To perform in an efficient way, LIBs require operation conditions which are within a specific range of current density, voltage and temperature. Nevertheless, when they are subjected to abuse conditions, exothermic reactions can take place, leading to a fast increase in internal temperature and pressure. What does it mean? Well, our battery is likely to explode!
Current LIBs include external sensors to prevent overheating and overpressure but, unfortunately, temperature and pressure inside the cells can actually increase much faster than they can be detected by those external sensors. Because of that, several alternatives have been developed in order to include internal components to solve the problem. For example, ceramic coating has been proven to be an effective way to shut down the battery and improving the thermal tolerance. However, after the battery is shut down, it cannot be used again. Using solid-state electrolytes can be another option, but the overall performance of the battery is decreased due to their low ionic conductivity.
About a year ago, researchers from Stanford University published a very interesting paper where they presented a new alternative: introducing a new material internally into the electrodes. Their material consists of “conductive graphene-coated spiky nanostructured nickel particles as the conductive filler and a polymer matrix with a large thermal expansion coefficient”. The spiky particles have a high electrical conductivity at a low filler fraction and a high thermal sensitivity, whereas the graphene provides high electromechanical stability to prevent oxidation and the decomposition of the electrolyte.
According to the authors, batteries that include this new graphene-based nanocomposite can work efficiently in a wide voltage window at normal temperature, whereas they can also shut down incredibly fast when the operating conditions are off limits. The best part is that these batteries can actually be reused right after the overheating event takes place, which is a major advantage!
Basically, the previously mentioned nickel nanoparticles promote conductive percolation and, once the temperature reaches a well defined value, the whole nanocomposite becomes insulating due to the increase in volume of the polymer matrix, interrupting the conductive pathways. Then, when the temperature decreases, the matrix contracts again, thus allowing the material to recover its conductivity. In addition, it has to be highlighted that the temperature at which the material will “cut” the current is a design parameter which can be controlled by modifying the composition of the material.
I highly recommend reading this paper from the Nature journal (“Fast and reversible thermoresponsive polymer switching materials for safer batteries”) to learn more about this topic. Since my work is related to crashworthiness of vehicles, I always tend to look for connections between new technologies and its potential applications to my specific field. Therefore, in this case it is pretty obvious that this research will help to improve the safety levels of electric vehicles in the event of accidents, where the batteries are very likely to catch fire due to the energy which is released during the impact.
