From：CTECHI GROUP Limited Release time：2018-10-24
Overview：The internal reason for the safety of lithium-ion batteries is the thermal runaway inside the batteries and the continuous accumulation of heat, resulting in the continuous rise of the internal temperature of the batteries. The external performance of the lithium-ion batteries is violent energy release such as combustion and explosion. The thermal runaway process can be divided into three stages.
The internal reason for the safety of lithium-ion batteries is the thermal runaway inside the batteries and the continuous accumulation of heat, resulting in the continuous rise of the internal temperature of the batteries. The external performance of the lithium-ion batteries is violent energy release such as combustion and explosion. The thermal runaway process can be divided into three stages.
Stage 1: Overheating
Thermal runaway starts with overheating of battery systems. Battery overheating can be caused by overcharging, high ambient temperature, external short circuit or internal short circuit. Among them, internal short circuit is the main reason for thermal runaway, and is relatively difficult to control. The internal short circuit will occur when the battery is damaged, the lithium dendrites are formed, and the diaphragm is defective. When the internal temperature of the battery begins to rise, phase 1 ends and stage 2 begins.
Stage 2: Heat Build-up And Gas Release Process
As phase 2 begins, the temperature inside the cell rises rapidly, and the cell undergoes the following reactions (which do not occur in exact order; some of them can occur simultaneously: (1) The solid electrolyte interface (SEI) decomposes due to overheating or physical infiltration. (2) As the decomposition temperature of SEI increases, lithium metal will react with organic solvents in the electrolyte to release flammable hydrocarbon Gases, which is an exothermic reaction and further promotes the temperature rise. (3) Polyethylene (PE) / polypropylene (PP) diaphragm began to melt when T > 130 C, resulting in short circuit between the positive and negative electrodes. (4) In the end, heat causes the decomposition of lithium metal oxide cathode materials and leads to the release of oxygen. During stage 2, temperature rises and oxygen accumulates in the battery. Once enough oxygen and heat are accumulated, the battery will burn, and the thermal runaway proceeds from stage 2 to stage 3.
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Stage 3: Combustion And Explosion
In stage 3, combustion starts. The electrolytes of LIBs are organic, highly volatile, and are highly flammable in nature. The oxygen and heat released in stage 2 provide the necessary conditions for the combustion of combustible organic electrolytes, causing fire or explosion. In stages 2 and 3, exothermic reactions occur near adiabatic conditions.
It should be pointed out that the above reactions do not happen one after another in strict order. Thermal runaway is a rather systematic and complex problem. Based on the understanding of the thermal runaway of batteries, a variety of methods are being studied to reduce potential safety hazards through the rational design of battery components.
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