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Why do lithium-ion batteries burn or even explode

Time:24-04-2024 author:

Time:2023-11-28 Views:938

Lithium ion batteries are an energetic component mainly composed of positive electrode, negative electrode, electrolyte, and separator. After charging, the positive electrode is generally a transition metal oxide, which has strong oxidizing properties; The negative electrode is graphite with a large amount of lithium embedded inside, which has strong reducibility. Electrolytes are generally organic esters with characteristics such as low melting point and flammability.

The firecrackers in our daily lives are also an energetic device, and many people know that the components containing gunpowder are sulfur (sulfone, chemical formula S), nitrate (stone, chemical formula KNO3), and charcoal. Among them, nitrate is a strong oxidant, and sulfur and charcoal are reducing agents. When the external stimulus exceeds 120 degrees, the redox reaction inside the firecrackers occurs violently, releasing a large amount of gas and heat, causing the gunpowder to burn and the firecrackers to explode.

From this, it can be seen that theoretically, lithium-ion batteries may undergo high exothermic oxidation-reduction reactions, and the combustible electrolyte contained in them can also promote this reaction, leading to combustion and even explosion consequences. How powerful is the combustion or explosion of lithium-ion batteries? From the perspective of storing electrical energy, the electrical energy of a regular lithium-ion battery with an energy density of 150Wh/kg is approximately 1/10 of the thermal energy density generated by the explosion of TNT explosives.

In lithium-ion battery accidents, the positive and negative electrodes can directly undergo severe redox reactions under special circumstances, and even aluminum and copper collectors can directly participate in the reaction in the form of reducing agents, generating significantly higher heat than the energy corresponding to battery storage. Generally speaking, in a safety accident involving lithium-ion batteries in a confined space, the maximum temperature can reach over 800 ℃, while the explosion heat of a 43.4g heavy lithium-ion battery is equivalent to 5.45g TNT, reaching 1/8 of the TNT equivalent.

The reason why lithium-ion batteries convert their internal chemical energy into electrical energy in a controllable and continuous manner through electrochemical reactions instead of violent oxidation-reduction reactions is because the separator effectively isolates the positive and negative electrodes physically and electrically (as well as the presence of conductive electrolyte). However, when various internal or external factors cause the diaphragm to fail, resulting in direct contact between the positive and negative electrodes, this internal short circuit can instantly release electrical energy, generate a large amount of heat and bring high temperatures, instantly disrupt the stability of the internal chemical system of the battery, leading to the oxidation-reduction reaction between the negative electrode electrolyte, positive electrode electrolyte, negative electrode and positive electrode, and even the fluid collector participating in the instantaneous exothermic heating The process of causing the electrolyte to instantly vaporize and then mix with positive and negative active material powder to spray out of the battery shell, resulting in combustion or even explosion, is called thermal runaway (TR).

The spontaneous thermal runaway is currently the biggest safety anxiety of electric vehicles. If every battery is completely consistent from microscopic electrode material particles and separators to macroscopic electrode plates and shell packaging, then a battery pack made of thousands or hundreds of thousands of such batteries will definitely have better safety characteristics. You may notice that the 100% expression here is a bit different, followed by a dozen or so zeros, which represents an ideal expectation – high consistency across the entire battery scale. As is well known, the consequence of battery inconsistency is that batteries with degraded performance will decay faster, with some passivation and deactivation leading to direct failure; Some have also taken a completely different path – internal short circuits leading to thermal runaway, combustion, and explosion