How destructive formations develop in batteries
Tiny formations inside lithium batteries can severely limit the operating life of an energy storage device. A research team at the Helmholtz-Zentrum Berlin (HZB) has now investigated the process behind these formations in greater detail. Their results provide anchor points for the future development of longer-lasting and safer lithium batteries.
Dendrites can form inside lithium batteries. These small needles or trees resemble the branched extensions of our nerve cells, from which they get their name. Dendrites form when the ions of an alkali metal like lithium encounter tiny crystallisation nuclei as the ions migrate back and forth between the internal plus and minus poles of a battery during the charge/discharge cycles.
These dendrites grow during each charge/discharge cycle and eventually short-circuit the battery, destroying it - and in some cases even causing an explosion. It is not yet clear how this danger can be averted, and how the service life of energy storage devices can be increased, because we do not yet fully understand how the dendrites develop and grow.
High-resolution insights in 3D
To unravel the mystery of this nucleation and growth of dendrites in lithium-ion batteries, a research team took a look deep inside a battery using two specialised methods at the HZB. "While conventional investigations with scanning or transmission electron microscopes generally provide a two-dimensional image, we use focussed ion-beam scanning electron microscopy to penetrate into the third dimension", explains Kang Dong, a postdoc who works in Ingo Manke's research group involved with imaging methods at the HZB Institute for Applied Materials Research.
"We also employed cryogenic transmission electron microscopy from Prof. Yan Lu's research group at the HZB. The low temperatures minimise the damage caused to our samples by the electron beam, and we obtain nearly realistic resolution in the nanometre range of the structure and chemistry of the lithium deposits."
The researchers obtained high-resolution images of the internal lithium deposits accurate in every detail. "We discovered that the dendrites have extremely varied features that depend strongly on the local current densities", explains Manke, who heads the research group. "At low current densities, they look like small spheres that clump together over time. At higher currents densities, they more closely resemble moss-like, fractal dendrites."
During their research, the team recorded images at different stages of the development of the lithium spheres and dendrites, which appear like whiskers. These three-dimensional images represent a milestone in understanding the mechanisms at work during deposition.
Anchor points for development of longer-lasting batteries
"We also found that the dendrites always commence at specific contamination points and/or structural inhomogeneities on the surface of the lithium anode", Manke tells us about an additional discovery. "The way the lithium reacts with the separator layer inside the battery has not yet been fully understood", adds Kang Dong. They already suggest in the published paper the direction research could take: "We think that optimising the electrolytes and the engineering of the internal surfaces are important points for keeping the lithium deposits more spherical and amorphous. This could prevent the growth of the branched dendrites, helping improve the operating stability of the batteries."
Source: Helmholtz-Zentrum Berlin (HZB)