For the first time, prototypes of sodium-ion batteries have been developed with performance such as to make them competitive with traditional lithium batteries, and moreover capable of short charging times. This was achieved by a team of researchers from the Department of Materials Science and Engineering at KAIST (Korea Advanced Institute of Science and Technology) led by Jeung Ku Kang, who achieved this result using a particular combination of materials for the two battery electrodes. This is a remarkable step forward in the relatively recent field of sodium batteries, which are increasingly emerging as more sustainable, economical, and safer alternatives to traditional lithium batteries, considered a critical raw material, especially in key sectors such as electric vehicles whose batteries currently depend mainly on lithium.
How the Korean sodium battery is made and how it recharges in a few seconds
Like regular batteries, sodium batteries work through redox reactions that produce ions which are exchanged from the positive electrode to the negative one (anode and cathode respectively), thus generating a potential difference that can power an electric current in a circuit. As you might guess, lithium batteries produce lithium ions (Li+) while sodium batteries produce sodium ions (Na+).
As we will see later, the main current limitations of sodium batteries are low power density (that is, how much power 1 kg of reagents can deliver) and long charging times. The "loophole" that KAIST took to solve this problem is very simple, but at the same time effective. At the moment there are two main applications of sodium technology: sodium batteries and sodium capacitors. The latter, like all capacitors, deliver a lot of power and therefore can recharge very quickly, but have a very low energy density (how much energy 1 kg of reagents can deliver) compared to a battery, and therefore low range. The idea is therefore to create a "hybrid" with one battery electrode and one capacitor electrode, in order to have the best of both worlds.
Attempts made in recent years had a big problem: the two electrodes store energy at very different speeds. Following the principle of "you go at the pace of the slowest," the electrode with the lowest kinetics acts as a bottleneck. As you can imagine, this imbalance is a serious limitation that prevents direct competition with commercial lithium batteries. To overcome this problem, KAIST researchers tested an anode with "battery-type" materials, but with improved reactivity thanks to active materials embedded in a porous carbon structure: this avoids the "bottleneck" effect between the two electrodes, thus drastically reducing charging times. The cathode instead uses materials typical of supercapacitors, ensuring high power density.
Result: KAIST's sodium battery prototypes achieved a maximum energy density of 247 Wh/kg (comparable to that of a commercial lithium battery) with a power density of 34.75 W/kg (higher than other sodium batteries) and 100% stability (i.e., no capacity loss) after 5,000 charge and discharge cycles. As for the versions with higher power density, the minimum charging time was about 5 seconds (but versions with higher energy density, i.e., greater range, have longer charging times, on the order of an hour). The graph below shows that overall the prototype represents a significant step forward compared to other "hybrid" sodium batteries. If this path continues, in the future it could also be applied to electric vehicles, although in these cases the conditional is always necessary.
The red line indicates the performance of various versions of the KAIST sodium battery in terms of energy density and power density, compared with other "hybrid" sodium batteries (blue), non-hybrid (lilac), and sodium supercapacitors (green). Source: KAIST
Sodium vs. lithium batteries: pros and cons
To understand the differences between the two types of batteries, we can take a look at the periodic table of elements. Here we see that lithium and sodium are chemically related: they both belong to the first group (i.e., the first column of the periodic table), in the second and third rows respectively. This means they have the same outer electron configuration: in particular, they have only one electron in the outermost s orbital. In fact, the chemical properties of lithium and sodium are very similar: they tend to lose that single electron very easily.
The periodic table of elements. Lithium is the first element of the second period (the second row) and sodium is the first element of the third period (the third row).
However, there are differences that have important practical implications from a technological development point of view. Compared to lithium, sodium is at least 500 times more abundant in nature, less expensive, and more recyclable. That's why it was chosen as an alternative to lithium: it has similar electrochemical behavior but allows for cheaper, greener, and also safer batteries, because they are more stable and therefore less at risk of catching fire.
However, sodium batteries also have disadvantages compared to lithium ones. Again from the periodic table, we note that sodium is heavier than lithium, whose ion, however, carries the same net electric charge. This means that sodium batteries have a lower energy density, i.e., they carry less energy for the same mass, and this translates into lower range. To give an idea, a typical energy density for a sodium battery can be 160 Wh/kg, while that of a commercial lithium battery is around 250 Wh/kg. In addition, the electrodes tend to be less reactive and this means longer charging times.
Why the sodium battery from KAIST is important news
For these reasons, sodium batteries are used in situations where high performance and fast charging are not needed, such as pedal-assist bicycles, but they struggle to find application in key sectors such as electric mobility, where lithium for now remains essential despite heavy environmental and geopolitical consequences. From this point of view, the goal is to advance sodium technology enough to make it competitive, so as to have an advantageous alternative and reduce the "lithium dependence" of Western and developing countries. However, a battery is needed that has high capacity, high energy density, high power density, and fast kinetics, and from this point of view the prototype developed by KAIST is very promising, from an environmental, economic, and even geopolitical perspective.
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