Batteries for electric cars Part 3: The future - sodium instead of lithium

Inhaltsverzeichnis

(Hier finden Sie die deutsche Version des Beitrags)

For many decades, the focus in the further development of the powertrain in cars was on the engine. With the switch to battery electric drive, this is shifting, as the battery cell is now the focus of interest. Huge sums are currently being invested globally to achieve progress in this area. Energy density, durability, charging performance and, above all, costs must be optimized if the battery-electric drive is to have a chance anywhere in the world.

In a series of articles, we want to show where we are today and where the development is heading. The European Union has set the political framework: From January 1, 2035, only passenger cars with no direct CO₂ emissions may be newly registered. According to current estimates, this will largely be battery-electric cars. Because the industry cannot simply be switched on and off, there will be a continuous ramp-up in the almost twelve years until the deadline.

Sodium can replace lithium as an active material in battery cells. At least that's what CATL and BYD say. The Chinese companies are convinced that they will succeed in replacing one alkali metal with the other. The advantage of sodium: experts assume that the cost per kilowatt hour of energy content in the traction battery will drop by at least 20 percent compared to lithium-based systems. Sodium occurs frequently and is easy to obtain. The basic production processes are similar to those used in the past. BYD plans to mass-produce electric cars with sodium-ion cells this year. Should this succeed, it would be above all a success for materials research.

Cost pressure is particularly high in the price-sensitive segments, i.e. electric compact, small and micro cars. Formally, sodium-ion batteries can be used anywhere; however, it is likely that this will happen first in low-cost electric cars. For example, the Chinese portal Auto Later reports that BYD's Qin, Dolphin and Seagull models could soon be equipped with them. BYD foresees the start of production for sodium-ion cells in the second quarter of 2023. According to this source, a subcompact car like the BYD Seagull, which has not yet been officially unveiled, could cost around 14,000 euros.

Initially low energy density

An inherent disadvantage of sodium-ion cells is their low energy density. CATL quoted up to 160 watt-hours per kilogram (Wh/kg) when it unveiled them in July 2021. A value that would have been typical for lithium iron phosphate (LFP) cells just a few years ago. However, according to CATL, a hybrid system with a mixture of sodium and lithium-ion cells will be used first, in order to balance the weaknesses of each with the strengths of the other. Nevertheless, CATL is already targeting 200 Wh/kg for an exclusively sodium-based second-generation traction battery.

In Germany, because of the low energy density, it is often said that this cell chemistry is only suitable as stationary storage for solar power in houses. In other words, where the space requirement is not important. This thesis was exactly the same a few years ago about LFP cells. When Tesla equipped the Model 3 with them, this view changed abruptly.

Good low-temperature performance

Apart from the price, sodium-ion cells have another strength: their low-temperature properties are better than those of LFP cells, for example. Thus, according to CATL, they still have 90 percent of their capacity at minus 20 degrees. In addition, CATL says the percentage of active material in the overall system can be increased to 80 percent. That's even more than the 72 percent the Chinese global market leader is targeting with the lithium-based cells in the new Qilin battery.

If the potential is so great, the question arises as to why not all battery producers build sodium-ion cells in addition to lithium cells. The answer lies in the anode: this is where amorphous carbon is found, usually referred to by the English term hard carbon.

Hard carbon anode

In the past, 20 percent of the capacity was lost during the very first charging. When CATL and BYD now announce that they are going into series production, this allows the interpretation that precisely this problem has been fixed. Both companies are very large; so they do not rely on inflating their own research results like startups do. On the contrary, CATL and BYD cannot afford not to follow up the announcement made with a product.

Prussian blue, a non-toxic dye, is found at the cathode of sodium ion cells. It, too, is inexpensive and can be made in Europe; expensive metals such as nickel or cobalt are not necessary. Sodium-ion cells have long been known in principle, and the advance is in the anode. How exactly the developers from China got a handle on the properties of the hard carbon, they won't say.

For the masses, it's the price that counts

If the electric car is to replace the internal combustion engine worldwide, low-cost cell chemistry is the most important prerequisite. It's all about money. A realization that Elon Musk voiced early and loudly, and on the German market Tesla has always been a pioneer in cell chemistry. Now, however, Chinese companies are at the forefront: CATL as a supplier and BYD as a cell producer and car manufacturer at the same time. We can look forward to seeing when the first electric car with sodium-ion cells will be launched on the market. My guess is a small car in 2025.

(mfz)