Atomic-level images of dendrites, finger-like growths that are capable of penetrating the barrier that separates battery compartments and cause batteries to blow up, shed light on why batteries fail.
Cryo-EM
Dendrites hamper the ability of batteries to store more energy posing a stumbling block for battery-operated devices and electric cars to work longer.
Researchers examined the inner workings of batteries using cryo-electron microscopy, or cryo-EM, a technique that involves firing beams of electrons at frozen proteins and "biological machines" to deduce the structure of their molecules.
"A beam of electrons is sent through a biomolecular sample that has been frozen, typically with liquid ethane," Stu Borman of Chemical & Engineering News explained.
"Electron beams physically damage biomolecules, but freezing them, the 'cryo' part of cryo-EM, protects them from electron damage and prevents them from getting dehydrated in the electron microscope's vacuum chamber."
Jacques Dubochet, Joachim Frank and Richard Henderson won the 2017 Nobel Prize in chemistry for developing this biomolecule-imaging technique.
Dendrites
The images showed lithium metal dendrites are long six-sided crystals and are not characterized by irregularly pitted shape depicted in earlier electron microscope shots.
By being able to see at this level of detail, scientists can better understand how batteries and their components basically work. They can also investigate why high-energy batteries that are commonly used in everyday devices, cars and machines, fail.
For the study published in the journal Science, Yuzhang Li, from the Department of Materials Science and Engineering, Stanford University, and colleagues used a cryo-EM instrument to take a closer look at lithium metal dendrites exposed to various electrolytes.
The researchers looked at the metal part of the dendrites as well as the solid electrolyte interphase, or SEI. SEI is a coating that forms on metal electrodes as battery charges and discharges. It develops when dendrites react with the surrounding electrolyte. Controlling the growth and stability of SEI is crucial for batteries to operate efficiently.
The images revealed that the crystalline dendrites prefer to grow in particular directions. Some of these develop kinks as they grew, but the crystal structure are still intact.
The researchers also looked at how electrons bounced off the atoms in dendrites, which unveil the locations of each atom in the crystal and the SEI coating.
When the researchers added a chemical used to boost battery performance, they noticed that the atomic structure of the SEI coating became more orderly, which can help explain the effectiveness of this chemical.
"We observe that dendrites in carbonate-based electrolytes grow along the <111> (preferred), <110>, or <211> directions as faceted, single-crystalline nanowires," the researchers wrote in their study. "We reveal distinct SEI nanostructures formed in different electrolytes."
