The big beginner’s guide explains the sodium-ion battery in simple terms and discusses the potential of this young technology. No prior knowledge is required and the functionality, advantages and disadvantages, state of development and chances of success are discussed in detail.
What comes after lithium-ion batteries? For a long time, it looked as if Lithium-ion batteries were unbeatable and no other technology could hold a candle to them. Since CATL, the world’s largest cell manufacturer, announced in 2021 that it would be entering the mass production of sodium-ion cells, this has changed. Like any new technology, it has been presented as the new wunderkind that will soon replace lithium-ion batteries. Electric cars and home storage with sodium-ion batteries? No problem – if you believe the numerous press reports on the subject. But for which areas of application is the sodium-ion battery actually suitable? What can this still young technology achieve? Which cell concepts are the most promising? The answers can be found in the big beginner’s guide to sodium-ion batteries:
How is a sodium-ion battery constructed?
Figure 1: Exemplary chemical structure of a sodium-ion battery (Cathode: Layered oxide, anode: hard carbon). Illustration of the discharge process. Own representation
The basic structure of a sodium-ion battery differs only slightly from lithium-ion batteries. Figure 1 shows an example of the structure. Just like lithium-ion batteries, sodium-ion batteries also consist of two active electrodes: The anode and the cathode, in which the chemical storage of energy takes place. The anode and cathode are separated by a separator to prevent short circuits between the active materials.
The anode and cathode each contain the active material, which is in powdered form and usually has grain sizes in the micrometre range. The exact choice of material for the active material is decisive for the properties of the Na-ion cell. Hard carbon is generally used for the anode. There are a variety of possible material combinations for the cathode, which can be classified as either layered oxides, polyanion compounds or Prussian blue analogs (more on this later).
A good electrical, mechanical and ionic connection of the components must be achieved for the cell to function. As with Li-ion batteries, this is the task of the electrolyte, which is made up of solvent, conductive salt (e.g. NaPF6) and other additives (carbon black for electrical conductivity).
There is also a special feature with the current collectors: these are there to conduct the current from the active material out of the cell. While aluminum is used for the cathode and copper for the anode in Li-ion batteries, both collectors are made of aluminum in sodium-ion batteries. What at first sounds unspectacular is, at second glance, one of the great advantages of the sodium-ion battery: copper is actually a very poor material as a collector because it starts to decompose at low voltage levels and can cause short circuits in the cell. This is the reason why lithium batteries should only be discharged up to approx. 3 V, because at lower voltages (see [1]) decomposition reactions begin which destroy the cell. This limitation does not apply to sodium-ion batteries! It is therefore possible to discharge the cell to 0 V without damaging the battery. This is extremely important for the transportation of batteries. There are currently very strict regulations for transporting lithium cells because the cells can potentially thermally run away at >3 V (see e.g. Ohneseit [2]). With sodium-ion batteries, the cell can simply be discharged to 0 V so that there is no longer any danger from the system.
What happens in the battery when it is discharged?
The discharge process is also shown in Figure 1. In the charged state, the sodium ions on the anode side are embedded in the intermediate layers of the hard carbon particles (blue spheres). If a load is now connected to the cell, the sodium ions in the particles first move to the respective particle boundaries (grain boundary), where the sodium ion and an electron are released in a chemical reaction (1). The sodium ion moves with the help of the conducting salt via the separator to the cathode (2a). The electron moves via the current collector and the load (e.g. a lamp) and then travels via the cathode current collector to the cathode (2b). There, the electron and sodium ion react again and are deposited in the cathode active material (green spheres) (3). This process is simply reversed during charging.
What materials are used for the anodes and cathodes?
There are essentially three material groups to choose from as cathode materials:
- Sodium ion layer oxides
- Phosphate-based polyanionic compounds
- Prussian blue analogues (PBAs)
The highest energy densities are currently achieved with layered oxide cathodes and, depending on the cell configuration, even reach the energy density of LFP. During charging and discharging, the material undergoes several phase transformations, which places a high mechanical load on the material, which is why a very long service life is not achieved. Phosphate-based polyanionic compounds, on the other hand, have a very long service life of several thousand cycles due to their stable oxygen compounds, but only achieve low energy densities. The third group of Prussian blue analogues is in the middle of the field. Prussian blue analogue is a material that has been used for centuries as a colorant, e.g. for paintings. The production process is correspondingly simple and well proven, even if the production of larger quantities represents a challenge. [3]. Figure 2 provides a more detailed overview of the individual materials.
Figure 2: Cathode materials for sodium-ion batteries, own illustration.
Graphite is generally used for anodes in lithium-ion cells. This is not possible for sodium-ion batteries, as the ions cannot be deposited cleanly in the graphite interlayers. Although there are attempts to use modified graphite, hard carbon is generally used. While graphite has a layered structure, hard carbon is more disordered, contains more structural defects and thus allows the sodium to penetrate the many cavities in the material. The energy density of hard carbon tends to be somewhat lower (200-400 mAh/g for hard carbon vs. 372 mAh/g for graphite), but the energy density is basically similar. From an ecological point of view, hard carbon is particularly interesting because it can be obtained from all kinds of organic waste materials such as cotton or sugar compounds. However, the purity of the source materials is usually problematic, which is why high energy densities are often not achieved in practice. As the hard carbons based on waste materials are also more expensive, it can be assumed that the cheaper synthetically produced hard carbon is more likely to be used [1].
How ecological and sustainable is the sodium-ion battery?
In fact, it is not so easy to make sweeping statements about the sustainability of sodium-ion batteries. In principle, the impact on the environment can be assessed with the help of a life cycle analysis. However, it is too early for a final assessment as there is still too little data on the service life and recycling of the cells. However, initial studies indicate that the greenhouse gases emitted per kilowatt hour over the life cycle are higher than for lithium batteries, which is mainly due to the lower energy density [4]. It can be assumed that the transition to mass production will result in considerable increases in efficiency, meaning that significant improvements can be expected.
Figure 3: Scarcity of raw materials required for lithium and sodium-ion batteries, The materials required depend on the subtype. Vanadium, for example, is only required for some polyanion cathodes; other systems do not need it, Own illustration.
However, the criticality of the raw materials can already be assessed today. Figure 3 shows how abundant the raw materials for lithium and sodium-ion batteries are in the earth’s crust. The materials that only occur in lithium-ion batteries are shown on the left and the materials that only occur in sodium-ion batteries are shown on the right. The elements that occur in both cells are listed in the middle. The lithium-ion battery performs significantly worse than the sodium-ion battery due to its scarce raw materials lithium and cobalt. It should be noted that there is no uniform composition of the chemistries. In the sodium-ion battery, vanadium is often used in phosphate-based polyanionic cathodes, which is comparatively rare and also toxic. On the other hand, there are also lithium-ion cathodes such as LFP, which do not require cobalt and therefore perform better in terms of criticality. Overall, however, the sodium-ion battery requires significantly fewer critical raw materials, making it clearly superior.
So how ecological is the sodium-ion battery? Fewer or no critical raw materials are used. One of the most criticized aspects of lithium-ion batteries is the use of lithium (high water consumption during extraction) and cobalt (child labour). These points are completely eliminated for sodium-ion batteries. In terms of greenhouse gas emissions, the sodium-ion battery is not yet superior, so further efforts are still needed here.
What are the first fields of application?
Figure 4: Comparison of the different sodium ion chemistries in the network diagram, own illustration.
It cannot be assumed that sodium-ion batteries will completely replace lithium-based systems. Instead, it is more likely that the two areas will coexist with different emphases. The network diagram in Figure 4 clearly shows how the individual chemistries differ from one another. Not all cell chemistries are suitable for all applications.
The exact cell chemistry – in particular the choice of cathode – has a significant influence on its potential area of application. Phosphate-based polyanionic cathodes tend to be suitable for home and industrial storage systems due to their long service life and safety. Cells with layered oxide cathodes are most likely to be used in automotive applications, where a shorter service life is acceptable if a high energy and power density is achieved. With their balanced properties, Prussian blue analogue cathodes lie in the middle between mobile and stationary storage systems, so that neither area is excluded.
However, as sodium-ion technology has only come into focus in recent years, a clear trend is not yet foreseeable and it remains to be seen which cell variants can actually be produced cheaply in large quantities. Due to the generally poorer energy density and the high demands made in the automotive sector, it can be assumed that the home and industrial storage market will be the main focus in the coming years. Electric cars with sodium-ion batteries can only be realized with improved energy densities and large-volume cells.
Which companies already exist and how far along are they?
Figure 5: Overview of the most important companies working on the commercialization of the sodium-ion battery and representation of the respective cathode materials, own illustration.
At the beginning of 2024, the market for sodium-ion batteries is divided into two parts. On the one hand, there are already companies that produce sodium-ion cells in large quantities and already have several years of market experience. These include NGK-Insulators and Natron Energy. NGK-Insulators specializes in high-temperature sodium-ion storage systems that are operated at a temperature of over 300 °C [5] and are used as industrial storage systems to stabilize grids. Natron Energy uses a cathode based on Berliner blue analogues and has optimized its cells primarily for uninterruptible power supplies, offering an extremely long service life but with a very low energy density [6]. Although both companies have found their respective niches, the compromises they have made mean that it is unlikely that these technologies will find widespread use.
On the other hand, there are companies such as CATL, BYD and Northvolt. Although their products will not yet be available in large quantities in 2024, they are expected to meet the requirements of the market much better. Manufacturers are currently relying predominantly on layered oxide or Prussian blue analogue cathodes. Hard carbon is generally used as the anode. The systems are optimized for the temperature range currently required by the home storage and automotive industries (-20 °C to 60 °C) and should achieve energy densities of 140-160 Wh/kg, which is slightly below the energy densities that can be achieved with LFP.
Figure 5 shows a map of the largest (known) sodium-ion projects.
Not all manufacturers have provided information on the service life of their cells. However, the information available suggests that a cyclical service life of over 3000 cycles is usually targeted, meaning that the cells will be particularly suitable for the stationary energy storage market (see HiNa Battery [7]) and less so for fully-fledged electric cars with long ranges.
It is not yet entirely clear how long it will be before sodium-ion cells can be produced in large quantities. Large, financially strong companies that started scaling up the technology early on will tend to achieve high volumes the fastest. However, the manufacturers’ announcements indicate that higher quantities can be expected in the second half of the 2020s at the latest.
What will determine success?
As of 2024, sodium-ion batteries are still at the beginning of industrialization and it will be a few years before sodium-ion cells are available in large numbers and another few years before they reach a relevant share of the market.
Drop-in technology
How quickly the cells will establish themselves on the market depends above all on how well the step from prototype to pilot and then series production will succeed. Most concepts attempt to develop sodium-ion batteries as drop-in technology: Attempts are being made to convert existing lithium-ion production facilities to sodium-ion technologies with only minimal effort, which on the one hand saves the cost of new facilities and can also be implemented much more quickly.
Advantages at system level
The sodium-ion battery would be only the second technology to succeed in establishing itself on the market alongside lithium-NMC cathode-based cells. So far, only lithium LFP cathodes have managed to claim large market shares for themselves, although they have a significantly lower energy density at cell level. However, they have compensated for this with their other properties, as they are very safe and it is possible to produce very large (and therefore cheap) cells and pack them very densely in modules, so that large storage systems can still be produced at very low cost at system level. Such “killer features” could ultimately also make the difference for the sodium-ion battery as to whether this cell type will find widespread use or not.
The possibility of discharging the cell to 0 V and the resulting favorable transport properties could represent such a “killer feature”. However, it is still too early to judge this conclusively.
Price
Finally, success depends on the costs. In the automotive industry in particular, there is very high price pressure and even small price differences can justify the switch to sodium-ion cells due to the high volumes.
However, it is not only the direct manufacturing costs of the cells that have an influence on the chances of success of this still young technology: the price of lithium carbonate also indirectly influences whether and how quickly the technology will become established. The higher the price, the greater the tendency to drive forward the development of cheaper technologies such as sodium-ion batteries in order to reduce costs. There is currently still a considerable supply gap for lithium until 2030 and the planned new mining projects for extracting the material will probably not be enough to close it (see [8]). Sodium-ion batteries can lead to a shift in demand and thus contribute to solving the problem.
Conclusion: Sodium-ion batteries are coming, but probably not for everyone
So what are the key takeaways on sodium-ion batteries? This can be summarized in 5 key statements:
1. Sodium-ion batteries will reach market maturity.
At present, almost everything indicates that series production of sodium-ion batteries is within reach. The construction of large production facilities is in full swing and the first gigafactories, i.e. production facilities that manufacture cells with a cumulative capacity of more than 1 GWh/year, are being planned or are already under construction.
2. The success of the sodium-ion battery depends on the lithium-ion battery.
In the end, the price will decide whether sodium-ion batteries can secure a broad market share. The more expensive lithium-ion batteries and their raw materials are, the more attractive the sodium-ion battery becomes. Ramping up the production of sodium-ion cells will be quickest if as many processes as possible can be taken over from lithium-ion production.
3. Sodium-ion batteries are more environmentally friendly if you want them to be.
Sodium-ion batteries generally use less scarce and critical resources than their lithium counterparts. However, this does not apply to all variants of the technology and they may contain rare and toxic substances such as vanadium. Part of the development process must therefore be to replace these with environmentally optimized materials.
4. It is unclear which exact chemistry will prevail.
So far, there is no clear trend as to which battery chemistry variant will prevail. Although almost all manufacturers rely on hard carbon as the anode material, there is still no clear preference for cathodes, with the focus being placed either on layered oxides with a higher capacity or Prussian blue analogues with a slightly longer service life. Phosphate-based polyanionic cathodes are only being focused on by a few manufacturers so far.
5. It is unclear whether sodium-ion batteries will also be suitable for electric cars.
Sodium-ion batteries are perfect for home and industrial storage. The gravimetric and volumetric energy density are less relevant. Instead, the focus is on cost/kWh and service life. From a cost perspective, sodium-ion batteries are unbeatable and a long service life can be achieved with Prussian blue analogue cathodes or phosphate-based polyanionic cathodes.
Whether sodium-ion batteries are also suitable for electric cars remains to be seen. In order to be competitive, the cells must achieve at least a similar energy density to lithium LFP cells. In addition, it must be possible to manufacture very large cells in order to achieve a high packing density, or other ways must be found to save volume and weight at system level (e.g. by omitting the heating/cooling). Only if progress is made here will sodium-ion batteries be able to achieve relevant market shares in the automotive market.
Sources
[1] Hendricks C. et al.: Copper Dissolution in Overdischarged Lithium-ion Cells: X-ray Photoelectron Spectroscopy and Xray Absorption Fine Structure Analysis, Journal of The Electrochemical Society, 2020
[2] Ohneseit, S. et al.: Thermal and Mechanical Safety Assessment of Type 21700 Lithium-Ion Batteries with NMC, NCA and LFP Cathodes–Investigation of Cell Abuse by Means of Accelerating Rate Calorimetry (ARC), 2023, Materials Design for Electrochemical Energy Storage
[3] Gupta, P et al.: Understanding the Design of Cathode Materials for Na-Ion Batteries, 2022, ACS Omega
[4] Titirici, Adelhelm, Hu: “Sodium Ion Batteries Materials, Characterization, and Technology Volume 1 & 2”, WILEY-VCH, 2023
[5] NGK-Insulators: About NAS Batteries, https://www.ngk-insulators.com/en/product/nas-about.html
[6] Natron Energy: Better performance through better chemistry, https://natron.energy/technology/
[7] HiNaBattery: R&D Achievements, https://www.hinabattery.com/en/index.php?catid=15
[8] Advanced Propulsion Centre UK: Q1 2022 Automotive industry demand forecast, 2022, https://www.apcuk.co.uk/wp-content/uploads/2022/07/Q1-2022-automotive-industry-demand-forecast-report.pdf
