Whether in an electronic car, cell phone, or cordless screwdriver, many everyday devices now use rechargeable batteries. However, the trend also has its downsides. For example, some cell phones were forbidden to be carried on planes, or e-cars caught fire. Modern commercial lithium-ion batteries are sensitive to mechanical stress.
So-called “solid-state batteries” can provide the remedy. They no longer have a liquid core – the so-called electrolyte – but consist entirely of a solid, such as a ceramic ionic conductor. As a result, it is mechanically strong, non-flammable, easy to miniaturize, and insensitive to temperature fluctuations.
But solid-state batteries show their problems after several charge-discharge cycles: While the positive and negative poles of a battery are still electrically separated from each other at first, they are electrically connected to each other by the battery’s internal processes: “Lithium” dendrites” slowly grow in the battery These lithium dendrites grow step by step during each charge until the two electrodes are connected.The result: the battery short-circuits and “dies.”To date, however, the exact physical processes that occur in this process are not yet well understood.
A team led by Rüdiger Berger from the Hans-Jürgen Butt Department tackled the problem and used a special microscopy method to investigate the processes in more detail. They investigated the question of where the growth of lithium dendrites begins. Is it like a cave of flowing stone where stalactites from the ceiling and stalagmites from the floor grow until they join in the middle and form what is called a “stagnation”? There is no top and bottom in a battery – but do the dendrites grow from negative to positive or from positive to negative? Or does it grow evenly from both poles? Or are there special places in the battery that lead to nucleation and then dendrite growth from there?
Rüdiger Berger’s team looked in particular at so-called “grain boundaries” in the solid ceramic electrolyte. These boundaries are formed during the production of the annealed layer: the atoms in the ceramic crystals are arranged very regularly. However, due to small and random fluctuations in crystal growth, line-like structures form where atoms are irregularly arranged – so-called “grain boundaries”.
The grain boundaries can be seen using a microscopy method – the ‘Kelvin prop force microscope’ – in which the surface is scanned with a sharp tip. “If a solid-state battery is charged, Kelvin Probe Force Microscopy sees electrons accumulating along grain boundaries — especially near the negative electrode,” says Chao Zhu, a PhD student working with Rüdiger Berger. The latter indicates that grain boundaries change not only the arrangement of the ceramic atoms, but also their electronic structure.
As electrons accumulate – that is, negative particles – the positively charged lithium ions that pass in the solid electrolyte can be reduced to metallic lithium. The result: lithium precipitates and lithium dendrites form. If the charging process is repeated, the dendrite will continue to grow until the battery electrodes are finally connected. The formation of such initial stages of dendrite growth was observed only at the negative pole – it was also observed only at this pole. No growth was observed at the opposite anode.
Scientists hope that by carefully understanding the growth processes, they will also be able to develop effective ways to prevent or at least limit growth in the negative electrode, so that in the future safer lithium solid-state batteries can also be used in broadband applications.
