Salt battery that can recharge in 5 seconds
- h2worlddaa
- Sep 4, 2024
- 4 min read
For the first time, prototypes of sodium-ion batteries have been developed with performances that make them competitive with traditional lithium batteries, and are also 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 electrodes of the battery. This is a significant step forward in the relatively new field of sodium batteries, which are increasingly emerging as more sustainable, economical and safe alternatives to traditional lithium batteries, considered a critical raw material, especially in key sectors such as electric vehicles whose batteries currently rely mainly on lithium.
How the Korean sodium battery is made and how it can recharge in a few seconds
Like regular batteries, sodium batteries work through oxidation-reduction reactions that produce ions that are exchanged from the positive electrode to the negative electrode (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 limitations of sodium batteries at present are their low power density (i.e. how much power can 1 kg of reagents deliver) and long charging times. The “workaround” 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 they have a very low energy density (how much energy can 1 kg of reagents deliver) compared to a battery, and therefore a low autonomy. The idea is therefore to create a “hybrid” with a battery electrode and a capacitor electrode, in order to have the best of both worlds.
Attempts made in recent years had a major problem: the two electrodes accumulate energy at very different rates. Following the principle of “go slowest first”, 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, researchers at KAIST tested an anode with “battery” 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 a high power density.
Result: The KAIST sodium battery prototypes achieved a maximum energy density of 247 Wh/kg (comparable to 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 5000 charge and discharge cycles. For the higher power density versions, the minimum charging time was around 5 seconds (but the higher energy density versions, i.e. longer 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, it could also be applied to electric vehicles in the future, although in these cases the conditional is always a must.
The red line shows the performance of various versions of the KAIST sodium battery in terms of energy density and power density, compared to other “hybrid” (light blue), non-hybrid (purple) and sodium supercapacitor (green) sodium batteries. Source: KAIST
Sodium vs. Lithium Batteries: The 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 that they have the same external electronic 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 one electron very easily.
The periodic table of elements. Lithium is the first element in the second period (the second row) and sodium is the first element in the third period (the third row).
However, there are differences that have important practical implications from the point of view of technological development. Compared to lithium, sodium is at least 500 times more abundant in nature, less expensive and more recyclable. This is why it was chosen as an alternative to lithium: it has a similar electrochemical behavior but allows for the creation of cheaper, more ecological and also safer batteries, because they are more stable and therefore less at risk of developing fires.
However, sodium batteries also have disadvantages compared to lithium batteries. 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, that is, they carry less energy for the same mass, and this translates into a lower autonomy. 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/kh. Furthermore, the electrodes tend to be less reactive and this means longer charging times.
Why KAIST 's Sodium Battery Is Big News
For these reasons, sodium batteries are used in situations where high performance and rapid charging are not needed, such as pedal-assisted 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, in order to have an advantageous alternative and reduce the "lithium dependence" of Western and developing countries. However, a battery with high capacity, high energy density, high power density and rapid kinematics is needed, and from this point of view the prototype created by KAIST is very promising, from an environmental, economic and also geopolitical point of view.
Quick-charge starter batteries are already available
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