Sodium enters (again) the battery race

At the end of 2022, C2W | Mens & Molecule published a preview of 2023. According to An Hardy and Dries De Sloovere of Hasselt University, we should at the very least expect a commercial breakthrough in sodium-ion batteries. They were right. Last November, Swedish battery manufacturer Northvolt presented their latest product: a battery using sodium instead of lithium as the ‘swimming’ ion between the poles.

But sodium-ion batteries are inherently larger and heavier than lithium-ion batteries. Their suitability for electric vehicles, for instance, therefore still seems miles away. If it ever becomes feasible at all. Nevertheless, research is being conducted worldwide, and start-ups promising interesting sodium-ion batteries are springing up like mushrooms.

A pressing problem

Since the 1970s, batteries based on sodium and lithium ions have attracted considerable interest. But the latter quickly pulled ahead. Lithium-ion batteries offer higher capacity and energy density, and can be charged and discharged more often than their sodium counterpart. As a result, they proved far more rapidly applicable on a large scale, and the sodium-ion battery faded from view. Nevertheless, the geographically limited supply of lithium gnaws at the reputation of this super-battery. Major lithium deposits are found primarily in South America and Australia. For sizeable economies such as Europe’s, dependence on just a handful of countries could become dangerously significant.

This problem becomes all the more pressing as the market for electric vehicles grows strongly, driving up demand for lithium. Sodium faces no such problem; it is available in large quantities virtually everywhere. ‘The demand for energy storage is now so great that you can see producers are unable to keep up with it,’ says Mark Huijben, Professor of Nanomaterials for Energy Conversion and Storage at the University of Twente. ‘Producers are saying: I’ll make batteries that may perform slightly below lithium-ion batteries, but at least I can actually deliver something.’ Moreover, sodium-ion batteries could even be the better option in certain domains. A battery with Na⁺ appears particularly attractive for large-scale energy storage in stationary systems, for example. The volume and mass of such a battery are then less relevant. The sodium-ion battery is, on the other hand, potentially cheaper and safer than its lithium counterpart.

‘The step from lab scale to industrial level was the bottleneck’

Mark Huijben

Prussian white

Northvolt has now succeeded in bringing one to market. Somewhat surprisingly, it is less widely known in Europe that Chinese battery manufacturer CATL announced an electric car at the Shanghai Motor Show in the summer of 2023, powered by those same sodium-ion batteries. ‘They use exactly the same materials and have the same energy density of 160 watt-hours per kilogram,’ says Huijben. In collaboration with Chinese car brand Chery, they plan to manufacture electric vehicles with these batteries, with the intention of entering the European market as well. A significant advantage of the Northvolt and CATL battery is that all components are available in large quantities.

‘The fact that all these companies are making different choices shows that various materials can be attractive’

An Hardy

Unlike lithium-ion batteries, for instance, no cobalt, nickel, or graphite is required, Huijben explains. That last material might seem simple to produce since it consists solely of carbon, but its manufacturing process is delicate and production does not take place on a global scale. Although the materials in this new sodium battery have been known for some time, it is nonetheless a breakthrough, says Huijben. The positive electrode — the cathode — in the Northvolt and CATL battery long presented a barrier to commercial production. It is made of Prussian white; Na_2-xFe[Fe(CN)₆]_1-y.zH₂O. ‘We could already make this at lab scale — the step to industrial level was the bottleneck,’ Huijben explains. That step has now been successfully taken in both Sweden and China.

Does this, then, make it the holy grail? ‘Every material naturally has its positive and negative sides,’ says Professor of Materials Chemistry An Hardy of UHasselt. With Prussian white, for example, the capacity is on the low side. Dries De Sloovere, R&D Project Manager at research centre imec, goes on to explain that this is precisely why he has been investigating polyanionic materials. These are salts in which the anion takes a tetrahedral form, such as, in his case, the phosphate ion (PO₄^3-). De Sloovere: ‘This has, in principle, a higher energy density than Prussian white.’

Hard carbon

The negative electrode — the anode — in Northvolt’s battery consists of carbon. Not in the form of graphite as in lithium-ion batteries, however, but as hard carbon. This ‘hard carbon’ has a disordered structure made up of fragments of graphene and graphite with considerable space between them, which allows sodium to bind effectively. In terms of stability, there are better anode materials than hard carbon, says De Sloovere. He has investigated titanium oxide as an alternative anode material, which belongs to the broad group of transition metal oxides. ‘In such a battery, sodium ions are continuously inserted into the anode and extracted again,’ he explains. ‘These materials need to be extremely robust. The wear on carbon may set in slightly more quickly.’ But when looking purely at energy density, hard carbon is the best material, De Sloovere concedes.

As for the substance between the poles — the electrolyte — Northvolt uses a conventional organic solution, as in lithium-ion batteries. Researchers around the world are also developing other electrolytes. Ideally, these would be non-flammable, unlike the inflammable Li-ion batteries, and safe for both the environment and human health. Hardy and De Sloovere are working on electrolytes in solid form. These are not only fire-safe, but cannot leak either.

‘The liquid binds with the solid material and is thus trapped within the pores’

Dries De Sloovere

Within their research group, they have developed an electrolyte consisting of two components. The main structure is made from a solid ceramic material with extremely fine pores. Into those pores, they incorporate a deep eutectic liquid — a mixture of a sodium salt and an organic compound with a melting point lower than that of its individual constituent components. ‘Macroscopically, it really is a solid material,’ De Sloovere explains. ‘The liquid binds with the solid material and is thus trapped within the pores.’ Despite the fact that the — initially — liquid component of their electrolyte is organic, it is barely flammable, says Hardy. They have carried out various standardised tests to determine flammability. ‘The liquid has a much poorer ignitability. A small flame can appear when heated, but it goes out immediately.’

Europe’s first commercialised sodium-ion battery certainly still has plenty of room for improvement. And the competition is watching closely. There are many other types of battery currently undergoing commercialisation, says Hardy. British company Faradion, for example, makes batteries with transition metal oxides as the cathode, whilst American firm Natron uses Prussian blue for both electrodes. Hardy: ‘Seeing all these companies make different choices is already an argument for thinking that various materials can be attractive.’ The availability of the required materials will remain crucial in the search for new sodium-ion batteries, she expects. ‘In the current climate, there is a reflex in Europe to be autonomous in certain areas.’ In that respect, Northvolt’s battery already has a head start.