How Do Batteries Work?
In today’s society, we depend on batteries as portable sources of energy to power our many mobile devices from hearing aids and pacemakers to smartphones, laptops, and even cars. The same basic concept is true of all batteries regardless of type; that is, batteries store chemical energy that can be converted into electrical energy. This electrical energy provides an electric current, or the movement of electrons through a conductive path called a circuit, that we use to power our devices. Batteries have four main components inside the casing, which include an anode and a cathode (also called the negative and positive terminals, respectively); the electrolyte in between the two electrodes (a general term encompassing anodes and cathodes) that allows ions to move; and a separator which is located between the two electrodes that only allows ions to pass through and prevents electrical short circuits. It is important to note that the anode and the cathode are made of two different conductive materials, typically metals.
When a battery is put into a device, a flashlight, for instance, contact is made to the terminal at each end of the battery to complete a circuit. This initiates chemical reactions at the electrodes that generate electrical energy and a flow of electrons through the device. A chemical reaction between the electrolyte and the anode causes electrons to accumulate at the anode, which becomes negatively charged, while producing an equal amount of positively charged ions in the electrolyte. Meanwhile, a different reaction occurs simultaneously at the cathode that causes this electrode to become positively charged. This difference in charge between the two electrodes causes negatively charged electrons to move from the anode to the cathode. However, the electrolyte acts as an insulator and electrons cannot move directly through it. Instead, the electrons travel out of the battery from the negative terminal, through the bulb of the flashlight, causing it to light up, before returning to the battery through the positive terminal. As electrons migrate through the external circuit, the positively charged ions produced at the anode migrate through the electrolyte to the cathode as it receives electrons in order to balance the charge. The electrons and positive ions then recombine to complete the circuit. The battery relies on this charge balancing chemistry to continue running until the chemicals are depleted and the battery goes flat.
The exact chemical reactions that occur in a given battery depend on the materials used the electrodes and the electrolyte. For disposable, or single-use, batteries, the discharge process is irreversible and the battery cannot be recharged. However, in rechargeable batteries, this process can be driven in reverse by connecting the battery to an external source of electrical energy to send the electrons and ions back to their initial state. Recharging the battery allows the system to operate as before, converting chemical energy into electrical energy, until the chemicals are once again depleted.
Why do batteries require two different electrode materials? Chemical elements vary in their ability to attract electrons or hold onto electrons. The key is to have a difference in electronegativity between the two materials, such that one electrode more readily loses electrons and the other more strongly attracts or accepts electrons. This enables electrons to move from one electrode to the other. As a consequence, the reaction at each electrode has a specific standard potential, and it is the difference in potentials that determines with what force the electrons will travel between the two electrodes. This difference in standard potentials determines the battery’s voltage. A higher voltage means that more work can be done by the same number of electrons.
Indeed, there are a number of metrics that characterize a battery, such as voltage, capacity, and energy and power densities. Many different types of batteries exist in order to best match the needs of a multitude of applications. Battery technology continues to be heavily researched with new advances focused on improving performance, in addition to the safety, durability, and affordability of rechargeable batteries.
Jonah W. Jurss, Ph.D. is a chemistry professor at the University of Mississippi who does research in the field of renewable energy. He joined the Department of Chemistry and Biochemistry in 2014.