1. Chemical structure damage
During the thermal runaway of the battery, a series of violent chemical reactions occur inside the battery. In lithium-ion batteries, for example, high temperatures cause a change in the crystal structure of the positive electrode material. Like the common lithium nickel cobalt manganese (NCM) ternary cathode material, at the thermal runaway temperature, the lithium ions in the lattice will lose the original ordered arrangement, resulting in a decline in the performance of the cathode material. This structural damage is irreversible and will greatly reduce the capacity and performance of the battery.
At the same time, the negative electrode material of the battery will also be affected. In the case of the negative electrode of graphite, high temperatures may rupture the solid electrolyte interface (SEI) film on the graphite surface. The SEI film originally played a role in protecting the negative electrode and controlling the transmission of lithium ions, but the rupture will lead to direct contact between the negative electrode and the electrolyte, further causing side reactions and accelerating the aging of the battery.
2. Decomposition and loss of electrolyte
Thermal runaway will cause a large amount of decomposition of the electrolyte. The electrolyte is the medium of ion transport in the battery and is essential for the normal operation of the battery. At high temperatures, the organic solvent in the electrolyte (such as vinyl carbonate, dimethyl carbonate, etc.) will decompose. These decomposition reactions not only consume the electrolyte, but also produce gas, resulting in increased pressure inside the battery.
After the decomposition of electrolyte, its composition changes and the ion transport ability decreases. For example, decomposition products may form a passivation film on the surface of the electrode, preventing the insertion and removal of lithium ions, thus reducing the efficiency of the battery's charge and discharge. With the continuous loss of electrolyte and the deterioration of performance, the cycle life of the battery will be significantly shortened.
3. The occurrence and aggravation of internal short circuit
During the thermal runaway process, the diaphragm inside the battery may melt and shrink due to high temperature. The main function of the diaphragm is to prevent direct contact between the positive and negative electrodes of the battery, and once the diaphragm is damaged, it will lead to a short circuit of the positive and negative electrodes. For example, in the initial phase of thermal runaway, local defects in the diaphragm may cause a slight short circuit, generating a small amount of heat. With the development of thermal runaway, short circuits will become more and more serious.
Short circuit will cause the battery in the charge and discharge process of abnormal large current, according to Joule's law (Q = I²Rt, Q is heat, I is current, R is resistance, t is time), large current will generate a lot of heat, further accelerate the damage of the battery, so that the battery life is sharply reduced. This vicious cycle will make the battery quickly lose its original function.
4. Shedding and loss of active substances
Drastic temperature changes and internal pressure changes can cause the active material on the battery electrode to fall off. For example, at high temperatures, particles of the positive electrode material may fall off the collector. These shed active substances will accumulate inside the battery and can no longer participate in electrochemical reactions.
For lithium-ion batteries, the loss of active substances directly affects the capacity of the battery. As the active material continues to fall off, the available capacity of the battery will gradually decrease. Moreover, the shed material may block the channel inside the battery, affecting the flow of electrolyte and ion transport, further reducing the performance and life of the battery.
