Batteries and energy storage

A battery stores energy as chemical potential and releases it as electricity. Each cell has three main parts: a negative electrode called the anode, a positive electrode called the cathode, and an electrolyte between them that lets ions move but blocks electrons.

When the battery is connected to a circuit, a chemical reaction releases electrons at the anode. They cannot cross the electrolyte, so they travel through the external circuit, powering the device, to reach the cathode. Meanwhile ions move through the electrolyte to keep the charges balanced.

In 1800 Alessandro Volta built the first true battery, the voltaic pile, from stacked discs of zinc and copper separated by cloth soaked in salt water. The unit of electrical potential, the volt, is named after him.

The lead-acid battery, invented by Gaston Planté in 1859, was the first rechargeable type and is still used to start most petrol and diesel cars. The zinc-carbon dry cell, and later alkaline cells, made cheap, portable, single-use batteries for torches, radios and toys.

In a rechargeable battery, the chemical reaction can be driven backward. Pushing current in the opposite direction forces ions and electrons back to where they started, restoring the stored chemical energy.

Each charge and discharge causes small, irreversible changes, such as side reactions and physical stress on the electrodes, so capacity slowly fades. How fast depends on the chemistry, temperature, how deeply the battery is discharged and how quickly it is charged.

Lithium is the lightest metal and gives up electrons very readily, which makes it ideal for storing a lot of energy in little weight. In a lithium-ion battery, lithium ions shuttle between a graphite anode and a metal oxide cathode, slotting into the layered structure of each material.

John Goodenough, Stanley Whittingham and Akira Yoshino shared the 2019 Nobel Prize in Chemistry for developing the technology, and Sony sold the first commercial lithium-ion batteries in 1991. Their energy density made smartphones, laptops and practical electric cars possible, and prices have fallen dramatically since.

• Energy density: even the best batteries store far less energy per kilogram than petrol.
• Heat: if damaged, overcharged or badly made, lithium-ion cells can undergo thermal runaway and catch fire.
• Ageing: capacity falls with cycles and time, especially when kept fully charged in hot conditions.
• Materials: cobalt, nickel and lithium mining raises environmental and human rights concerns.
• Recycling: recovering materials from spent batteries is possible but not yet widespread enough.

Lithium iron phosphate batteries avoid cobalt and nickel and are safer and longer lasting, though somewhat less energy dense; they are increasingly used in cars and grid storage. Sodium-ion batteries use abundant sodium and are beginning to reach the market for applications where weight matters less.

Solid-state batteries replace the flammable liquid electrolyte with a solid, promising greater safety and energy density, but manufacturing them cheaply at scale remains a challenge. For the power grid, batteries compete with other storage methods such as pumped hydro, compressed air and hydrogen.