Archive for November, 2018

Recently, the topic of spontaneous combustion of lithium-ion batteries often flashes in the news headlines: the smartphone lights up, the hover board, or the car. So what happens inside the battery during thermal acceleration and why does spontaneous combustion occur?

Elements of Lithium-ion batteries:

Lithium-ion batteries consist of an anode and a cathode, separated by a porous polymer separator. The cathode active material is most often transition metal oxides with lithium ions embedded in the crystal. Graphite is commonly used in the anode. The electrolyte, which is filled with an electrochemical cell, is an organic solution of lithium salts. When the manufacturer first charges, when lithium is embedded in the anode on the electrodes (especially on the anode), a protective ion-conducting layer (SEI) is formed, consisting of decomposed electrolyte. This layer protects the electrodes from parasitic reactions with electrolyte.

Causes of explosion:

The most common cause of spontaneous combustion of batteries is a short circuit inside the electrochemical cell. Electrical contact between the anode and the cathode can occur for many reasons. This may be, for example, mechanical damage to the cell. Another internal short circuit arises due to the violation of production technology in case of uneven cutting of electrodes or metal particles between the anode and cathode, which leads to damage to the porous separator. Also, the cause of the internal short circuit may be the “germination” of metal lithium chains (dendrites) through the separator. This effect occurs if lithium ions do not have time to integrate into the anode crystal when charging too fast or at low temperatures, as well as if the capacity of the cathode active material exceeds the capacity of the anode. Right now MapQuest explores the different aspects of the complications that arises.

Since reactions with electrolyte are exothermic, the temperature and pressure inside the battery continue to rise. When the temperature reaches 180–200 ° C, the cathode material, usually a transition metal oxide with lithium embedded in the crystal, reacts to disproportionate and liberates oxygen. This is where the spontaneous combustion and even more sharp temperature jumps occur. In parallel, there is a thermal decomposition of the electrolyte (200-300 ° C), which also produces heat. It looks like this:

Fully loaded:

The batteries are also equipped with fuses and valves, which, when the pressure and temperature inside increase, either disconnect the electrodes from the circuit, or contribute to the release of accumulated gas. In the latter case, since the gases are flammable, a flame arises when it comes into contact with oxygen outside. An example of the action of safety valves could be observed in an accident involving the Tesla car Model S, where the battery was punctured by a large metal object. Since in Tesla the battery valves were aimed down on the asphalt and the individual blocks were well isolated from each other, only the front of the battery burned (as Elon Musk said, if the same metal object broke through the gasoline tank, the car would burn completely).