How Do Rechargeable Batteries Work? Inside a Lithium-Ion Battery

Phones, laptops, wireless earbuds, electric cars: almost everything we carry runs on the same kind of battery. Every night, billions of people plug in and refill them. But what actually happens inside when a battery charges, and why does it hold less and less after a couple of years?

A battery is a controlled chemical reaction

A battery has two electrodes: a negative one and a positive one. Between them is an electrolyte, a substance that lets charged atoms, called ions, move through it but blocks electrons. Electrons can only get from one side to the other by travelling through an outside circuit, such as your phone. That flow of electrons is the electric current that powers the device.

In an ordinary disposable battery, the chemicals are used up and that is the end of it. In a rechargeable battery, the reaction can be run backwards.

Inside a lithium-ion battery

• The negative electrode is usually graphite, layers of carbon atoms stacked like pages of a book.
• The positive electrode is a metal oxide containing lithium, for example with cobalt, nickel, manganese or iron.
• A thin porous plastic separator keeps them apart, and a liquid electrolyte soaks everything.

Charging: the charger pushes electrons into the graphite. To balance the charge, lithium ions leave the metal oxide, cross the electrolyte and slip between the layers of graphite. The battery is now “full”: lots of lithium is stored in an uncomfortable, high-energy position.

Using: when you switch on the phone, the lithium ions flow back to the metal oxide, and their electrons travel through the phone’s circuits to get there. The battery is “empty” when most of the lithium is back on the positive side.

The ions simply shuttle back and forth, which is why some engineers call these “rocking-chair” batteries.

Why lithium?

Lithium is the lightest metal and gives up its electron very readily. That means a lot of energy can be stored in a small, light package. In 2019 the Nobel Prize in Chemistry went to John Goodenough, M. Stanley Whittingham and Akira Yoshino for developing the lithium-ion battery. Sony sold the first commercial version in 1991.

Why batteries wear out

Every cycle causes small, irreversible changes. A thin film called the solid electrolyte interphase grows on the graphite, trapping some lithium so it can no longer move. The electrodes expand and contract slightly with each charge, causing tiny cracks. Over hundreds of cycles the battery holds less charge and can deliver less power, which is why an old phone can shut down suddenly when it is cold or the battery is low.

According to research on lithium-ion degradation, the biggest factors are high temperature, keeping the battery at a very high state of charge for long periods, and deep discharges. That leads to some simple habits:

• Keep your phone out of heat: hot cars, direct sun and pillows while charging.
• Use your phone’s optimised or limited charging setting if it has one, which often holds the battery around 80%.
• Prefer frequent partial top-ups to running it down to zero.

In short

• A lithium-ion battery stores energy by moving lithium ions between two electrodes.
• Charging pushes ions into the graphite; using the battery lets them flow back to the metal oxide.
• Electrons travel through the device’s circuit, providing the current.
• Lithium is the lightest metal, so it stores a lot of energy in a small space.
• Batteries age because of small irreversible side reactions with every cycle.
• Heat, long periods at 100% and deep discharges speed up ageing.