Silicon electrodes could help to increase the storage capacity of lithium-ion batteries. Oldenburg chemists have now observed for the first time how wafer-thin films grow on silicon electrodes.
Rechargeable lithium-ion batteries provide energy for hearing aids, mobile phones, e-bikes or electric cars and serve as storage for wind and solar power. Chemists have long been looking for ways to increase the performance of the batteries so that electric cars, for example, can cover longer distances. They are experimenting with different electrode materials, including silicon. "Silicon can store more lithium than graphite, which is currently used as the negative electrode in most batteries," says Prof Dr Gunther Wittstock, head of the Physical Chemistry working group at the university. This increases the energy storage capacity of the battery. However, silicon has a decisive disadvantage: during the charging process, the electrodes swell to more than two and a half times their original volume. This causes stresses in the material and the electrodes crumble quickly. For battery applications, silicon electrodes made of nanoparticles are therefore being tested, which are better able to cope with the expansion.
Wafer-thin films that form on the electrodes during operation are crucial for the service life and performance of a battery. These films protect the electrodes and the battery fluid from degradation, but the thicker they become, the more they reduce the performance of the batteries. So far, however, little is known about their formation, as it is difficult to observe their growth directly. A team led by Wittstock and his colleague Eduardo dos Santos Sardinha has now observed for the first time how silicon electrodes are gradually covered by such a film during the first charging cycle. The researchers proved that, contrary to previous assumptions, the protective layer does not form on the entire surface at the same time, but grows in patches. Islands of growth formed on the extremely smooth surface of a platelet made of crystalline silicon, from which the film continued to expand. In the journal ACS Applied Energy Materials, the team reports that the control of film formation can now be specifically investigated.
A separating layer that acts like a bouncer
The films form because chemical reactions take place in the electrochemical cells of the batteries during charging and discharging: At high electrical voltages, the electrically conductive electrolyte liquid between the two electrodes, which consists of lithium-containing salts and an organic solvent, decomposes. The decomposition products form a solid, thin, complex layer on the negative electrode, which acts like a bouncer: it separates the electrolyte liquid from the reactive electrode material, but allows lithium ions to pass through.
The films represent one of the greatest challenges on the way to more powerful lithium-ion batteries. In the case of silicon electrodes, the strong volume change means that the films tear during the first charging cycle and are subsequently formed again and again, consuming the electrolyte fluid. Wittstock, dos Santos Sardinha and two colleagues from the University of Graz have now succeeded in observing the film formation during the very first charging cycle. "If you only analyse an electrode after several charging cycles, the original film is already heavily altered because the electrode has already expanded and contracted several times," explains Wittstock.
Snapshots in the nanoworld
The team slowed down the charging process using a special programme. They used a virgin electrode made of crystalline silicon and analysed the film formation using electrochemical scanning microscopy (SECM). This method uses a microelectrode to scan the silicon surface piece by piece. The measured values are translated into a colour scale and combined to form an image. These images revealed to the researchers where there was already a film and where there was not. "In principle, we were able to take snapshots of the surface during film formation," says Wittstock. The results of the study now enable chemists to systematically investigate and subsequently optimise film formation on silicon electrodes.