What is the reason for the short circuit inside the battery?

Internal short circuit in lithium-ion batteries refers to the phenomenon where the active materials of the positive and negative electrodes come into contact with each other, resulting in discharge and heat generation when the battery separator fails. Internal short circuit is the most common characteristic of thermal runaway, and almost all abuse conditions are accompanied by the occurrence of internal short circuit. There are various causes of internal short circuit faults, including internal defects, mechanical compression, overcharging, overdischarging, and overheating, as shown in the following figure. Through comprehensive analysis of lithium-ion battery safety accidents, combined with existing relevant research, the main factors causing internal short circuits can be divided into internal defects, mechanical abuse, electrical abuse, and thermal abuse.

According to the classic battery internal short circuit model, there are four forms of internal short circuits:
Positive negative short circuit, positive Cu foil short circuit, AI foil negative short circuit, and AI foil Cu foil short circuit.
Among them, the third situation has the highest risk of short circuit due to its low resistance and the possibility of continuous occurrence. It is most likely to cause thermal runaway of the battery. Considering the conductivity and thermal conductivity of the material, the short circuit between the AI foil and the negative electrode is usually considered the most dangerous.

The evolution process of internal short circuit can be divided into early, middle and late stages. In the later stage, internal short circuit is prone to causing thermal runaway, which poses great harm to battery and vehicle safety, as shown in the following figure. As shown in the figure, in the early stage of the internal short circuit evolution process, the voltage change is more significant than the temperature change. Therefore, in the early stage of the internal short circuit evolution process, the electrical characteristic parameters can be used as the characteristic parameters of the detection method; During the mid-term of the internal short circuit evolution process, both voltage and temperature changes are quite drastic, which can be detected by adding temperature as a characteristic parameter in the detection method; In the later stage of the internal short circuit evolution process, irreversible thermal runaway is prone to occur, causing accidents.

The production of lithium-ion batteries requires a series of complex manufacturing processes, each of which affects the performance of the battery. Internal defects and other issues generated during the production process can cause internal short circuits. WUYi et al. conducted research on internal defects in the manufacturing process of lithium-ion batteries, and the results showed that internal defects are mainly divided into two categories: structural deformation during the manufacturing process and defects caused by manufacturing. Structural deformation mainly occurs on the battery separator and electrode, such as separator cracking and electrode burrs; Manufacturing induced defects mainly refer to the introduction of foreign objects during the manufacturing process, including metal particles and non-metal particles. Pinholes or cracks inside the battery separator can cause the separator to fail, resulting in direct contact between the positive and negative electrodes and causing an internal short circuit. During the production and manufacturing process of lithium-ion batteries, non-standard operating environments may introduce foreign objects into the interior of the battery.

There are two main mechanisms that cause internal short circuits for metal foreign particles: one is that the metal foreign particles directly puncture the diaphragm, causing internal short circuits; Another method is for the metal particles mixed in the positive electrode to dissolve first, and then precipitate in the negative electrode to form dendrites that pierce the separator, causing an internal short circuit.
The study on the influence of metal foreign particles shows that
(1) Even if there are metal foreign objects in the battery with a particle size greater than 20-28 times the thickness of the separator, and the battery is cycled or stored under constrained conditions, the metal foreign objects will not puncture the separator and cause internal short circuits in the battery;
(2) Metal foreign objects located in the center of the negative electrode cannot cause internal short circuits in the battery;
(3) Only metal foreign objects located in the center of the positive electrode or near the positive electrode tab may cause internal short circuits in the battery;
(4) For batteries containing larger metal particles, they can generally be screened out in the early stages through aging and self discharge screening;
(5) Metal foreign objects with a particle size less than 100 μ m will not significantly affect battery cycling or self discharge, but metal foreign objects with a particle size of 150 μ m will significantly deteriorate cycling.

Electricity abuse is one of the most common safety hazards in the use of lithium-ion batteries, and overcharging and overdischarging are the main factors leading to electricity abuse. Research has shown that overcharging can precipitate metallic lithium at the negative electrode of the battery, promote the growth of lithium dendrites, and then penetrate the separator, causing internal short circuits.
Overcharging refers to the situation where a battery is forced to continue charging even after reaching the nominal cut-off voltage during charging conditions. When overcharged, the negative electrode material is destroyed and decomposed, and the electrolyte decomposes to produce gas. The gas produced changes the distance between the positive and negative electrodes, and lithium deposition is more likely to occur in areas near the graphite edge of the negative electrode, promoting the continuous growth of lithium dendrites and ultimately penetrating the separator, causing internal short circuits.
Overdischarge refers to the situation where the battery continues to discharge even when its voltage drops below the cut-off voltage. Research has shown that over discharge can cause irreversible chemical reactions, and at the same time, the internal structure of the battery can also be damaged, leading to internal short circuits.

High temperature is one of the most common causes of thermal abuse in lithium-ion batteries. Lithium ion batteries have a narrow operating temperature range and poor heat resistance. If they work in a high-temperature environment for a long time, it can cause irreversible damage and, in severe cases, may even lead to dangerous accidents such as fire and explosion. Therefore, the battery thermal management system must strictly control the operating temperature of lithium-ion batteries. Research has shown that a malfunction in the battery thermal management system may cause the battery to overheat and trigger an internal short circuit. At high temperatures, unstable substances will decompose at the negative electrode of the battery, generating heat and oxygen. The generated heat further promotes temperature rise, oxygen oxidizes the electrolyte, intensifies side reactions, raises the battery temperature, and triggers internal short circuits. High temperature can also promote hydrolysis and pyrolysis reactions inside the battery, accelerating the decomposition of the separator. Without the protection of the separator, the reaction between the positive electrode and the electrolyte becomes more intense, generating more heat. The large amount of heat causes the separator to melt, leading to an internal short circuit.

Electric vehicles are inevitably prone to collision accidents during daily use. When a collision occurs, strong external forces can cause deformation and displacement of the battery, known as mechanical abuse. Mechanical abuse mainly includes squeezing and puncturing. When mechanical abuse occurs, it can cause the battery separator to puncture or fail, leading to internal short circuits. In severe cases, it can cause major accidents such as thermal runaway, fire, and explosion.
Squeezing can cause serious damage to the battery, such as battery deformation and electrolyte leakage. When the battery is compressed by external force, the shell of the battery will first be subjected to external force and deform; Squeezing further develops, and the squeezing force will be transmitted to the inside of the battery, causing deformation of the internal components of the battery.
Puncture is another common phenomenon under mechanical abuse. Research has shown that when a puncture occurs, an external conductor will penetrate the diaphragm, directly connecting the positive and negative electrodes, causing an internal short circuit.