Explosions are a common hazard in power battery systems, and their impact is more severe, causing not only property damage and environmental destruction, but also personal injury or even death.
Possible causes of combustion or explosion in a power battery system include:
An exothermic side reaction in a power battery (cell) leads to thermal runaway, igniting the electrolyte and other flammable materials;
If the local connection impedance in the high-voltage circuit of the power battery system is too high, a large current flowing through it will cause the temperature to rise to the ignition point, igniting the flammable materials inside the power battery system;
Combustion occurs outside the power battery system, causing the internal temperature of the power battery system to rise continuously until it reaches the ignition point temperature, igniting the combustible materials inside.
Analysis of electric vehicle usage shows that the first scenario has a higher probability of occurrence and a higher risk factor. Thermal runaway caused by the exothermic side reaction of the battery cell is the main cause of combustion or explosion of the power battery system.
The main exothermic reactions inside a lithium-ion battery include:
The decomposition temperature range of ESI membranes is 90~120℃;
The reaction between the negative electrode and the electrolyte reaches temperatures above 120℃.
The electrolyte decomposes at a temperature of approximately 200℃.
The reaction between the positive electrode and the electrolyte, accompanied by the decomposition of oxygen at the positive electrode, occurs in the temperature range of 180~500℃;
The reaction between the negative electrode and the binder occurs at approximately 240 degrees Celsius or higher.
The root cause of thermal runaway (combustion, explosion) of battery cells is the accumulation of heat due to exothermic side reactions inside the cell. The rate of heat exchange between the cell and the outside is less than the rate of heat accumulation, causing the temperature to rise continuously until it reaches the ignition point, leading to combustion and explosion.
The thermal processes inside the battery cell follow the law of conservation of energy: Qp = Qe + Qa
In the formula, Qp represents the heat generated by various negative reactions within the battery cell, Qe represents the heat exchanged between the battery cell and the environment (i.e., heat dissipation), and Qa represents the heat absorbed and accumulated by the battery cell itself. If Qe ≥ Qp, then Qa is negative or zero, the internal temperature of the battery cell will not rise, and thermal runaway will not occur; if Qe ≥ Qp, then Qa is negative or zero.
The above analysis shows that if the exothermic side reactions inside the battery cell cannot be blocked, the internal temperature of the telecommunications equipment will continue to rise until a thermal runaway event occurs. To reduce the risk of such an accident, the following measures can be taken:
Take protective measures to reduce the probability of external emergencies (such as overcharging, over-discharging, overheating, short circuits, crushing, punctures, etc.);
To block the positive feedback process of exothermic side reactions, such as by using bonding fuse technology in the PACK module, or by adding PTC material between the positive and negative electrode materials and the current collector;
Reduce the heat generated by exothermic side reactions, such as by selecting lithium iron phosphate cathode materials and changing the organic solvent composition of the electrolyte;
Increase the ignition point temperature, such as by adding flame-retardant materials to the electrolyte or using ceramic diaphragms;
To improve heat dissipation and avoid heat accumulation, some solutions, such as the high-efficiency liquid cooling design used in Lilang batteries, involve immersing the entire battery in coolant.
The thermal runaway mechanisms and preventive measures summarized above have been applied in the design and manufacturing of all types of batteries. However, different material systems have different chemical properties, resulting in different thermal runaway mechanisms in the cells. Different system designs will also lead to different system-level hazards and solutions.
