Lithium-ion batteries often experience various failure phenomena during use or storage, including capacity decay, increased internal resistance, reduced rate performance, gas generation, leakage, short circuit, deformation, thermal runaway, and lithium plating. These failures severely degrade the performance, reliability, and safety of lithium-ion batteries. These failures are caused by the interaction of a series of complex chemical and physical mechanisms within the battery.
Lithium-ion battery systems are complex, involving thermodynamics, kinetics, microstructure, inter-component interactions and reactions, and surface and interface reactions. A correct analysis and understanding of failure phenomena plays a crucial role in improving lithium-ion battery performance and technological advancements. This article begins with battery failure phenomena and provides a brief introduction to failure mechanisms.
Failure behavior and failure mechanism of lithium-ion batteries
1. Capacity decay
Capacity decay is mainly divided into two categories: reversible capacity decay and irreversible capacity decay. Reversible capacity decay can be recovered by adjusting the battery charging and discharging regime and improving the battery usage environment; while irreversible capacity decay is caused by irreversible changes inside the battery, resulting in unrecoverable capacity loss.
The root cause of battery capacity degradation lies in material failure, which is also closely related to objective factors such as battery manufacturing process and battery usage environment. From a material perspective, the main causes of failure include structural failure of the positive electrode material, excessive growth of SEI on the negative electrode surface, electrolyte decomposition and deterioration, current collector corrosion, and trace impurities in the system.
2. Increased internal resistance
The internal resistance of a lithium-ion battery is related to the electron and ion transport processes within the battery system, and is mainly divided into ohmic resistance and polarization resistance. Polarization resistance is primarily caused by electrochemical polarization, which can be either electrochemical or concentration polarization. The main factors leading to increased internal resistance in lithium-ion batteries are the battery's key materials and the battery's operating environment.
3. Internal short circuit
Short circuits can manifest in the following four ways:
Short circuit between copper/aluminum current collectors;
If the diaphragm fails and loses its electronic insulation or its porosity increases, the positive and negative electrodes may become in close contact, resulting in severe localized heating. During further charging and discharging, this heating may spread to the surrounding areas, leading to thermal runaway.
If transition metal impurities in the positive electrode slurry are not completely removed, they may puncture the separator or promote the formation of lithium dendrites in the negative electrode, leading to an internal short circuit.
Lithium dendrites can cause internal short circuits.
In addition, unreasonable design or excessive local pressure during battery design and manufacturing or battery pack assembly can also lead to internal short circuits; internal short circuits can also occur under the induction of overcharging and over-discharging of batteries, mainly due to corrosion of the current collector, which causes deposits on the electrode surface, and in severe cases, the positive and negative electrodes can be connected through the separator.
4. Gas production
Gas production in lithium-ion batteries is mainly divided into normal gas production and abnormal gas production. Normal gas production occurs during the battery formation process when electrolyte is consumed to form a stable SEI film. Gas production during the formation stage mainly results from ester single/double electron reactions producing H2, CO2, C2H2, etc. Abnormal gas production mainly occurs during battery cycling, such as excessive electrolyte consumption releasing gas or oxygen release from the positive electrode material. This often occurs in pouch cells, causing excessive internal pressure leading to deformation, rupture of the aluminum encapsulation film, and internal cell contact problems.
5. Thermal runaway
Thermal runaway refers to a rapid increase in temperature in a localized area or throughout a lithium-ion battery, where heat cannot dissipate in time, accumulates in large quantities inside, and induces further side reactions.
To prevent lithium-ion batteries from causing serious safety problems due to thermal runaway, measures such as PTC, safety valves, and thermal conductive films are often used. At the same time, systematic consideration is needed in battery design, battery manufacturing process, battery management system, and battery usage environment.
6. Lithium plating
Lithium plating is a relatively common aging failure phenomenon in lithium-ion batteries. It manifests primarily as a layer of gray, grayish-white, or grayish-blue substance appearing on the surface of the negative electrode; this substance is metallic lithium deposited on the negative electrode surface.
