18 July 2018
The cause of performance-reducing “voltage fade” of cathode materials called Lithium-rich NMC (nickel magnesium cobalt) layered oxides has been identified by a team of researchers.
The research, led by a University of California San Diego team, points to nanoscale defects or dislocations in Lithium-rich NMC cathode materials as the batteries charged at a range of voltages going up to 4.7V.
“The dislocations are extra atomic layers that don’t fit into the otherwise perfectly periodic crystal structure,” said Andrej Singer, lead author who performed this work as a postdoctoral researcher at UC San Diego. “Discovering these dislocations was a big surprise: if anything, we expected the extra atomic layers to occur in a completely different orientation.”
By combining experimental evidence with theory, the research team concluded that the nucleation of this specific type of dislocation results in voltage fade.
Knowing the origin of voltage fade, the team showed that heat treating the cathode materials eliminated most of the defects and restored the original voltage. They put the heat-treated cathodes into new batteries and tested them at a range of voltages going up to 4.7V, demonstrating that the voltage fade had been reversed.
While the heat treating approach to reversing the defects is labour-intensive and not likely to scale, the physics and materials science-based approach to characterizing and then addressing the nano-scale defects offers promise for finding new solutions to the voltage fade problem.
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