Whether smartphones or electric cars: Wherever electrical power is to be available on the go, it usually comes from rechargeable lithium-ion batteries. One of the two electrodes consists of graphite, in which lithium ions are stored to store energy. The disadvantage of the carbon material: its capacity is quite low - which requires frequent battery charging. That is why researchers worldwide are looking for alternative electrode materials for batteries with longer charging cycles.
Silicon pushes the storage capacity
Silicon is a promising candidate. Because it can absorb more than ten times as much ions as graphite. "In addition, silicon is one of the most common elements in the earth's crust and is available in almost inexhaustible quantities," says Dr. Sebastian Risse, who deals with the analysis of storage materials at the HZB. Some battery manufacturers are already using small silicon particles to improve the energy storage capacity of graphite electrodes. But this trick has limits. "When lithium ions are stored, the silicon expands to a multiple of its normal size," explains Risse. The result: the material gradually becomes brittle.
A team of researchers at HZB headed by Sebastian Risse has now investigated what is going on in the material in detail and in high resolution on electrodes made of crystalline silicon - and followed the physical processes during charging and discharging live as in a film. The basis for this is the multidimensional operando analysis - a technique that the Berlin researchers have developed in recent years. "We can use it to measure different properties at the same time and thus track changes in the shape of the material - while the battery is being operated in the usual way," says Risse.
Battery operation in coherent X-ray light
In addition to electrical measurements and recordings with an electron microscope, the Berlin researchers also rely on the phase contrast imaging method. "It uses a coherent X-ray beam that only a synchrotron can deliver," emphasizes Sebastian Risse. The physicist and his colleagues used X-rays from the HZB's BESSY II synchrotron storage ring for their experiments. In doing so, they targeted an electrode made of crystalline silicon as it went through several charging and discharging cycles. In the coherent X-ray radiation - in which all waves of the X-ray light vibrate in unison - the subtleties of the material structure lead to characteristic phase shifts. "This way, much more details can be observed than with an analysis with normal X-ray light," says Risse. The necessary know-how was managed by Dr.
The results of the high-resolution operando imaging of crystalline silicon electrodes provide new insights into the promising system. In this way, the researchers were able to show that when loading and unloading, a checkerboard-like fracture pattern emerges and disappears again. "Although the breaks get a little bigger each time they are unloaded, the pattern is retained," reports Risse. "There are no new breaks."
This is good news for using silicon in batteries. Additional hope gives another discovery. The storage of lithium ions in the crystal lattice of silicon takes place in two steps: First, a phase weakly loaded with lithium is formed, then a second phase, rich in lithium. The process is reversed when unloading. The Berlin researchers have now found that the material only breaks in the second step when the low-lithium phase is also discharged.
The foundation for persistent electricity donors
"If only one part of the silicon were used to store ions at a time, the macroscopic damage to the material could be avoided," concludes Sebastian Risse. And although they would not use up the full storage capacity, lithium-ion batteries with electrodes made of silicon could absorb much more energy than those made of graphite. Mobile phones would then have to be plugged in less frequently, and electric cars could cover longer distances with one battery charge. "There is still a long way to go," says HZB physicist Risse. But the scientific foundation for this has now been laid.