Batteries for electric cars: Sodium cells on the advance?

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Three percent of all new electric cars in 2030 will run on sodium-based traction batteries. This is the prediction of Prof. Dr. Markus Hölzle from the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW). For his analysis, the materials specialist compiled data from various sources. However, the world of cell chemistry is in constant flux. There are several arguments that replacing lithium with sodium could be more successful than current estimates suggest. The most important: money.

One advantage: lower costs

Traction batteries with sodium instead of lithium-ion cells are less expensive. Specifically, Professor Hölzle quotes 151 euros per kilowatt hour of energy content for 2022 for high-quality NMC811 cells, 122 euros per kWh for LFP cells and 83 euros per kWh for sodium-ion cells from CATL. These figures refer to the system level, i.e. the finished traction battery including all components such as the cooling and heating, the crash-proof housing and the management device.

NMC remains technologically ahead

The percentage of the cost of the battery system becomes lower the higher the vehicle segment. Put another way: In a Mercedes EQS or comparable electric cars, a cost of 17,365 euros - exemplified by the EQS's approximate gross capacity of 115 kWh at 151 euros/kWh - is less significant. The properties, on the other hand, are: when it comes to energy density, no one can beat the NMC cells. In terms of both weight (gravimetric in watt hours per kilogram) and installation space (volumetric in watt hours per liter), battery cells with a cathode mixture of nickel, manganese and cobalt are at the top.

Mass production only with low raw material prices

If electric cars are to be built en masse, they will have to become cheaper. This is a problem for vehicles in the price-sensitive segment. It is logical that one of the first electric cars with sodium-based cells will be the BYD Seagull small car. It starts at around 11,000 euros and 30 kWh of energy content. The logic behind this application is the same as for LFP cells. Lithium iron phosphate cells do without the expensive metals nickel and cobalt. They have high cyclic durability and very low risk of thermal runaway. But the energy density is lower than NMC cells, and in freezing conditions, the cells must first be heated before they can be charged.

Thousands of drivers of the Tesla Model 3 can apparently live well with these compromises. Considered on its own, 491 km WLTP range in the entry-level version is perfectly okay for many applications. The Model 3 Long Range with all-wheel drive offers a standard range of 602 km thanks to high-priced cell chemistry; the additional price of about 9000 euros is an announcement that deters quite a few buyers. Traction batteries with LFP cells are currently becoming very widespread. In Germany, they are offered by MG or BYD, in addition to the entry-level versions of Tesla Model 3 and Model Y. Other manufacturers such as VW are following with the base model of the ID.2.

LFP on the losing track

Sodium-based cells have a similar weakness as LFP cells with the relatively low energy density. But they have better characteristics in almost all other aspects. The logical conclusion: sodium cells are a potential replacement for LFP cells. Or phrased as a question: If sodium cells are better in several respects, why would the market share be only three percent in 2030? LFP cells, after all, are forecast at 35 percent. Global market leader CATL cited a gravimetric energy density of 160 Wh/kg when it unveiled the sodium ion cell in July 2021. This is still a design that additionally uses lithium-ion cells. CATL's goal is to achieve 200 Wh/kg in a second generation that does not use this mixed construction.

The manufacturer is not precise in the announcement whether these values refer to the cell or the system level. Nevertheless, it is obvious: If BYD officially states 140 Wh/kg for the Atto 3 with LFP cells that is sold today, this does not seem to be unattainable at all for sodium-based traction batteries, even if the data from CATL should apply to the cell instead of the system. Sodium-ion cells have advantages over LFP cells not only in terms of cost, but also in terms of even better cyclic durability and low-temperature performance. Lower price, longer service life, stronger performance at low temperatures - on paper, there is not much more to be said for LFP.

Strategic opportunity for Europe

Current question marks: How well can sodium-ion cells be integrated into battery systems? There is no practical experience from large-scale production on this yet. Perhaps the effort required for heating and cooling is even lower, which could mitigate the disadvantage of energy density. That would be ideal and is a plausible assumption, but not a determination. Also an open question is how quickly supply chains can be established for production. It is possible to transfer current processes to a large extent; no radical changes are needed, such as in all-solid-state batteries. But manufacturing the anode from amorphous carbon (hard carbon) and the cathode with dyes such as Prussian White has not yet been practiced in large-scale production. The ramp-up is just taking place - in China.

From a European perspective, replacing lithium with sodium offers the opportunity to become less dependent on certain raw materials. However, an industrial policy strategy is not discernible in the EU. Admittedly, there are so many announcements on the production of battery cells that the need for full electrification of all rolling vehicles could be met as early as 2030. However, these public declarations of intent should be taken with a grain of salt until action follows. It still seems as if industry and politics in Europe have not finally grasped what is happening in the world. There is a great deal of activity, and yet the final will to counter the strategic actions of the USA and China quickly and forcefully is lacking.

(mfz)