Battery Technology How Can Solid-State Batteries Be Made More Efficient?

The Max Planck Institute for Polymer Research, in collaboration with Japanese universities, is working on the question of the efficiency of solid-state batteries. The focus has been on studying space charge effects that cause additional resistance during charging and discharging.

Solid-state batteries are particularly promising for electromobility, as they offer more storage capacity and enhanced safety. Unlike conventional batteries, they use a solid rather than a liquid electrolyte, preventing leaks and significantly reducing the risk of fire. The Max Planck Institute for Polymer Research, in collaboration with Japanese universities, is now working to improve the performance of solid-state batteries. "A battery is like a pump," explains Rüdiger Berger, group leader at the MPI-P. "Inside, ions—charged atoms—move, and this movement must be balanced externally by an electron flow, which creates a current." As the ions travel within the battery, space charge layers can form at the internal interfaces. These layers repel the other moving ions, creating additional resistance and resulting in energy losses within the battery—hindering both charging and discharging processes. The Max Planck Institute discovered that this effect primarily occurs at the positive electrode, where a charge layer measuring less than 50 nanometers thick forms. Furthermore, they determined quantitatively that the space charge layer is dynamic, meaning it depends on the battery's state of charge. This space charge layer accounts for about 7 percent of the battery's total resistance but can be larger depending on the materials used for the electrolyte.

Two New Microscopic Methods

Until now, little was known about the size of this charge layer and its impact on the current flow. Various research teams worldwide have already studied this effect, but depending on the method used, they arrived at completely different estimates for the thickness of the charge layer. The research team therefore studied where and how the charge layer forms using two microscopic methods. The challenge lay in analyzing the interface of a model battery with microscopic methods during operation and at different states of charge.

For this purpose, they constructed a thin-film model battery and analyzed it using Kelvin probe force microscopy and nuclear reaction analysis. With the Kelvin probe force microscopy, they were able to scan the cross-section of the battery with a fine needle, learn more about the local influence of voltage, and observe electric potentials in real time. The nuclear reaction analysis method allowed them to detect the enrichment of lithium at the interface with the battery's positive electrode. "Both techniques are new to battery research and can also be applied to other questions in the future," emphasizes Taro Hitosugi from the University of Tokyo. Through further investigations, the team hopes to find ways to reduce resistance and enhance the performance of solid-state batteries by modifying the material or structure of the electrode. (se)

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