Internal mechanisms and external factors contributing to the lifespan degradation of power lithium batteries
Internal Mechanism Analysis Affecting Lithium Battery Life
Lithium-ion batteries convert chemical energy into electrical energy through normal chemical reactions. Theoretically, the internal reaction is a redox reaction between the positive and negative electrodes, which generates current through the insertion and extraction of ions. Therefore, the lithium-ion concentration usually remains constant. However, in actual battery cycling, lithium ions undergo many side reactions in addition to the normal reactions, such as the formation and growth of the SEI film and the decomposition of the electrolyte. Any reaction that generates or depletes lithium ions will disrupt the internal balance of the battery, and this imbalance will severely impact the battery. The internal factors that cause lithium-ion battery capacity reduction and lifespan degradation are as follows:
1. Changes in cathode materials
The dissolution and structural changes of cathode materials are important reasons for these changes. As the number of battery cycles increases, lithium ions continuously intercalate and deintercalate within the cathode, causing the lattice volume of the cathode material to expand and contract, thus altering its structure and reducing its ability to intercalate and deintercalate lithium ions. Dissolution of the cathode material generally occurs during deep discharge. This dissolution forms a solid film on the surface of the cathode material, hindering the intercalation and deintercalation of lithium ions, which also leads to battery capacity degradation.
2. Electrolyte decomposition
The positive electrode typically decomposes into insoluble products such as lithium fluoride. These insoluble products can clog pores, consume active materials, and reduce battery capacity. The decomposition voltage of the positive electrode is usually greater than 4.5V, so the electrolyte is not easily decomposed at the positive electrode. The electrolyte is not very stable in the graphite layer and is prone to side reactions, reducing the internal lithium-ion concentration and causing battery capacity decay. During charging, the electrolyte is prone to reduction reactions at the negative electrode, which can cause electrolyte decomposition and solvent reduction. During the initial charge and discharge, a passivation film (SEI film) forms inside the battery to prevent further oxidation of the electrolyte and negative electrode. All of these factors can adversely affect the battery capacity.
3. Formation and growth of SEI membrane
The SEI film forms during the initial charge and discharge of the battery. Its formation consumes some active material. The SEI film protects the negative electrode, preventing direct contact and reaction between the electrolyte and the negative electrode, thus preventing the loss of active lithium and extending the battery's cycle life. However, in subsequent cycles, the continuous expansion and contraction of the electrode material exposes new positioning points. The SEI film continues to grow as these new positioning points are exposed, leading to continuous lithium-ion loss, which macroscopically manifests as a decrease in capacity.
4. Formation of lithium dendrites
If a battery is charged at a current higher than its rated current for an extended period or charged and discharged at low temperatures, lithium dendrites are likely to form on its negative electrode. These lithium dendrites can easily pierce the separator, causing direct contact between the positive and negative electrodes and resulting in an internal short circuit. This is a devastating failure for the battery, and lithium dendrites are difficult to detect before a short circuit occurs.
5. The impact of inactive ingredients
As the number of battery cycles increases, the binder inside the battery decomposes, causing the active materials inside the battery to continuously detach. This reduces the amount of active material participating in the reaction between the positive and negative electrodes. The current collector also corrodes after multiple charge-discharge cycles, and the corrosion products passivate the active materials, leading to a continuous increase in the battery's internal resistance. Most failure mechanisms inside lithium batteries are caused by the formation of lithium dendrites, changes in the positive electrode material, and the decomposition of the electrolyte. In particular, the formation of lithium dendrites can easily cause short circuits, leading to thermal runaway of the cell, which, if not properly controlled, can result in a cell explosion.
Lithium-ion battery failure research essentially involves studying battery failure modes and mechanisms to optimize batteries and improve their safety. Therefore, battery failure research not only provides important guidance for actual production and operation but also plays a crucial role in improving battery life, the safety and reliability of electric vehicles, and reducing the operating costs of electric vehicles.
Analysis of external factors affecting lithium battery life
Studies have shown that external factors affecting lithium battery capacity and lifespan degradation include temperature and charge/discharge rate, which are determined by the user's operating conditions and actual usage. The following are some of the most common external factors affecting battery aging.
1. Depth of Discharge (DOD)
Depth of discharge (DOD) refers to the percentage of a battery's rated capacity that it can discharge from a fully charged state. When a battery discharges to its cutoff voltage, the discharge rate is 100% DOD. 60% DOD refers to a battery between 100% SOC and 40% SOC. The greater the DOD, the greater the amount of electricity discharged. Studies show that under DOD conditions (20%–80%), the increase in AC internal resistance during charging and discharging is relatively small. However, deep discharge increases the battery's internal resistance, thus reducing its lifespan.
2. Overcharging
When a battery is overcharged, the negative electrode is already saturated with lithium ions. However, continued lithium ion insertion causes lithium ions to deposit on the negative electrode surface, blocking the porous material and making lithium ion extraction more difficult. This also reduces the concentration of lithium ions inside the battery, resulting in a loss of battery capacity. Overcharging leads to an increase in battery voltage. When it exceeds a critical value, the electrolyte oxidizes, generating insoluble substances and gases. These insoluble substances block the pores of the porous material, reducing ion transport rates. Overcharging is generally prevented by setting charging cutoff voltage and charging cutoff current. Regardless of whether it's a nickel-metal hydride or lithium-ion battery, when overcharging occurs, a large amount of heat from the current conversion is dissipated, causing numerous reactions within the battery, such as reactions between the positive and negative electrodes and the electrolyte. This reduces the maximum capacity of the lithium-ion battery. When the accumulated heat is difficult to dissipate, it can even lead to fires and explosions.
3. Self-discharge
Lithium-ion batteries are prone to self-discharge, which typically manifests as a loss of battery capacity. While most self-discharge is reversible, irreversible self-discharge still occurs. Irreversible self-discharge can be caused by various factors, such as losses due to irreversible lithium-ion reactions, increased internal resistance caused by insoluble substances generated from electrolyte oxidation blocking micropores, and the rise of the SEI film. These chemical reactions reduce the internal lithium-ion concentration of the battery, leading to capacity decay.
4. Ambient temperature
Lithium-ion batteries experience performance changes at excessively high or low temperatures. Low temperatures affect the activity of the electrolyte, reducing charge and discharge efficiency. High temperatures disrupt the battery's internal chemical balance, causing irreversible side reactions, deforming the electrode structure, reducing battery capacity, and decreasing the number of battery cycles.
5. Pressure
Pressure. To facilitate the diffusion of lithium ions within the battery, the separator and positive and negative electrodes of lithium batteries typically have porous structures. Pressure can affect the porosity and tortuosity of porous materials, thus indirectly affecting the diffusion rate of lithium ions between the positive and negative electrodes and the separator, thereby impacting the discharge performance of the lithium battery. Without pressure, the battery would be difficult to hold in place; however, excessive pressure can cause the interlayer spacing of the graphite in the negative electrode to become too small, increasing the interlayer van der Waals forces. This increases the resistance to lithium ion insertion, resulting in a decrease in the number of inserted lithium ions and consequently, a reduction in capacity. Therefore, research on battery pressure is essential.
