Lithium ion battery is an energetic component mainly composed of positive electrode, negative electrode, electrolyte, and separator. After charging, its 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 extremely strong reducibility. Electrolytes are generally organic esters, which have characteristics such as low melting point and flammability.
The firecrackers in our daily lives are also a type of energetic device. Many people know that the components of the gunpowder contained in them are monosulfur (sulfur, chemical formula S), dinitrogen (stone, chemical formula KNO3), and three wood charcoal. Among them, saltpeter is a strong oxidant, and sulfur and charcoal are reducing agents. When the external stimulus exceeds 120 degrees, the oxidation-reduction reaction inside the firecrackers occurs violently, releasing a large amount of gas and heat. The gunpowder burns and the firecrackers explode.
From this, it can be seen that theoretically, lithium-ion batteries may inherently undergo highly exothermic oxidation-reduction reactions, and the combustible electrolyte contained within them can also promote this reaction, leading to consequences such as combustion or even explosion. 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 energy density generated by the explosion of TNT explosives.
In lithium-ion battery accidents, the positive and negative electrodes can undergo severe oxidation-reduction reactions directly under special circumstances, and even aluminum and copper current collectors can participate in the reaction as reducing agents, generating significantly more heat than the energy corresponding to battery storage. Generally speaking, in the event of a safety accident involving a lithium-ion battery in a confined space, the maximum temperature can reach over 800 ℃, and the explosion heat of a 43.4g heavy lithium-ion battery is equivalent to 5.45gTNT, which is 1/8 of TNT equivalent.
The reason why lithium-ion batteries do not undergo violent oxidation-reduction reactions but instead convert their internal chemical energy into electrical energy in a controllable and continuous manner through electrochemical reactions is because the separator effectively physically isolates the positive and negative electrodes and isolates electronic conduction (as well as the presence of ion conducting 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 will instantly release electrical energy, generate a large amount of heat, and bring high temperatures, instantly disrupting the stability of the internal chemical system of the battery, leading to oxidation-reduction reactions between the negative electrode electrolyte, positive electrode electrolyte, negative electrode and positive electrode, and even the current collector. The instantaneous heat release and temperature rise cause the electrolyte to vaporize instantly, which then mixes with the positive and negative electrode active material powder and sprays out of the battery shell, causing the consequences of combustion or even explosion. This process is called thermal runaway (TR).
Spontaneous thermal runaway is currently the biggest safety anxiety for electric vehicles. If every battery is completely identical from microscopic electrode material particles and separators to macroscopic electrode sheets and shell packaging, then battery packs made from thousands or hundreds of thousands of such batteries will definitely have better safety characteristics. You may have noticed that the 100% expression here is a bit different, with around ten zeros at the end, representing an ideal expectation of 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.
There is still much work to be done to improve safety while respecting objective reality. Firstly, there is innovative warning technology, such as Stanford University's recent report that the sensitive capture of hydrogen signals can push forward the time for warning of lithium-ion battery thermal runaway by 5 minutes, which is enough for people on electric vehicles to escape. In addition, the "self poisoning" technology of batteries is also quite effective. Its mechanism is that in the early stage of thermal runaway, some special chemicals can be released to passivate and "paralyze" the inside of the battery, breaking the chain of thermal runaway.
Pay attention to the safety of lithium-ion batteries, vigorously develop innovative and efficient safety improvement technologies, and continuously improve the consistency of battery manufacturing.