What are lithium-ion batteries?

Lithium-ion batteries are found in most electric vehicles, but what exactly are they and how do they work?

If you're looking into buying an electric car or a plug-in hybrid vehicle (PHEV), the chances are you've come across the term 'lithium-ion battery'.

Frequently used in laptops, phones and electronic devices, lithium-ion batteries are used incredibly widely: the technology is used so widely in fact, its inventors recently won a Nobel Prize for the contribution they've made to modern life. Lithium-ion cells commonly feature in electric and hybrid cars now too, as a method of powering the electric motors found in each type of vehicle.

They hold several advantages over conventional lead acid batteries as they are lighter, more energy dense and come with a longer lifespan.

What is a lithium-ion battery?

A lithium-ion battery is made up of multiple lithium-ion cells. The lithium-ions from these cells move from the negative electrode to the positive during discharge (i.e. when you’re driving the vehicle) and then back again while charging. The whole process is made possible by the electrolyte.

There are several advantages that lithium-ion batteries have over conventional lead-acid or other types of batteries. They’re lighter than most other battery technologies, which makes them ideal for electric vehicles that require a large number of battery cells in order to deliver the kind of range drivers need.

They’re also very energy dense. In comparison, a typical lithium-ion battery can store 150 watt-hours of electricity in a one kilogram battery. A lead-acid battery – like the one found in normal petrol and diesel cars that operates the alternator and starter motor – can only store 25 watt-hours per kilogram.

Lithium-ion batteries can also go through a number charge and discharge cycles with only a small deterioration in performance. What this means is that you can charge and use your electric vehicle hundreds of times before seeing a noteworthy fall in battery performance. You'll notice this as the maximum estimated range of a fully charged vehicle drops over time.

This is unrelated to the fall in range you'll typically see as a result of cold weather, although the temperature of a battery – particularly when it's charging – does have an effect on it's performance in the long-term.

It’s important to note that the lithium-ion battery in an electric vehicle isn’t a single unit, but rather is made up of multiple battery cells stacked together. The more cells, the higher the capacity of the battery is: this is measured in kilowatt-hours (kWh).

Hybrid batteries are often between five to 10kWh, rising to between 10kWh and 20kWh for plug-in vehicles. Electric vehicles can come with anywhere from 20kWh to 100kWh units.

Generally speaking, the higher the battery capacity, the higher the range, although factors like vehicle weight and aerodynamic efficiency have an effect too. An electric car's efficiency – that is to say, the amount of distance it travels per unit of energy – is often given in miles per kilowatt-hour (mi/kWh).