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How Do Lithium-Ion Batteries Work?

First invented more than 30 years ago, lithium-ion or Li-ion batteries have become a ubiquitous part of our daily lives, from the tiny versions in cell phones to the tenfold stacks used to power electric cars. They are the subject of intense research efforts all over the world as a solution to the pressing challenge of electricity storage.

Image of a lithium-ion battery from the Toyota Prius hybrid car.
Hundreds of lithium-ion cells are grouped together to power electric or hybrid vehicles, such as the Toyota Prius pictured here. ©TOSHIFUMI KITAMURA / AFP

How Batteries Work

Batteries are devices that convert chemical energy into electrical energy. Rechargeable batteries, also known as accumulators, can accept and store electric energy and release it when needed. This means they can be discharged and charged up again in a reversible process. Single-use electric batteries supply electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... to an external circuit until they run out of charge.

Lithium-ion batteries can store three to four times more energy per unit mass than batteries using other technology

Batteries contain two electrodes immersed in an electrolyte – a conductive liquid or solid – and connected outside the electrolyte by a conductive wire. When discharging, the negative electrode (anode) releases electrons that travel along the wire and are absorbed by the positive electrode (cathode). This movement of electrons creates an electric current, which can then be converted to powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... a motor or electronic device. To balance out the electronMatter is made up of atoms. An atom comprises a nucleus made of protons (positively charged particles) and neutrons (neutral charge)... exchange, positive ions travel through the electrolyte between the two electrodes. When the battery is charging up again with an external electricity source, this process is reversed.

 

Different Types of Rechargeable Batteries

Rechargeable batteries use combinations of materials that can easily and durably exchange electrons and positive ions. Internal combustion engine vehicles most often use lead-acid batteries, which contain a negative electrode made of lead, a positive electrode made of lead oxide, and an electrolyte consisting of sulfuric acid and water. Other materials used in batteries include nickel, cadmium, sodium and sulfur1.

Scientists became particularly keen on lithium for batteries, since it is a very lightweight metal (the third element in the periodic table, after hydrogenThe simplest and lightest atom, the most abundant element in the universe. and helium). Lithium atoms can readily release one of their three electrons, creating positively charged Li+ ions. Manufacturers initially used lithium metal for the negative electrode, which emits electrons. However, they noticed that the repeated use and recharging cycles altered the metal. To avoid this, the cathodes are now often made from cobalt oxide and a small proportion of lithium, with a graphite anode. The electrolyte is made of lithium salts in a solvent, meaning that it contains a great many lithium ions. That is where the name “lithium-ion battery” comes from.

 

Lithium-Ion Cells

The core component of a lithium-ion battery is a cell that looks a bit like puff pastry, with an aluminum plate to collect the current, followed by the cathode, electrolyte, anode, and finally a copper plate (see diagram)

Diagram of how a lithium-ion cell works

When the battery is being charged up, Li+ lithium ions leave the positive electrode (cathode) and are stored in the negative electrode (anode). When it is discharged to produce an electric current, the Li+ ions move in the opposite direction2.

These cells, each with a voltage of a few volts, can be grouped together in varying numbers, depending on the capacity required to power a cell phone or car battery.

Advantages and Disadvantages

Lithium-ion batteries have a high energy densityThe amount of energy stored in an object, expressed in watt-hours per kilogram (1 Wh/kg = 3.6 kJ/kg)..., meaning that they can store three to four times more energy per unit mass than batteries using other technology. They are quick to recharge and can be used over and over again, with at least 500 discharge/charge cycles at 100%.

However, they are at risk of suddenly catching fire and releasing toxic gases due to the electrolyte overheating to above 100°C, known as thermal runaway. This has led to thousands of cell phones and tablets being recalled by manufacturers in recent years. In 2013, a battery in a Boeing 787 aircraft caught fire after landing.

Investigations have shown that overheating is most often caused by a short circuit brought on by incorrect assembly or impact. As a result, manufacturers are now required to follow rigorous processes and fit the lithium-ion batteries they make with an electronic battery management system (BMS), which turns the power off if it detects an anomaly.

In addition, manufacturers are looking into innovative technology that could help prevent overheating, such as solid electrolytes made of ultrathin polymer films.

The battery market is dominated by Asian companies, which could be considered a geopolitical risk given how strategic this equipment is for the world economy. A further challenge is the highly uneven distribution of lithium resources around the world, potentially leading to environmental complications when they are extracted. See the feature report on rare metals.

 

(1) Battery comparison – Lycée Eiffel (in French)

(2) Watch the animation by BASF