechanisms in recent years, analyzes in detail the influencing factors and modes of action of lithium-ion battery aging, and elaborates on the modeling methods of aging side reactions.
The effect of electrolyte decomposition on capacity degradation
Electrolyte is an ionic conductor that can conduct lithium ions between positive and negative electrodes. As the number of cycles increases, the electrolyte undergoes certain oxidation or decomposition reactions over time, which weakens its mass transfer ability and increases the internal resistance of the battery.
In addition to reacting with the positive and negative electrode surfaces of the battery, the electrolyte also undergoes a series of reactions under lithium deposition and heating; Under heating, the electrolyte may decompose and generate gases such as CO2, and further increase in temperature may even lead to combustion and explosion.
Research has shown that when the operating voltage exceeds the electrochemical stability window of the electrolyte, an oxidative decomposition reaction occurs between the electrolyte and the positive electrode material. The formation of SEI film between electrolyte and negative electrode, as well as the reaction process of electrolyte during lithium evolution, are often studied in conjunction with other forms of aging. Organic solvents in the electrolyte undergo ester exchange and polymerization reactions during battery operation, and conductive salts such as LiPF6 degrade in the reaction to form organic phosphates and fluorites. Henschel et al. analyzed the aging of lithium-ion battery electrolytes from five automotive manufacturers and found that as lithium-ion batteries age, the electrolyte in both energy and power batteries will experience varying degrees of loss, and the concentration of LiPF6 will significantly decrease.
The impact of diaphragm decomposition on capacity degradation
The separator is a key material for lithium-ion batteries, which can isolate electrons. During the charging and discharging process, lithium ions diffuse and propagate, physically separating the positive and negative electrodes. Therefore, the separator is crucial for the safe operation of the battery. To meet the performance requirements of lithium-ion batteries, the separator should have high chemical stability, good wettability, good thermal stability, high mechanical strength, and high porosity. The high porosity of the membrane can meet the requirements of ion transport, while the aging form of the membrane is mainly due to blockage of the membrane pores, which hinders ion transport between electrodes, resulting in power attenuation and impedance increase.
The reason for membrane aging comes from the decomposition products of electrolyte and the blockage of membrane pores by active materials, which can lead to an increase in impedance and a decrease in power capacity. The main reasons for membrane aging are not only electrolyte erosion, lithium dendrites passing through membrane pores, and structural degradation caused by high temperature or cycling, but also uneven deposition of electrolyte decomposition products on the membrane surface, which can lead to a decrease in membrane ion conductivity. Wu et al. analyzed the mechanism of membrane damage and aging, and believed that the main cause of membrane damage is that dendrites generated during lithium evolution may pierce the thin film, leading to a decrease in battery capacity or even internal short circuit. Asymmetric modification on the surface of the membrane can effectively suppress the growth of lithium dendrites and improve the lifespan of the membrane.
4 Temperature+charge discharge rate+overcharge
This article comprehensively analyzes the mechanism of capacity degradation in lithium-ion batteries, classifies and organizes the factors that affect the aging and lifespan of lithium-ion batteries, and elaborates on various mechanisms such as overcharging, SEI film growth and electrolyte, self discharge, active material loss, and current collector corrosion. It summarizes the research progress of scholars in various fields in battery aging mechanisms in recent years, analyzes in detail the influencing factors and modes of action of lithium-ion battery aging, and elaborates on the modeling methods of aging side reactions.
The temperature environment has a significant impact on the performance, safety, and lifespan of lithium-ion batteries. Some studies suggest that lithium-ion batteries are suitable for operating in the temperature range of 15-35 ℃. In practical applications, various thermal management techniques are generally used to regulate the operating temperature of lithium-ion batteries, thereby extending their cycle life and improving the safety of the entire battery life cycle. At low temperatures, the electrochemical reaction rate slows down, the electrolyte conductivity decreases, the SEI film impedance increases, the lithium ion transfer impedance increases, and the polarization voltage increases under charging and discharging conditions. Therefore, lithium deposition is prone to occur during charging, resulting in irreversible decrease in battery capacity and even causing safety risks.
When working at higher temperatures, due to reaction kinetics (Arrhenius effect), the electrochemical reaction rate of lithium-ion batteries increases, the internal resistance decreases, and the capacity increases; Continuous high temperature will accelerate internal side reactions in the battery, causing electrolyte oxidation and decomposition and promoting the formation of SEI film, resulting in irreversible capacity loss and impedance increase. During the operation of lithium-ion batteries, due to the low thermal conductivity of internal components such as electrodes and separators, temperature gradients are generated inside the battery cells. The temperature gradient phenomenon is more pronounced in high rate and low temperature environments, and this spatial temperature distribution difference may exacerbate the non-uniform distribution of current density, thereby accelerating battery degradation.
Charge discharge rate
The current rate can also lead to a decrease in the capacity of lithium-ion batteries. The increase in charge discharge rate will accelerate the capacity decay rate and the growth rate of ohmic resistance and polarization resistance of high-energy lithium-ion batteries, with the growth rate of polarization resistance being higher than that of ohmic resistance. The impact of charge discharge rate on battery pack aging and consistency is mainly manifested in accelerating the aging of single cells with small capacity. For small capacity batteries, under high charge and discharge rates, overcharging and overdischarging phenomena occur more frequently, which accelerates the capacity decay of small capacity batteries and forms positive feedback. This can lead to a decrease in the available capacity of the battery pack, and even pose thermal safety issues due to phenomena such as overcharging and discharging. The mechanism of battery aging caused by high rate charge and discharge cycles is mainly due to the loss of positive electrode active material caused by diffusion induced stress generated during high rate charge and discharge; Considering the decrease in the volume fraction of positive electrode active material during battery aging, it will lead to an increasing trend in the current density per unit area of the electrode material. Therefore, under high rate charge discharge cycle conditions, battery aging will exhibit an accelerated trend.
Dubarry et al. conducted aging experiments on composite positive lithium-ion batteries using multiple charging and discharging rates, and the results showed that high rate charging and discharging would accelerate battery performance degradation; After analyzing the degradation results, it is believed that the aging process can be divided into two stages. The capacity loss in the first stage comes from the loss of active lithium ions caused by the formation of SEI film on the negative electrode surface, while the degradation in the second stage comes from the loss of electrode active materials. Cheng et al. studied the aging characteristics of NCM lithium-ion batteries and found that capacity loss increases with the number of cycles, accompanied by structural damage to the positive electrode material and the formation of negative electrode SEI film during the aging process. Barcellona and Piegari, through Peltier suppression of temperature changes during charge and discharge processes, believe that there is no significant relationship between battery aging and current rate within a certain current rate and specific SOC conditions. Yang et al. discussed the relationship between battery performance degradation and the number of cycles using an electrochemical thermal combined model that includes side reactions. They believed that as the number of cycles increased, there would be a turning point in battery aging, showing a process of transition from approximately linear to nonlinear. The main reason for the later nonlinear accelerated aging was the occurrence of lithium deposition on the negative electrode surface.
Analysis of the impact of overcharging on capacity degradation
The capacity degradation of batteries caused by overcharging mainly includes lithium deposition due to negative electrode overcharging, gas production due to positive electrode overcharging, and intensified side reactions during electrolyte overcharging.
When the negative electrode is overcharged, lithium evolution reaction occurs, leading to the deposition of metallic lithium, which is more likely to occur when there is an excess of positive electrode active material compared to negative electrode active material. However, in the case of high rate charging, even if the ratio of positive and negative electrode active materials is normal, lithium evolution may still occur. The deposition of metallic lithium may cause capacity degradation in batteries from the following aspects: ① leading to a decrease in the amount of recyclable lithium in the battery; ② The precipitated metallic lithium undergoes side reactions with solvents or electrolytes, forming other by-products and consuming the electrolyte, resulting in a decrease in discharge efficiency; ③ Lithium metal mainly deposits between the negative electrode and the separator, which may cause blockage of the separator pores and increase the internal resistance of the battery.
When the ratio of positive electrode active material to negative electrode active material is too low, positive electrode overcharging is prone to occur. Positive electrode overcharging mainly causes capacity degradation of batteries through the generation of electrochemical inert substances, oxygen loss, and other forms. Due to the disruption of capacity balance between electrodes, irreversible loss of battery capacity can occur. At the same time, the oxygen released by the positive electrode reaction may also pose safety hazards to the use of lithium-ion batteries.
If the charging voltage of lithium-ion batteries is too high, it will cause oxidation reactions in the electrolyte and generate insoluble substances (such as Li2CO3) and gases. These by-products will block the electrode micropores, hinder the migration of lithium ions, and cause a decrease in cycling capacity. Moreover, as the electrolyte is consumed, its mass transfer capacity weakens, leading to an increase in the internal resistance of the battery. In addition, if solid products are generated, a passivation film may form on the electrode surface, which will increase battery polarization and reduce the output voltage of the battery.
5 Battery inconsistency+charging method+depth of charge and discharge
This article comprehensively analyzes the mechanism of capacity degradation in lithium-ion batteries, classifies and organizes the factors that affect the aging and lifespan of lithium-ion batteries, and elaborates on various mechanisms such as overcharging, SEI film growth and electrolyte, self discharge, active material loss, and current collector corrosion. It summarizes the research progress of scholars in various fields in battery aging mechanisms in recent years, analyzes in detail the influencing factors and modes of action of lithium-ion battery aging, and elaborates on the modeling methods of aging side reactions.
Internal inconsistency of battery
To meet the energy and power requirements of the entire vehicle, lithium-ion battery cells usually need to be connected in series or parallel before they can be applied in electric vehicles. Due to differences in manufacturing processes, working environments, and other conditions, the cells may exhibit differences in capacity, impedance, cut-off voltage, and other characteristics. This inconsistency may lead to accelerated aging of the battery pack under complex vehicle operating conditions, thereby affecting the durability, reliability, and safety of electric vehicles.
The inconsistency of batteries is mainly caused by subtle differences in manufacturing processes and materials at the factory, as well as differences in the usage environment during subsequent battery use. Inconsistencies are mainly reflected in parameters such as battery voltage, internal resistance, and capacity. The impact of voltage inconsistency on lifespan is mainly reflected at the end of discharge. Cells with lower voltage will reach the cut-off voltage earlier and reach a completely empty state, while other batteries have higher voltages than the cut-off voltage and still have some capacity internally. The discharge of batteries at low SOC has a significant impact on their lifespan, therefore, the aging rate of completely emptied cells will be faster than other batteries.
Research has shown that there is a strong correlation between the inconsistency of lithium-ion battery modules/systems and the inconsistency of lithium-ion battery cells. In general, the service life of a battery pack is less than the service life of the lowest single battery in the battery pack. Due to the inconsistency in the use of lithium-ion battery packs, the actual capacity of each individual cell is different. Therefore, under the same load current conditions, the actual depth of charge and discharge of each cell is also different. Battery packs used in deep discharge conditions for a long time have a shorter lifespan than those used in shallow discharge conditions; Charging and discharging power exceeding the optimal charging and discharging current can also affect the service life of the battery pack. Ziberman et al. studied the aging characteristics of series structured lithium-ion battery packs using differential voltage method combined with scanning electron microscopy. The results showed that a temperature gradient of 5 ℃ would lead to differences in battery aging rate, resulting in capacity degradation and performance decline of the battery pack.
Charging form and strategy
The charging process of lithium-ion batteries has a significant impact on the capacity degradation of lithium-ion batteries. The research results indicate that the charging cut-off voltage of lithium-ion batteries has a significant effect on the aging process. Taking the lithium manganese oxide system lithium-ion battery as an example, assuming its charging cut-off voltage is 4V, slightly reducing the cut-off voltage can effectively improve the available cycle life. But its available capacity will also decrease accordingly. This property can provide guidance for the design of fast charging strategies for lithium-ion batteries. On the other hand, fast charging of lithium-ion batteries also has a significant impact on aging. The research results indicate that aging under fast charging to 100% is more pronounced compared to aging under fast charging to 80%, and even aging under normal charging to 100% is more severe compared to aging under fast charging to 80%.
Pulse discharge can effectively improve charging efficiency and shorten charging time compared to classical constant current (CC) charging or constant current constant voltage (CC-CV) charging methods. The research results indicate that pulse charging can significantly reduce charging time, but increasing the pulse frequency does not significantly improve charging efficiency when using the same pulse charging method. However, pulse charging has a significant impact on battery aging. The experimental results of Li et al. showed that the internal resistance of lithium-ion batteries significantly increased under pulse charging conditions, and analysis based on scanning electron microscopy revealed more severe loss of negative electrode active materials.
Depth of charge and discharge
The research results indicate that during the charging and discharging process of lithium-ion batteries, deep charging and discharging will accelerate the capacity degradation of lithium-ion batteries, and at this time, the ohmic resistance and polarization resistance of lithium-ion batteries will both increase; On the other hand, under the same depth of charge and discharge, lithium-ion batteries cycled in the high SOC range are more prone to aging compared to those cycled in the low SOC range, which may be due to the problem of lithium deposition in the high SOC range. In addition, during the accelerated cycle aging process of lithium-ion batteries, the aging rate under constant current charging conditions is higher than that under constant current and constant voltage charging conditions. Therefore, extending the idle time during charging and discharging or using extremely low current charging at the end of charging is beneficial for prolonging the battery life.