What makes a battery rechargeable?

Batteries power our mobile phones, start our cars, and save us from having to get up to change the TV channel. They provide the electrical power for our mobile technology to work and are essentially a chemical reaction housed in a container that, when you connect the negative and positive terminals (the anode and cathode), allows stored energy in the form of electrons to flow between the two electrodes.

Single-use batteries (also known as primary batteries), can only make the electrons take a trip from the anode over to the cathode one-way once. This is because the battery actually destroys itself over a discharge—either depleting electrodes as they discharge, or building up reaction products on the electrodes preventing the reaction from continuing. Once this happens, the battery ends up in the bin (or hopefully the recycling, but that’s a whole other topic).

However, with the appropriate choice of electrode materials, we can reverse the chemical reaction that occurs during discharge. All we need is some energy to drive the positive ions released from the anode into the electrolyte back to the anode, and the electrons that the cathode took in also back to the anode. The return of both the positive ions and electrons to the anode primes the system so it’s ready to run again: your battery is recharged.

The process isn’t perfect, however. The replacement of the negative and positive ions from the electrolyte back on to the relevant electrode as the battery is recharged isn’t as neat or as nicely structured as the electrode was in the first place.

Each charge cycle degrades the electrodes just a little bit more, meaning the battery loses performance over time, which is why even rechargeable batteries don’t keep on working forever.

Over the course of several charge and discharge cycles, the shape of the battery's crystals becomes less ordered. This is exacerbated when a battery is discharged and recharged at a high rate—for example, if you drive your electric car in big bursts of speed rather than steadily. High-rate cycling leads to the crystal structure becoming more disordered, with a less efficient battery as a result.