A Ray of Hope: Sodium-Ion Batteries

A pack of lithium-ion cells of the 18650 format folded with a positive terminal up. Image with flare effect.
23 Feb 2021

Lithium-ion batteries currently dominate the market for energy storage. However, the Earth’s reserves of lithium are limited, scarce and expensive. Sodium-ion batteries would be cheaper and more sustainable, since sodium occurs in large quantities in nature and is much simpler to acquire than lithium. The technology already works, but it could be a while before it enters mass production.

You find them in laptops, smartphones, ebikes, electric cars or as storage in the electricity network – lithium-ion batteries currently dominate the market for electrochemically-based energy storage. They are powerful, lightweight and offer a high energy density. The problem: supplies of lithium on our planet are limited, and could be used up in just a few years. Consequently, the costs will increase. Currently, the acquisition of lithium is very difficult and pollutes the environment. Another disadvantage: lithium-ion batteries use cobalt on the cathode, a metal which is as rare as it is toxic.

Sodium as an alternative conserves resources

In future, sodium-ion batteries will provide a possible alternative or supplement to lithium-ion batteries. In recent years, scientists and engineers across the world have made great progress with this technology, and have built some very promising prototypes. Sodium-ion batteries work, but the technology is not sufficiently mature for mass production. This is likely to change in the years to come. But why is sodium especially good for use in batteries?

As alkali metals, lithium and sodium are chemically very similar. They each only have one electron, which they can give up to allow electricity to flow. As they create alkali cations with a single positive charge at the same time, they make excellent charge carriers in electrolytes. However, in comparison to lithium, sodium is very common in nature, for example, as sodium chloride in sea salt. Production is not limited by scarce resources; plus, sodium is less expensive to extract.

As the concept is similar, it is relatively simple to reconfigure lithium-ion battery production lines for manufacturing sodium-ion batteries – without any extensive technical changes. The manufacture is actually cheaper, because aluminium can be used as the electrical connection for the anodes, instead of copper, which is more expensive. Brown coal, wood and other types of biomass (e.g. corn cobs, coffee grounds) have proven to be good materials for the anode, which stores the sodium-ions during charging. On the cathode, common substances such as manganese or iron can be used to replace the toxic and rare cobalt. In a nutshell: unlike lithium-ion batteries, sodium-ion batteries do not require any rare or expensive raw materials.

The disadvantage: a higher weight and lower energy density

But sodium has two disadvantages. Firstly, sodium-ions are around three times heavier than lithium-ions. This means sodium-ion batteries are also heavier, even though lithium only makes up about 5 per cent of the battery’s total weight. Due to their greater mass, sodium-ions have more inertia and cannot move as quickly in electrolytes. Here the challenge is to find materials for rapidly conducting solid electrolytes.

What’s more, sodium batteries are less powerful than lithium, because their cell voltage is 0.3 V lower, resulting in a loss of around 10% energy density. More energy density is lost due to their higher weight. Currently their energy density corresponds to that of lithium-ion batteries from 20 years ago. And because sodium metal is more reactive than lithium metal, developers must prevent undesired metal deposition on the surface of the electrodes, in order to ensure the safety of the system.

Prototypes and potential applications

To date, scientists and researchers have come up with numerous functioning prototypes. For example, a South Korean sodium-ion battery managed around 500 complete charging cycles before its capacity sank to 80 per cent. A battery from a US-Chinese research group with a slightly different chemical composition managed 450 charging cycles with a similar capacity.

In practice, these batteries would probably survive more charging cycles, because in everyday use, batteries are generally only partially charged and discharged. The complete charging and discharging in the experiment are much tougher on the battery. Start-ups, such as the French firm Tiamat or the Dutch company AquaBattery, are also working on sodium technology. It will be exciting to see how the development work progresses.

Due to their higher volume and weight, sodium-ion batteries are considerably larger than lithium-ion batteries providing the same performance. This makes them less suitable for batteries for electric cars or laptops and smartphones. In future, they will primarily be used in stationary storage systems to store wind and solar power for times when the wind isn’t blowing or the sun isn’t shining. This electricity storage decouples electricity generation and electricity consumption; smoothing over the varying availability of renewable energies. It can also deliver electricity at peak times. With these, size is less important: the critical factor is generally the price. Sodium-ion batteries do well here thanks to their lower manufacturing costs.

Stefan Winklhofer