Batteries work by storing and exchanging ions between two chemically different electrodes. This process is called intercalation. In co-intercalation, however, both ions and solvent molecules are stored in the electrode materials, which was previously considered unfavourable. However, an international team led by Philipp Adelhelm has now shown that co-intercalation in sodium-ion batteries can work with suitable cathode materials. This approach offers new development opportunities for batteries with high efficiency and fast charging capabilities. The results were published in Nature Materials.
The performance of batteries depends on many factors. In particular, it depends on how ions are stored in the electrode materials and whether they can be released again. As sodium ions are relatively large charge carriers, they can cause undesirable volume changes when they migrate into the respective electrode. This effect, known as "breathing", impairs the service life of the battery. The volume change is particularly pronounced when sodium ions migrate into electrode materials together with molecules from the organic electrolyte. This co-intercalation was previously regarded as undesirable and detrimental to the service life of batteries.
An international team led by Prof Dr Philipp Adelhelm from Humboldt-Universit?t zu Berlin has now investigated cathode materials in which the co-intercalation of ions with solvent molecules works well and enables faster charging and discharging processes.
Co-intercalation in graphite anodes
In previous studies, the team already investigated co-intercalation in graphite anodes and showed that sodium in combination with glyme molecules can migrate quickly and reversibly into and out of the electrolyte over many cycles. However, the same concept could not previously be applied to cathode materials. Adelhelm's team has now investigated cathode materials made from layered transition metal sulphides. "The process of co-intercalation 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 process works differently in cathodes
Results from the last three years have been incorporated into the study: Dr Yanan Sun, a postdoctoral researcher in Adelhelm's team, carried out the measurements of the volume change in the cathode materials, investigated the structure of the materials using synchrotron radiation at PETRA III (DESY) and determined the electrochemical properties for various combinations of electrodes and solvents. In collaboration with Dr Gustav ?vall and theoretical modelling, the team was able to identify the decisive parameters that make it possible to model co-intercalation reactions in advance. "The co-intercalation process in cathode materials differs significantly from that in graphite anodes," explains Yanan Sun.
Low capacity losses, fast kinetics
While co-intercalation reactions in graphite anodes typically reduce the capacitance, the capacitance loss caused by co-intercalation is very low in the cathode materials investigated. "Certain cathode materials offer an enormous advantage: the kinetics are super-fast, almost like a supercapacitor!" emphasises Sun.
Chemical "landscape" offers many options
"The real beauty of co-intercalation reactions lies in the huge "chemical landscape" that opens up for the development of novel layered materials for a wide range of applications," says Adelhelm. Researching the co-intercalation concept was risky, as it contradicted the classic findings on batteries. "I was therefore grateful to receive funding for this idea from the European Research Council (ERC) through an ERC Consolidator Grant. The results come from the collaboration of many talented people and would not have been possible without the opportunities provided by the joint research group on operando battery analysis funded by Helmholtz-Zentrum Berlin and Humboldt-Universit?t zu Berlin," he adds. "The recently announced Berlin Battery Lab between HZB, HU and BAM will provide even more opportunities for joint research projects in Berlin."
Further information
Press releases of the Helmholtz Centre Berlin