How do Li-ion batteries change their properties during discharge?
The Vienna University of Technology has succeeded in developing new models that can be used to describe very precisely the behavior of lithium-ion batteries during the charging and discharging process
From cell phone batteries to electric cars - lithium-ion batteries have long been part of our everyday lives. Even the Nobel Prize in Chemistry was awarded in 2019 for the invention of such batteries. However, describing the processes in lithium-ion batteries with technical precision is difficult. As the research team for electrochemistry at TU Wien has now shown, the material properties can change drastically during the charging process.
With the help of numerous experiments, the team succeeded in developing a practical mathematical description of these changes. This makes it possible to calculate how the battery voltage changes as a function of the state of charge. It is now even possible to derive information about the internal state of the electrode material from the electrical behavior of the battery.
The charging process changes the material
Crucial to the construction of a lithium-ion battery are electrode materials that contain mobile lithium ions. Initially, each of these ions sits in its intended place in one of the two electrodes, and the battery is fully discharged. Then, when an electrical voltage is applied, the positively charged lithium ions begin to move toward the other electrode, leaving vacancies in the crystal.
The smaller the number of remaining lithium ions becomes, the higher the voltages you have to apply to get the last ions out of the crystal until, ideally, all the lithium ions are removed - at which point the battery is fully charged.
"You can use different lithium-containing materials for this," says Andreas Bumberger, the first author of the current publication, who is working on his dissertation in Prof. Jürgen Fleig's team at the Institute of Chemical Technologies and Analytics at TU Wien.
People often try to describe the movement of lithium ions in such materials using a single parameter - the diffusion coefficient. But the TU Vienna team was able to show that this is not enough. "After all, the material changes quite fundamentally during charging," explains Andreas Bumberger. "We start with a crystal that contains a lot of lithium, and at the end we have a crystal that is almost lithium-free. The material properties change accordingly." So you have to understand the charging process as a dynamic material change.
A happy medium
One way to do this would be to calculate the course of the battery voltage at the atomic level using quantum physics formulas - for example, density functional theory. In practice, however, this is only of limited help if one wants to better understand battery materials. Such calculations are very time-consuming, and it is also difficult to establish a simple relationship between the voltage and the various material defects in this way.
The research team at the Vienna University of Technology therefore tried a middle way: they developed a model that, on the one hand, does not merely describe the materials with one parameter, but takes into account the continuous transformation of the material during the charging process, but which, on the other hand, is simpler and more descriptive than a quantum physical description of the material at the atomic level.
A look into the material, without a microscope at all
"As we show in our current paper, our model fits quite excellently with the data we obtained from impedance spectroscopic measurements," says Andreas Bumberger. "We analyze how the electrical behavior of the battery changes as its state of charge changes. And from the measured data, we can use our model to gain valuable information about the atomic processes during charging."
In this way, it is possible to see, for example, what kind of defects the material has, whether there are perhaps incorrect atoms in some places or whether the crystal lattice has certain irregularities - and all without looking through a microscope, just on the basis of electrical measurements. "This now makes basic research on novel batteries much easier for us," says Prof. Fleig. "It's an important step in understanding many materials that will continue to play a major role in the development of rechargeable batteries in the future."
Note: This article has been translated using a computer system without human intervention. LUMITOS offers these automatic translations to present a wider range of current news. Since this article has been translated with automatic translation, it is possible that it contains errors in vocabulary, syntax or grammar. The original article in German can be found here.
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