The "super battery" that can store 800,000 kWh of electricity is here. How does it work?

Produced by: Science Popularization China

Produced by: Hanmu Diaomeng

Producer: China Science Expo

Dalian has built a "super battery". The first phase of the project has entered the final stage of grid connection and commissioning, and is expected to be officially put into use in mid-October. Currently, it can store 400,000 kWh of electricity at a time, which can be used by 200,000 residents for one day. When the project is completed, it can store 800,000 kWh of electricity within 4 hours.

This "super battery" is not made up of a large number of lithium batteries. It is another type of battery - the all-vanadium liquid flow battery, or vanadium battery for short. Today, let's talk about how it charges and discharges.

Three pictures to understand vanadium

Vanadium is relatively unfamiliar to everyone, but on the periodic table, it is a neighbor of the well-known titanium element, and it is not that far away from iron element, with only two elements between them.

The position of vanadium in the periodic table

(Photo source: Siriudie)

Vanadium, with atomic number 23 and symbol V, is a hard, silvery-grey, ductile transition metal.

99.95% purity vanadium square rod

(Photo credit: eigene Aufnahme)

The "gray" pure vanadium does not look good, but its solutions in various valence states are very bright.

Vanadium solutions with different valence states

(Photo credit: Hyung Kim)

Valence is key

We must talk in detail about the various valence states of vanadium.

In a compound, the party that loses electrons, or gives away electrons, has a positive charge, while the party that gains electrons has a negative charge.

For example, in carbon dioxide CO2, because carbon has lost 4 electrons, its valence is positive 4, written as C4+. Another example is carbon monoxide CO, at this time, carbon has only given 2 electrons, so its valence is positive 2, written as C2+.

The reason why we talk about valence is that the potential difference between vanadium ions of different valence states is the reason why it can be used as a battery. For example, the potential difference between positive divalent vanadium ions and positive pentavalent vanadium ions is 1.259 volts. The potential difference between the positive and negative electrodes of our common No. 5 batteries is usually only 1.5V or 1.2V.

Where there is a potential difference, there is chemical energy, just as where there is a height difference, there is gravitational potential energy.

For example, the height difference between the upper and lower reservoirs is 1,259 meters. What is the first thing that comes to your mind? It must be the release and storage of energy, which is a pumped storage power station.

Chemical energy and gravitational potential energy have different names, but the energies they contain are essentially the same after transformation.

Therefore, when there is a potential difference between the two barrels of solution, the two barrels can release the energy through conversion, which constitutes a battery. At this time, the two barrels of solution are the positive and negative electrodes of the battery respectively.

How do vanadium batteries discharge?

To help you understand the discharge principle of vanadium batteries, you can imagine a picture like this:

On a vertical cliff, two large barrels of solutions are hung one above the other, one containing pentavalent vanadium ion solution and the other containing divalent vanadium ion solution.

(Image source: self-made by the author)

It is not difficult to see that there is a potential difference of 1.259 volts between the pentavalent vanadium ion solution and the divalent vanadium ion solution, with the presence of trivalent and quadrivalent vanadium ions, just like there are two pools on the cliff with a height difference of 1259 meters.

The vanadium ions in the pentavalent vanadium ion solution have lost five electrons, so they have a strong urge to gain electrons to make themselves more complete. The question is, where do they grab electrons from? From the weak, that is, the divalent vanadium ion solution below.

Therefore, the continuous discharge process of the vanadium battery is actually the process in which the divalent vanadium ion solution below continuously loses electrons and then turns itself into a trivalent vanadium ion solution. At the same time, it is also the process in which the pentavalent vanadium ion solution above continuously obtains electrons and then turns itself into a quadrivalent vanadium ion solution.

When the discharge is finished, the status of the two becomes like this:

(Image source: self-made by the author)

At this point, we can also use springs to help us understand vanadium batteries. A fully charged vanadium battery is like a fully stretched spring.

(Image source: self-made by the author)

A vanadium battery that has completed its discharge returns to its original state like a spring.

(Image source: self-made by the author)

How is a vanadium battery charged? After using a spring to help imagine the discharge process of a vanadium battery, its charging process is easy to understand. It is similar to the process of using external energy to pull the "spring" open again.

The tetravalent vanadium ion that has lost four electrons can still bear the pain and lose another electron, but there is a price to pay, and this price is energy, that is, a continuous external input of electricity.

Under the action of the electric field, the 4-valent vanadium ion is forced to lose another electron and then become a 5-valent vanadium ion. Since electric charge will not be created or destroyed out of thin air, the electron lost by the 4-valent vanadium ion needs to go somewhere, that is, it needs a place to carry it, which is the 3-valent vanadium ion solution. When the 3-valent vanadium ion continues to gain electrons, it changes from a 3-valent vanadium ion to a 2-valent vanadium ion. So, when the charging is finished, they become like this again:

(Image source: self-made by the author)

The above is the general principle of vanadium battery charging and discharging. It should be noted that in order to facilitate the explanation of the principle, we actually overlooked an extremely important detail in the description.

Forget about conservation of charge!

As mentioned earlier, the discharge process of the vanadium battery is as follows: in the positive electrode solution, the pentavalent vanadium ions continuously obtain electron replenishment, and then gradually transform themselves into quadrivalent vanadium ions.

But think about it, can such a scenario be realized? Almost impossible. Because if you simply input electrons continuously into a large bucket of solution, the solution will carry a lot of static electricity. The more electrons you input, the stronger the static electricity will be, and eventually it will be too strong to be realized.

Therefore, according to the law of conservation of charge, if the positive electrode solution wants to maintain electrical neutrality, there is only one way to go, that is: for every negatively charged electron obtained, a positive charge must also enter the house at the same time, the positive and negative cancel each other out, and the solution becomes neutral.

Where does this positive charge come from? It can only come from the negative electrode solution. The reason is that during the discharge process, the positive electrode solution is the party that gains electrons, while the negative electrode solution is the party that loses electrons. Then, according to the law of conservation of charge, the negative electrode solution cannot simply output electrons, otherwise it will also carry a large amount of static electricity. Therefore, while outputting electrons, it must also output an equal amount of positive charge.

In summary, the discharge process of vanadium batteries is: the negative electrode solution simultaneously provides equal amounts of electrons and positive charges to the positive electrode solution .

The charging process is the opposite: the positive electrode solution simultaneously provides an equal amount of electrons and positive charge to the negative electrode solution.

Here comes another question: what is this positive charge?

It is actually the hydrogen ion, or proton, in the solution. Hydrogen has only one electron, and when it loses one electron, it is essentially a proton with a positive charge.

Therefore, in the real all-vanadium flow battery, the positive electrode solution and the negative electrode solution are actually next to each other, but they are separated by a layer of proton exchange membrane. This proton exchange membrane only allows protons to shuttle back and forth, and other substances are not allowed to pass.

Take charging as an example. During charging, the 4-valent vanadium ion in the positive electrode solution loses one electron and becomes a 5-valent vanadium ion. The lost electron goes through the external circuit into the negative electrode solution, while the hydrogen ion (proton) passes through the proton exchange membrane into the negative electrode solution. During discharge, the process is the opposite.

Now that you know the principles of vanadium battery charging and discharging, you must be more interested in its advantages. Pay attention to Science Popularization China to learn more about vanadium batteries.

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