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Scanning and transmission electron micrographs of the cathode material at different magnifications. These images show that the 10-micron spheres (a) can be hollow and are composed of many smaller nanoscale particles (b). Chemical "fingerprinting" studies found that reactive nickel is preferentially located within the spheres' walls, with a protective manganese-rich layer on the outside. Studying ground up samples with intact interfaces between the nanoscale particles (c) revealed a slight offset of atoms at these interfaces that effectively creates "highways" for lithium ions to move in and out to reach the reactive nickel (d).
Using an aberration-corrected scanning transmission electron microscope—a scanning transmission electron microscope outfitted with a pair of "glasses" to improve its vision—the scientists saw that the particles had facets, flat faces or sides like the cut edges of a crystal, which allowed them to pack tightly together to form coherent interfaces with no mortar or cement between the bricks. But there was a slight misfit between the two surfaces, with the atoms on one side of the interface being ever so slightly offset relative to the atoms on the adjoining particle.
"The packing of atoms at the interfaces between the tiny particles is slightly less dense than the perfect lattice within each individual particle, so these interfaces basically make a highway for lithium ions to go in and out," Xin said.
Like tiny smart cars, the lithium ions can move along these highways to reach the interior structure of the wall and react with the nickel, but much larger semi-truck-size electrolyte molecules can't get in to degrade the reactive material.
Using a spectroscopy tool within their microscope, the CFN scientists produced nanoscale chemical fingerprints that revealed there was some segregation of nickel and manganese even at the nanoscale, just as there was in the micron-scale structures.
"We don't know yet if this is functionally significant, but we think it could be beneficial and we want to study this further," Xin said. For example, he said, perhaps the material could be made at the nanoscale to have a manganese skeleton to stabilize the more reactive, less-stable nickel-rich pockets.
"That combination might give you a longer lifetime for the battery along with the higher charging capacity of the nickel," he said.
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technologies Office, of the U.S. Department of Energy. The Center for Functional Nanomaterials at Brookhaven Lab and the Stanford Synchrotron Radiation Lightsource at SLAC are both DOE Office of Science User Facilities supported by the DOE Office of Science (BES).
About Brookhaven National Laboratory
Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
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