How Lithium-ion Batteries Store and Release Energy

A lithium-ion battery stores energy in chemical form. When it is charged, lithium is packed into one electrode, usually graphite. When you use it, the lithium moves to the other electrode, a metal oxide, and the battery releases energy as an electric current.

Two things move at once. Lithium ions travel inside the battery, through a material called the electrolyte. Electrons travel outside, through the wires and the device. Charging pushes both of them back to where they started. Because the reactions can run backwards, the battery can be recharged many times.

A single unit is called a cell. Lithium-ion cells power phones, laptops and electric cars, and each one has the same main parts:

• Negative electrode (anode): usually graphite. Graphite is made of flat sheets of carbon atoms stacked in layers, and lithium can slip in between the layers. This is called intercalation.
• Positive electrode (cathode): a lithium metal oxide, such as lithium cobalt oxide. Others use manganese oxide or iron phosphate. It also has spaces where lithium can sit.
• Electrolyte: a substance that lets lithium ions move between the electrodes.
• Separator: a very thin sheet full of tiny pores. Ions can pass through it, but it keeps the two electrodes from touching.

In a charged cell, most of the lithium sits between the graphite layers. Lithium is more stable in the metal oxide, so it has a tendency to move there. That difference is the stored energy.

When you switch on a device, the circuit is complete. Each lithium atom in the graphite gives up one electron and becomes a positive lithium ion (Li⁺). The ion drifts through the electrolyte and the separator to the cathode, where it slots into the metal oxide.

The electron cannot follow that inside route. Instead it leaves the anode through the negative terminal, travels along the wire, passes through the device and enters the cathode. There it meets a lithium ion again. This flow of electrons is the electric current, and it does useful work on the way, lighting a screen or turning a motor.

A lithium-ion cell gives about 3.6 to 3.7 volts. Its voltage stays nearly constant as it runs down, which suits electronics.

A charger is an outside power supply. It pushes electrons the opposite way round the circuit: out of the cathode and back into the graphite anode. To balance the charge, lithium ions leave the metal oxide, cross the electrolyte and separator, and slide back between the graphite layers.

The charger does work to move the lithium back to its higher-energy position. That is how the energy is stored again. Graphite is good at this because its layers hold lithium without being bent out of shape, so the process can be repeated many times.

Two numbers are often printed on a battery. Capacity, in milliampere-hours (mAh) or ampere-hours (Ah), is an amount of electric charge: a current multiplied by a time. Energy, in watt-hours (Wh), is how much work the battery can do.

To get energy, multiply charge by voltage. Two batteries can share the same voltage but store very different amounts of energy if one can move much more charge. So mAh alone only compares batteries of the same voltage.

Lithium-ion cells pack a lot of energy into a small mass. They can store up to about 330 Wh per kilogram, compared with about 75 Wh per kilogram for a lead-acid battery. They also lose charge only slowly when stored, around 1.5–2% a month.

A used lithium-ion battery still holds enough charge to cause injury or start a fire. The risk grows if it is damaged or crushed, for example in a rubbish lorry or at a sorting plant.

• Never put a used lithium-ion battery in household rubbish or recycling bins. Take it to a battery recycling or hazardous waste collection point.
• Before you drop it off, cover the terminals with tape and keep each battery in its own bag.
• If a battery is damaged, ask the maker of the device how to handle it.

Cells do not last for ever. A common reason lithium cells fail is that thin threads of lithium metal, called dendrites, grow on the anode, a little like tree roots pushing through a pavement.