First atomic-resolution images of dendrites


Rebecca Pool

Tuesday, October 31, 2017 - 15:00
Image: Cryo-EM image of a dendrite reveals a crystalline nanowire with well-defined facets, [Y. Li et al., Science].
US-based researchers have used cryo-electron microscopy to capture the first atomic-level images of lithium metal dendrite structures that can pierce the barrier between battery compartments and trigger short circuits or fires.
The new images from researchers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory reveal that each dendrite is a long, beautifully formed six-sided crystal, and not the irregular, pitted shape depicted in previous electron microscope images. 
“This is super exciting and opens up amazing opportunities,” says Professor Yi Cui from SLAC and the Stanford Institute for Materials and Energy Sciences (SIMES).
Left; room-temperature TEM image, exposure to air has corroded a lithium metal dendrite and the electron beam has melted holes in it. Right; a cryo-EM image of a dendrite shows that freezing has preserved its original state, revealing that it’s a crystalline nanowire with well-defined facets, [Y. Li et al., Science].
Dendrite formation has long been a stumbling block on the road to developing next-generation batteries for electric cars, mobile phones and more.
While researchers have used electron microscopy to study the structures, chemically-reactive lithium corrodes during sample transfer into the TEM column and is extremely unstable under an electron beam.
Given this, Cui and colleagues developed a novel cryo-transfer method based on cryo-EM procedures widely used in structural biology.
They first electrochemically deposited Li metal onto a copper TEM grid and then washed the grid with electrolyte, immediately flash-freezing specimens in liquid nitrogen.
Specimens were then placed onto the cryo-TEM holder while still immersed in liquid nitrogen and isolated from the environment - avoiding air exposure - by a closed shutter.
Imaging was carried out using a FEI Titan 80-300 environmental STEM, operated at 300 kV and equipped with an aberration corrector in the image-forming (objective) lens.
With cryo-EM, researchers resolved the locations of individual atoms in Li metal (left) and measured the distance between atoms (right), [Y. Li et al., Science].
As Cui highlights in Science: "At cryogenic temperatures, Li metal does not react with the liquid nitrogen or ice so that the dendrites retain their electrochemical state with the relevant structural and chemical information preserved."
The researchers went on to study thousands of lithium metal dendrites that had been exposed to various electrolytes.
How cryo-EM is able to get a close-up image of a lithium metal dendrite when other methods fail, [Yuzhang Li/Stanford University].
By directly visualising individual dendrite structures, rather than reconstructing 3D models,they reached atomic resolutions of 0.7 Å.
“We were really excited. This was the first time we were able to get such detailed images of a dendrite,” says Yanbin Li. “This tool can help us understand what different electrolytes do and why certain ones work better than others.”
The researchers now plan to focus on learning more about the chemistry and structure of dendrite coatings known as solid electrolyte interphase layers.
Research is published in Science.
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