Plug in your phone and nothing seems to happen. The screen shows a little lightning bolt, the percentage creeps up, and the phone gets slightly warm. But inside, something very physical is going on. Billions of billions of lithium atoms are being forced to move house, from a place where they are comfortable to a place where they are cramped, and that forced move is where the energy goes.
A lithium-ion battery has two ends, called electrodes, separated by a thin porous plastic sheet soaked in a liquid called the electrolyte. One electrode is usually graphite: sheets of carbon atoms stacked like the pages of a book. The other is a metal oxide, often containing cobalt, nickel or manganese, built as a framework with spaces in it. Lithium can sit in either. It can slide in between the pages of the graphite, or into the spaces of the oxide. This sliding in and out, without the electrode itself being destroyed, has a name, intercalation, and it’s the whole trick of the battery.
The key is that lithium is not equally happy in both places. In the metal oxide it sits in a low-energy, comfortable arrangement. Wedged between graphite sheets, it is in a higher-energy, less comfortable one. So left to itself, lithium in the graphite wants to go back to the oxide, just as water at the top of a hill wants to flow down. Charging is pumping the water uphill. Using the phone is letting it flow back down through a water wheel.
Here’s how the flow works. When you use your phone, a lithium atom in the graphite gives up its outer electron, becoming a positively charged lithium ion. The ion drifts through the electrolyte, across the separator, and into the metal oxide. But the electron can’t follow it. The electrolyte is designed to let ions through and block electrons completely. So the electron has to take the long way round: out of the battery, through the phone’s circuits, and back into the battery at the other end, where it rejoins the lithium. That flow of electrons through the circuits is the electric current that lights your screen. The battery has separated the paths of the ion and the electron, and made the electron do work on its way.
Charging runs the whole thing in reverse. The charger acts as a pump, pulling electrons out of the metal oxide side and pushing them into the graphite side. The positive lithium ions, attracted by those electrons, leave the oxide, cross back through the electrolyte, and squeeze in between the graphite sheets again. The energy from the wall socket hasn’t been stored as electricity. It has been stored in an arrangement: lithium, pushed uphill into the graphite, waiting to come back down.
How much energy? The voltage of a cell, about 3.7 volts in the middle of its range, measures how steep the hill is: how much energy each unit of charge releases on the way down. The capacity, in milliamp-hours, measures how much lithium can make the trip. Multiply them and you get the energy. A phone battery holds roughly fifteen to twenty watt-hours, around sixty thousand joules. That’s enough to lift a fifty-kilogram sack of rice onto a table more than a hundred times, all stored in a slab thinner than a biscuit.
Lithium is ideal for this. It is the lightest metal, so a battery full of it is light, and it gives up its outer electron very readily, which makes for a steep hill and a high voltage. The batteries were developed over decades: Stanley Whittingham found the first working intercalation idea in the 1970s, John Goodenough discovered the much better metal oxide in 1980, and Akira Yoshino built the first practical, safe version with a carbon electrode in 1985. All three shared the Nobel Prize in Chemistry in 2019.
Why does a battery wear out? Every trip uphill and back causes tiny damage. During the very first charges, some of the electrolyte reacts with the graphite and forms a thin crust on its surface, called the solid electrolyte interphase. A little crust is essential: it protects the graphite. But it keeps thickening slowly over the years, and every bit of it locks away lithium that can no longer make the trip. Meanwhile, the electrodes swell and shrink a little with every cycle as lithium goes in and out, and over hundreds of cycles they crack and wear. Heat speeds all of this up, and so does keeping the battery full, when the lithium is crammed in at its highest energy for hours on end. That’s why many phones now hold charging at around 80 per cent overnight and only top up just before you wake.
So the next time you plug in, picture it: a vast crowd of lithium ions, being marched uphill into the graphite, where they will sit, waiting, until you open an app and let them all run home.