Why Does Lithium Battery Capacity Decrease?

Positive and negative electrodes, electrolyte, and separator are the important components of a lithium-ion battery. Lithium insertion and extraction reactions occur at the positive and negative electrodes, and the amount of lithium inserted at each electrode is the main factor affecting the battery capacity. Therefore, maintaining the capacity balance of the positive and negative electrodes is essential to ensure 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 the main factor determining the battery's internal resistance and interface structure, directly affecting the capacity decay. A high-quality separator will significantly improve the capacity and overall performance of the lithium-ion battery. Typically, the separator mainly functions to separate the positive and negative electrodes, preventing short circuits caused by contact between them, while also allowing electrolyte ions to pass through, thus 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 from various fields at home and abroad have thoroughly studied the mechanisms of lithium battery capacity loss. Currently, the main factors causing lithium-ion battery capacity decay 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. Currently, the changes in lithium-ion battery capacity decay and their causes are still under continuous research.

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 exhibit varying electrochemical properties. Graphite, with its high conductivity, excellent layered structure, and high crystallinity, is well-suited for lithium intercalation and deintercalation. Furthermore, graphite is affordable and readily available, making it widely used.

During the initial charge and discharge of a lithium-ion battery, solvent molecules decompose on the graphite surface, forming a passivation film called SEI (Sediment Injection). This reaction leads to irreversible capacity loss. During overcharging, lithium metal deposition occurs on the negative electrode surface, particularly when the positive electrode active material is in excess relative to the negative electrode active material. Lithium metal deposition can also occur under high-rate conditions.

Generally, the formation of lithium metal leading to capacity decay in lithium batteries primarily stems from the following:

First, it reduces the amount of cyclic lithium in the battery;

Second, lithium metal reacts with the electrolyte or solvent, forming other byproducts;
Third, lithium metal mainly deposits between the negative electrode and the separator, causing pore blockage and increasing internal resistance. The mechanism by which different graphite materials affect the capacity decay of lithium-ion batteries varies. Natural graphite has a higher specific surface area; therefore, self-discharge reactions will lead to capacity loss in lithium-ion batteries. Furthermore, natural graphite, as the negative electrode, has a higher electrochemical resistance than synthetic 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-ion batteries.

Positive electrode overcharging mainly occurs when the proportion of positive electrode material is too low, leading to capacity imbalance between electrodes and causing irreversible capacity loss in lithium-ion batteries. 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 lithium-ion batteries.

ACEY-BCT506-512H battery charge discharge tester is used for battery formation, capacity measurement and performance parameter testing, etc. It is a master-slave type battery comprehensive testing system composed of a computer and a single-chip microcomputer. Its structure is advanced, performance is reliable, operation is convenient, efficiency is high, and it has an extremely high cost-performance ratio. It is an indispensable production testing equipment for lithium battery manufacturers, capacity measurement manufacturers and research institutions.

If the charging voltage of a lithium-ion battery is too high, it will cause oxidation of the electrolyte and generate some byproducts, which will clog the electrode micropores, hindering the migration of lithium ions and thus causing changes in cycle capacity decay. The stability of the electrolyte is inversely proportional to its concentration; higher concentrations lead to lower stability, thus affecting the capacity of the lithium-ion battery. During charging, electrolyte is consumed, requiring replenishment during assembly, which reduces the amount of active material and impacts initial capacity.

The electrolyte comprises electrolyte, solvent, and additives, and its properties affect battery lifespan, specific capacity, rate charge/discharge performance, and safety. Decomposition of both the electrolyte and solvent causes capacity loss. During the initial 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. The presence of impurities such as water or hydrogen fluoride in the electrolyte may cause the LiPF6 electrolyte to decompose at higher temperatures, and the resulting products react with the positive electrode material, affecting battery capacity.

Simultaneously, some products react with the solvent, affecting the stability of the SEI film on the negative electrode surface and causing lithium-ion battery performance degradation. In addition, if the products of electrolyte decomposition are incompatible with the electrolyte, they will block the positive electrode pores during migration, leading to battery capacity decay. In general, the occurrence of side reactions between the electrolyte and the positive and negative electrodes of the battery, as well as the resulting byproducts, are the main factors causing battery capacity decay.

Lithium-ion batteries generally experience capacity loss, a process known as self-discharge, which is divided into reversible and irreversible capacity loss. The solvent oxidation rate directly affects the self-discharge rate. The positive and negative electrode active materials may react with the solute during charging, leading to lithium-ion migration, capacity imbalance, and irreversible decay. Therefore, reducing the surface area of ​​the active material can reduce the capacity loss rate, and solvent decomposition affects battery storage life. Separator leakage can also cause capacity loss, but this is less likely. If self-discharge persists for a long time, it will lead to lithium metal deposition, further causing capacity decay in both the positive and negative electrodes.

During charging, the active material of the battery's positive electrode is unstable, causing it to react with the electrolyte and affecting battery capacity. Among the factors affecting battery capacity, structural defects in the cathode material, excessively high charging potential, and carbon black content are the main factors.

Spinel LiMn2O4 is abundant and inexpensive in my country, possesses good thermal stability, and is a primary material for battery cathodes. Storage of LiMn2O4 cathodes at high temperatures and during battery charge-discharge cycles lead to capacity decay, primarily due to the following factors: First, under high voltage conditions, electrochemical reactions occur in the electrolyte, generally above 4.0V; second, Mn in LiMn2O4 dissolves in the electrolyte, causing a disproportionation reaction that disrupts the crystal structure of the cathode material.

Since carbon black is an inactive substance and does not participate in the discharge reaction, excessive carbon black content in the cathode material will affect its strength and capacity. Therefore, it needs to be added appropriately. In addition, the transport carrier generates a catalytic substance on the carbon black surface, which can enhance the decomposition rate of metal ions and effectively promote the dissolution of active substances.

Acey New Energy is specialized in researching and manufacturing of high-end equipment for lithium-ion batteries. If you are interested in lithium battery capacity tester, please feel free to contact us at any time.