I. Analysis of Lithium-ion Battery Capacity Degradation
Positive and negative electrodes, electrolyte, and separator are the essential components of a lithium-ion battery. Lithium insertion and extraction reactions occur at the positive and negative electrodes, and the amount of lithium inserted into each electrode is the primary factor affecting the battery’s capacity. Therefore, maintaining a balance in the capacity of the positive and negative electrodes is crucial for optimal battery performance.
Generally, lithium-ion batteries use an electrolyte solution composed of an organic solvent and an electrolyte (lithium salt). This electrolyte solution should possess sufficient conductivity and stability, and be compatible with the electrodes. For the separator, its performance is a major factor determining the battery’s internal resistance and interface structure, directly impacting capacity decay. A high-quality separator significantly improves the capacity and overall performance of the lithium-ion battery. Typically, the separator primarily separates the positive and negative electrodes, preventing short circuits caused by contact between them, while also allowing electrolyte ions to pass through, maximizing battery efficiency.
The chemical reactions in lithium-ion batteries include not only the redox reactions during lithium-ion insertion and extraction, but also side reactions such as the formation and destruction of the SEI film on the negative electrode surface, electrolyte decomposition, and structural changes and dissolution of active materials. These side reactions are all causes of lithium-ion battery capacity decay.
Capacity decay and loss during battery cycling are inevitable phenomena. Therefore, in order to improve battery capacity and performance, scholars in various fields both domestically and internationally have thoroughly studied the mechanisms of lithium battery capacity loss. Currently, the main factors causing lithium-ion battery capacity decay are known to include the formation of SEI passivation films on the positive and negative electrode surfaces, lithium metal deposition, dissolution of electrode active materials, the occurrence of redox reactions or side reactions at the anode and cathode, structural changes, and phase changes. The study of lithium-ion battery capacity decay and its causes is still ongoing.
II. Overcharging
2.1 Overcharging Reaction of the Negative Electrode
Many types of active materials can be used as negative electrodes in lithium-ion batteries, with carbon-based, silicon-based, tin-based, and lithium titanate negative electrode materials being the main materials. Different types of carbon materials have different electrochemical properties. Among them, graphite has advantages such as high conductivity, excellent layered structure, and high crystallinity, making it suitable for lithium insertion and extraction. Graphite is also affordable and readily available, thus its application is widespread.
When a lithium-ion battery is first charged and discharged, solvent molecules decompose on the graphite surface, forming a passivation film called SEI. This reaction causes battery capacity loss and is an irreversible process. During overcharging, lithium metal deposition occurs on the negative electrode surface. This is more likely to occur when the positive electrode active material is in excess relative to the negative electrode active material. Furthermore, lithium metal deposition may also occur under high-rate conditions.
Generally, the formation of metallic lithium leading to capacity decay in lithium batteries mainly includes the following aspects: First, it reduces the amount of cyclic lithium in the battery; second, metallic lithium undergoes side reactions with the electrolyte or solvent, forming other byproducts; third, metallic lithium mainly deposits between the negative electrode and the separator, causing blockage of the separator pores and increasing the battery’s internal resistance. The mechanism of capacity decay in lithium-ion batteries varies depending on the graphite material. Natural graphite has a higher specific surface area; therefore, self-discharge reactions will lead to capacity loss in lithium batteries. Furthermore, natural graphite, as the negative electrode, has a higher electrochemical resistance than artificial graphite. In addition, factors such as the dissociation of the layered structure of the negative electrode during cycling, the dispersion of conductive agents during electrode fabrication, and the increase in electrochemical resistance during storage are all important factors contributing to capacity loss in lithium batteries.
2.2 Positive Electrode Overcharge Reaction
Positive electrode overcharging mainly occurs when the proportion of positive electrode material is too low, leading to capacity imbalance between electrodes and irreversible capacity loss in the lithium battery. Furthermore, the coexistence and continuous accumulation of oxygen and combustible gases released from the decomposition of the positive electrode material and electrolyte may pose safety hazards to the use of the lithium battery.
2.3 Electrolyte Reaction at High Voltage
If the charging voltage of the lithium battery is too high, it will cause oxidation of the electrolyte, generating some byproducts that clog the electrode micropores, hindering lithium-ion migration and causing changes in cycle capacity decay. The change trend of electrolyte concentration and electrolyte stability is inversely proportional; the higher the electrolyte concentration, the lower the electrolyte stability, thus affecting the capacity of the lithium-ion battery. During charging, some electrolyte is consumed; therefore, it needs to be replenished during assembly, leading to a reduction in battery active material and affecting the initial capacity of the battery.
III. Electrolyte Decomposition
The electrolyte comprises electrolyte, solvent, and additives, and its properties affect the battery’s lifespan, specific capacity, rate charge/discharge performance, and safety performance. Decomposition of both the electrolyte and solvent in the electrolyte leads to battery capacity loss. During the first charge/discharge cycle, the formation of an SEI film on the negative electrode surface by solvents and other substances results in irreversible capacity loss, which is inevitable. If impurities such as water or hydrogen fluoride are present in the electrolyte, the LiPF6 electrolyte may decompose at higher temperatures, and the resulting products react with the positive electrode material, affecting battery capacity. Simultaneously, some products may react with the solvent, affecting the stability of the SEI film on the negative electrode surface, causing lithium-ion battery performance degradation. Furthermore, if the decomposition products of the electrolyte are incompatible with the electrolyte, they will block the positive electrode pores during migration, leading to battery capacity degradation. In summary, the occurrence of side reactions between the electrolyte and the battery’s positive and negative electrodes, as well as the generated byproducts, are the main factors causing battery capacity degradation.
IV. Self-Discharge
Lithium-ion batteries typically experience capacity loss, a process known as self-discharge, which can be reversible or irreversible. The solvent oxidation rate directly impacts the self-discharge rate. During charging, the active materials of both the positive and negative electrodes may react with the solute, leading to lithium-ion migration, capacity imbalance, and irreversible degradation. Therefore, reducing the surface area of the active materials can decrease the capacity loss rate, and solvent decomposition affects battery lifespan. Separator leakage can also cause capacity loss, although this is less likely. Prolonged self-discharge can lead to lithium metal deposition, further causing capacity decay in both the positive and negative electrodes.
V. Electrode Instability
During charging, the active material of the battery’s positive electrode is unstable, which can lead to reactions with the electrolyte and affect battery capacity. Among the main factors affecting battery capacity are structural defects in the positive electrode material, excessively high charging potential, and carbon black content.
