Batteries Sodium-Ion Batteries: New Storage Mode for Cathode Materials

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Batteries work by storing and exchanging ions between two chemically different electrodes, a process known as intercalation. In co-intercalation, however, both ions and solvent molecules are stored in the electrode materials, which was previously considered unfavorable. With the appropriate cathode materials, it can work, as demonstrated by an international team.

The performance of batteries depends on many factors. In particular, it is crucial how ions are stored in the electrode materials and whether they can be released again. Since sodium ions are relatively large as charge carriers, they can cause undesirable volume changes when migrating into the respective electrode. This effect, known as "breathing," compromises the battery's lifespan. The volume change is especially pronounced when sodium ions migrate into electrode materials together with molecules from the organic electrolyte. This co-intercalation has so far been considered undesirable and detrimental to the lifespan of batteries.

Faster Charging And Discharging Processes

An international team led by Prof. Dr. Philipp Adelhelm has now studied cathode materials where the co-intercalation of ions with solvent molecules works well and enables faster charging and discharging processes.

In previous studies, the team had already investigated co-intercalation in graphite anodes and demonstrated that sodium, in combination with glyme molecules, can quickly and reversibly migrate into and out of the electrolyte over many cycles. However, the same concept could not yet be applied to cathode materials. The team led by Adelhelm has now examined cathode materials made of layered transition metal sulfides. "The co-intercalation process could be used to develop efficient batteries with very short charging times. That's why we wanted to investigate this topic in more detail," says Adelhelm.

The co-intercalation process in cathode materials differs significantly from that in graphite anodes.

Dr. Yanan Sun

The study incorporates results from the past three years: Dr. Yanan Sun, a postdoctoral researcher on Adelhelm’s team, conducted measurements of the volume changes in the cathode materials, examined the materials' structure using synchrotron radiation at PETRA III (DESY), and determined the electrochemical properties for various combinations of electrodes and solvents. In collaboration with Dr. Gustav Åvall and theoretical modeling, the team was able to identify the key parameters that enable co-intercalation reactions to be modeled in advance. "The co-intercalation process in cathode materials differs significantly from that in graphite anodes," explains Yanan Sun.

The true beauty of co-intercalation reactions lies in the vast "chemical landscape" they open up for the development of novel layered materials for diverse applications.

Prof. Dr. Philipp Adelhelm

Ultra-Fast Kinetics, Almost Like A Supercapacitor

While co-intercalation reactions in graphite anodes typically reduce capacity, the capacity loss caused by co-intercalation in the studied cathode materials is very low. "Certain cathode materials offer a tremendous advantage: The kinetics are ultra-fast, almost like a supercapacitor!" emphasizes Sun.

"The true beauty of co-intercalation reactions lies in the vast 'chemical landscape' it opens up for the development of novel layered materials for diverse applications," says Adelhelm. The exploration of the co-intercalation concept was risky as it contradicted classical knowledge about batteries. "I was therefore grateful to have been supported for this idea by the European Research Council through an ERC Consolidator Grant. The results are the product of collaboration among many talented individuals and would not have been possible without the capabilities of the joint research group on operando battery analysis, funded by the Helmholtz Center Berlin and Humboldt University," he adds. "The recently announced Berlin Battery Lab between HZB, HU, and BAM will offer even more opportunities for collaborative research projects in Berlin, Germany."

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