Sodium-ion batteries: New storage mechanism for cathode materials
The image illustrates a layered structure composed of transition metals (blue) and sulfur (yellow). The space between the layers can be occupied by sodium ions (purple) and organic solvent molecules (red, brown) through a process called co-intercalation. © Y. Sun et al., Nature Materials 2025
Li-ion and Na-ion batteries operate through a process called intercalation, where ions are stored and exchanged between two chemically different electrodes. In contrast, co-intercalation, a process in which both ions and solvent molecules are stored simultaneously, has traditionally been considered undesirable due to its tendency to cause rapid battery failure. Against this traditional view, an international research team led by Philipp Adelhelm has now demonstrated that co-intercalation can be a reversible and fast process for cathode materials in Na-ion batteries. The approach of jointly storing ions and solvents in cathode materials provides a new handle for designing batteries with high efficiency and fast charging capabilities. The results are published in Nature Materials.
The performance of batteries depends on many factors. In particular, it depends on how ions are stored in electrode materials and whether they can be released again. This is because the charge carriers (ions) are relatively large and can cause an undesirable change in volume when they migrate into the electrode. This effect, known as 'breathing', impairs the battery's service life. The volume change is particularly pronounced when sodium ions migrate together with molecules from the organic electrolyte. This so-called co-intercalation has generally been considered detrimental to battery life. However, an international research team led by Philipp Adelhelm has now investigated cathode materials that enable the co-intercalation of ions and solvent molecules, allowing for faster charging and discharging processes.
Co-intercalation in anodes
In earlier studies, the team investigated co-intercalation in graphite anodes, demonstrating that sodium could migrate quickly and reversibly into and out of the electrolyte over many cycles when combined with glyme molecules. Nevertheless, proofing the same concept for cathode materials remained difficult. To tackle this challenge, the team explored a range of layered transition metal sulfides and identified solvent co-intercalation processes in cathode materials. ‘The process of co-intercalation could be used for developing very efficient and faster-charging batteries. This is why we wanted to investigate this topic in more detail,’ says Prof. Philipp Adelhelm.
Co-intercalation in cathodes: a different process
The study incorporates detailed investigations from the last three years: Dr. Yanan Sun carried out volume change measurements in the cathode materials, performed structural analyses with synchrotron radiation at PETRA III at DESY, and investigated the electrochemical properties for a variety of combinations of electrodes and solvents. By support from theory, in collaboration with Dr. Gustav Åvall, important parameters could be identified that help predicting co-intercalation reactions in the future.
Advantage: Super-fast kinetics
‘The co-intercalation process in cathode materials differs significantly from what happens in graphite anodes,’ explains Yanan Sun. While co-intercalation reactions in graphite anodes typically result in low-capacity electrodes, the loss of capacity caused by co-intercalation in the investigated cathode materials is very low. ‘Above all, certain cathode materials offer a huge advantage: the kinetics are super-fast, almost like a supercapacitor!’ Sun emphasises.
Vast chemical landscape for novel materials
'The true beauty of co-intercalation reactions lies in their ability to offer a vast chemical landscape for designing novel layered materials for diverse applications.’ says Adelhelm. ‘Exploring the concept of co-intercalation was extremely risky because it is against classical battery knowledge. I was therefore very grateful to receive funding for this idea from the European Research Council through an ERC Consolidator Grant. The findings are the result of a collaborative effort from many talented people and would not have been possible without the opportunities provided by the joint research group on operando battery analysis financed by Helmholtz-Zentrum Berlin and Humboldt-University,’ he adds. ‘The recently announced Berlin Battery Lab between HZB, HU and BAM will provide even more opportunities for joint research projects in Berlin.‘
- Science in Berlin: collaborative, diverse, innovative, internationalScience is a key social and economic factor for Berlin. It creates jobs and adds value, attracts talent from all over the world, and enriches city life. To ensure that Berlin remains one of the world’s leading centres of science in the future, science must feature even more prominently on the political agenda. This is what BR50 stands for: over 50 non-university research institutions, including the HZB, are joining forces in this network. Following the election in Berlin, BR50 is now addressing politicians with a position paper, accompanied by the campaign “𝗪𝗶𝗿 𝗳𝗼𝗿𝘀𝗰𝗵𝗲𝗻 𝗳ü𝗿 𝗕𝗲𝗿𝗹𝗶𝗻.”
- Does African Architecture exist?The world’s first Pan-African Architecture Biennale took place this September in Nairobi, Kenya. People from the worlds of architecture, art, academia and politics from 54 African countries and the diaspora had travelled to Nairobi to exchange ideas on traditional and contemporary building methods, materials, experiences and projects. Samira Aden co-organised this biennial as lead curator. We spoke to her about it.
- Nanosilver as an electrocatalyst for CO₂ reductionVia electrolysis, CO2 can be reduced to CO, a raw material for further chemical products such as fuels. Within the GreenQuest Project, an internation team led by HZB chemist Prashanth Menezes has now systematically investigated catalyst layers made of silver nanoparticles, varying both the size of the particles and their density. The best yield was achieved with nanoparticles with diameters of around 10 nm, which were loosely distributed. Furthermore, they demonstrated how the economic efficiency of the electrochemical cell can be enhanced by integrating an additional chemical reaction at the anode, enabling the simultaneous production of a valuable formic acid, hydrogen, and CO in one device.