1. Factors affecting aging
The optimal operating temperature for power lithium batteries is 15℃-35℃, but in daily applications, it is impossible to fully meet the requirements of the battery. Therefore, the most common scenarios that affect battery aging are high temperature and low temperature.
Besides the environment, battery operating parameters can also accelerate or slow down aging, so the selection of cell charging and discharging parameters has a significant impact.
Under the external factors listed above, battery electrode materials and other components may undergo side reactions beyond normal charging and discharging during the electrochemical reaction process, leading to aging.
2. Typical aging process
The specific details of the aging process are closely related to the selection of positive and negative electrode materials, electrolyte, and separator. This article explains the general aging process and will not provide detailed explanations for specific materials.
2.1 High-temperature aging
50℃ to 60℃ is the upper limit of the allowable operating temperature range for general lithium-ion batteries. At higher temperatures, the electrolyte is more reactive and prone to decomposition. These decomposition products combine with the cathode material, consuming it; the cathode material corrodes, and the crystal lattice collapses due to insufficient support, reducing lithium-ion vacancies and decreasing the cathode's ability to hold lithium ions, thus resulting in a loss of battery capacity.
Meanwhile, the products of the positive electrode material reaction, floating in the electrolyte, may adhere to the surfaces of the positive and negative electrodes. The electrode surfaces are covered by substances that cannot participate in the charging and discharging process, hindering the smooth occurrence of the electrochemical process and increasing the internal resistance of the cell.
The effects of high-temperature processes on aging occur primarily at the positive electrode, with a relatively small impact on the negative electrode.
2.2 Low temperature aging
When the ambient temperature drops below 0°C, the performance of lithium-ion batteries begins to be significantly affected by the low temperature. The SiE film, or SiE layer, is a passivation film formed during the cell formation process between the negative electrode material and the electrolyte, serving to protect the negative electrode material.
During low-temperature operation, the SEI film grows, consuming some of the active lithium ions in the electrolyte. This reduces the concentration of conductive ions in the electrolyte, resulting in a permanent loss of usable battery capacity. The thickening of the SEI film increases the difficulty for lithium ions to penetrate the film and reach the negative electrode. Combined with the reduced concentration of conductive lithium ions, this leads to an increase in the cell's internal resistance.
Charging at low temperatures, especially with high charging current, can lead to another side reaction at the negative electrode: lithium deposition. At low temperatures, lithium-ion activity decreases, and forced charging causes excess lithium ions to accumulate around the negative electrode. These ions cannot penetrate the SEI film to intercalate and instead deposit on the negative electrode surface, forming a pure lithium layer. This process is prone to occur at excessively low charging temperatures and is irreversible. With repeated use, elemental lithium continues to accumulate, dendrites grow, and the risk of puncturing the separator increases steadily.
When lithium-ion batteries operate at low temperatures, aging issues primarily occur at the negative electrode. Side reactions also exist at the positive electrode, but their impact is not significant.
2.3 High Current Charging and Discharging
Discharging with a current exceeding the design discharge capacity has two main effects. First, the thermal effect of the current causes the battery temperature to rise, gradually exacerbating the side effects of high-temperature aging. Second, the large current causes an excessive amount of lithium ions to embed into the cathode material, impacting the material's stability.
High-current discharge also presents issues such as heat generation and the stability of the cathode material's insertion/extraction. Furthermore, excessive lithium ion transport to the anode, exceeding its capacity, leads to lithium deposition. This not only results in capacity loss but also increases the risk of thermal runaway during long-term use, causing even more serious damage.
2.4 Overvoltage and Undervoltage Charge/Discharge
Overvoltage charging and undervoltage discharging can both cause phase transitions in the cathode material, reducing the number of lithium-ion vacancies and affecting the maximum usable capacity of the cell.
2.5 Self-discharge
Self-discharge of battery cells occurs constantly, and it becomes more pronounced at higher temperatures and with higher charge levels. Self-discharge results in a loss of both reversible and irreversible capacity. The byproducts of self-discharge adhere to the electrode surface, blocking lithium-ion channels and reducing lithium-ion insertion sites, ultimately leading to permanent capacity loss in the battery cell.
3. Module aging
Lithium-ion batteries are connected in series and parallel to form modules. The aging of the module is directly affected by the aging of the individual cells. In addition, the aging of the cells leads to a deterioration in the consistency between the cells, which means that the aging of the module is amplified on top of the aging of the cells.
Besides the effects of cell aging, the aging process of the module can be accelerated by factors such as vibration and oxidation corrosion of conductive components. Inside the module, the cells and busbars, as well as the busbars and module terminals, are connected by welding or screws to maintain tight contact and ensure resistance is within a reasonable range. However, the increased connection resistance caused by vibration and oxidation alters the distribution of resistance within the module. These changes can affect the measured cell voltage, thereby impacting the charging, discharging, and equalization processes of the cells.
Harsh environments and excessive operating parameters make the aging process more pronounced and easier for researchers to observe. In reality, the aging process occurs silently throughout the entire process. A battery cell has two lifespans: calendar life and cycle life. The name "calendar life" itself suggests the relentless aging process. Therefore, research on factors influencing battery cell aging focuses on mitigating the accelerated aging caused by improper operation.
