How does a battery work?

Your watch, laptop, and laser-pointer are all powered by the same thing: chemistry…

By Mary Bates

There are a lot of different kinds of batteries, but they all function based on the same underlying concept. “A battery is a device that is able to store electrical energy in the form of chemical energy, and convert that energy into electricity,” says Antoine Allanore, a postdoctoral associate at MIT’s Department of Materials Science and Engineering. “You cannot catch and store electricity, but you can store electrical energy in the chemicals inside a battery.”  

There are three main components of a battery: two terminals made of different chemicals (typically metals), the anode and the cathode; and the electrolyte, which separates these terminals. The electrolyte is a chemical medium that allows the flow of electrical charge between the cathode and anode. When a device is connected to a battery — a light bulb or an electric circuit — chemical reactions occur on the electrodes that create a flow of electrical energy to the device.  

More specifically: during a discharge of electricity, the chemical on the anode releases electrons to the negative terminal and ions in the electrolyte through what’s called an oxidation reaction. Meanwhile, at the positive terminal, the cathode accepts electrons, completing the circuit for the flow of electrons. The electrolyte is there to put the different chemicals of the anode and cathode into contact with one another, in a way that the chemical potential can equilibrate from one terminal to the other, converting stored chemical energy into useful electrical energy. “These two reactions happen simultaneously,” Allanore says. “The ions transport current through the electrolyte while the electrons flow in the external circuit, and that’s what generates an electric current.” 

If the battery is disposable, it will produce electricity until it runs out of reactants (same chemical potential on both electrodes). These batteries only work in one direction, transforming chemical energy to electrical energy. But in other types of batteries, the reaction can be reversed. Rechargeable batteries (like the kind in your cellphone or in your car) are designed so that electrical energy from an outside source (the charger that you plug into the wall or the dynamo in your car) can be applied to the chemical system, and reverse its operation, restoring the battery’s charge.  

The Group Sadoway lab at MIT is working on creating more efficient batteries for multiple uses. For large-scale energy storage, the team is working on a liquid metal battery, in which the electrolyte, anode, and cathode are liquid. For portable applications, they are developing a thin-film polymer battery with a flexible electrolyte made of nonflammable gel. Another goal of the lab is to build batteries using previously unconsidered materials, focusing on abundant, cheap and safe substances that have the same commercial potential as popular lithium batteries.

Thanks to 18-year-old Steven Minkus from Glenview, IL, for this question.

Posted: May 1, 2012