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Explosion is a common manifestation of harm in power battery systems, and its impact is even more severe. It not only causes property damage and environmental damage, but can even cause personal injury or life-threatening situations.

Possible causes of combustion or explosion in the power battery system include:

The exothermic side reaction of a power battery (cell) leads to thermal runaway, igniting electrolyte and other combustible substances;

The local connection impedance in the high-voltage circuit of the power battery system is too high, causing a large current to flow through and causing the temperature to rise to the ignition point temperature, igniting combustible materials inside the power battery system;

The external combustion of the power battery system causes the internal temperature of the power battery system to continue to rise, reaching the ignition point temperature and igniting combustible materials inside.

Regarding the analysis of the use of electric vehicles, the first scenario has a high probability of occurrence and a high risk factor. The thermal runaway caused by the exothermic side reaction of the battery cells is the main cause of combustion or explosion in the power battery system.

The main exothermic reactions inside lithium-ion batteries are:

The decomposition of ESI membrane, with a temperature range of 90~120 ℃;

The reaction between the negative electrode and the electrolyte reaches a temperature above 120 ℃;

The electrolyte decomposes at a temperature of approximately 200 ℃;

The reaction between the positive electrode and the electrolyte is accompanied by the decomposition of the positive electrode and the precipitation of oxygen, with a temperature range of 180-500 ℃;

The reaction between the negative electrode and the adhesive is approximately above 240 degrees.

The fundamental reason for thermal runaway (combustion, explosion) of battery cells is the accumulation of heat caused by the exothermic side reaction inside the battery cell. The rate of external heat exchange of the battery cell is lower than the rate of heat accumulation, and the temperature continues to rise, directly reaching the ignition point temperature, causing combustion and explosion.

The thermal process inside the battery cell follows energy conservation: Qp=Qe+Qa

In the formula, Qp is the heat generated by various negative reactions inside the battery cell, Qe is the heat exchanged between the battery cell and the environment, that is, heat dissipation, and Qa is the heat absorbed and accumulated by the telecommunications company itself. If Qe ≥ Qp, Qa is negative or zero, and the internal temperature of the battery cell will not rise, resulting in thermal runaway; If Qe<Qpq, the internal temperature of the battery cell will continue to rise until it reaches the thermal runaway temperature of 200-300 ℃.

From the above analysis, it can be seen that if the exothermic side reactions inside the battery cannot be blocked, the temperature inside the telecommunications will continue to rise until a thermal runaway event occurs. To reduce the risk of accidents, measures can be taken:

Take protective measures to reduce the probability of external emergencies (such as overcharging, over discharging, overheating, short circuit, squeezing, puncture, etc.);

Blocking the positive feedback process of exothermic side reactions, such as using bonding fuse technology in PACK modules, or adding PTC materials between positive and negative electrode materials and current collectors;

Reduce the heat generated by exothermic side reactions, such as selecting lithium iron phosphate cathode materials, changing the organic solvent composition of the electrolyte, etc;

Increase the ignition temperature, such as adding flame retardant materials to the electrolyte, selecting ceramic membranes, etc;

Improve heat dissipation capacity and avoid heat accumulation. For example, the Lilang battery adopts an efficient liquid cooling design scheme, and there are also individual schemes that immerse the entire battery in coolant.

The mechanisms and preventive measures for thermal runaway summarized above have been practiced in the design and manufacturing of battery systems. However, in practice, different material systems may have different chemical properties, resulting in different mechanisms for thermal runaway in battery cells. Different system designs can also lead to different system level hazards and solutions.

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