How Safe Are the Batteries You Use?

There is occasional news about exploding smartphone batteries. As this is a safety related issue, the topic has generated a lot of interest. Several researchers, from the National Physical Laboratory, UK, the Imperial College, London, ESRF the European Synchrotron, and UCL, the University College, London have tried to find out the reasons and the mechanism behind batteries exploding. Their research reveals how damage to the internal structure of the batteries can spread to neighboring batteries.

Now, researchers at the Stanford University, San Francisco, have developed a safe lithium-ion battery. Based on the temperature inside, the battery can shut itself down to prevent starting a fire.

When lithium batteries are packed tightly, they can overheat and catch fire if they experience short circuits or damage in some way. In fact, fires from lithium batteries have brought down two cargo jets in the past decade. Tests conducted by the US Federal Aviation Administration have found that overheating batteries can cause major fires.

When punctured or shorted, traditional lithium-ion batteries can catch fire. Temperatures inside the battery under these conditions can rise to 300 degrees Fahrenheit, causing the battery to explode. Preventive techniques of adding flame-retardants to the electrolyte of the battery usually do not work because they make the battery nonfunctional, thus defeating the purpose.

Zhenan Bao, professor of chemical engineering, and Zheng Chen, a postdoctoral scholar, have turned to nanotechnology for solving the issue of explosion of lithium-ion batteries. For this, they used a wearable body temperature monitor that Bao has recently invented. The sensor, made of plastic material, has tiny particles of nickel embedded inside. Nano scale spikes protrude from the surface of these nickel particles. To use the sensor in batteries, researchers used a one-atom thick graphene layer to coat the spiky nickel particles. They embedded the coated particles in a thin film of elastic polyethylene.

The researchers attached the polyethylene film to one electrode of the battery such that the load current of the battery would flow through the film. Under normal temperatures, the spiky particles touch one another and allow conduction of electricity. If the temperature rises, the polyethylene stretches due to thermal expansion. This makes the particles to spread out leading to the film becoming non-conductive. That stops the flow of electricity through the battery, until it cools down.

The polyethylene film starts expanding above 160 degrees Fahrenheit. That causes the spikes on the particles to move apart, causing the battery to shut down. As temperatures drop below 160 degrees, the particles come into contact again with each other, allowing the battery to start functioning again and generate electricity. According to the researchers, they can tune the temperature based on the type of polymer used and the number of nickel particles.

With the film in place, the battery shut down as soon as it got too hot and stopped working. Moreover, it resumed operation quickly as soon as the battery cooled down. As there is no electricity flowing when the battery is hot, chances of it catching fire and exploding are practically nil.