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2016-01-29

National labs researchers find simple procedure to improve performance of NMC cathodes in Li-ion batteries



Electron microscope images zoom in to show details of the NMC cathode particles. From left: Hollow NMC spheres, just 10 millionths of a meter in diameter, are made up of much smaller particles about 100 billionths of a meter across, visible in the second image. The third image is a close-up of a few of these nanoscale particles. At right, the microscope zooms in on the interface between two nanoscale particles, revealing individual atoms. The particles are slightly offset in a way that allows lithium ions from the battery’s electrolyte to move i and out (arrow) during charging and discharging. (Brookhaven National Laboratory)

Lithium nickel manganese cobalt oxide (NMC) is one of the more promising chemistries for better lithium batteries, especially for electric vehicle applications, but scientists have been struggling to get higher capacity out of them.
Now, a team of scientists from the US Department of Energy’s (DOE) Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, and SLAC National Accelerator Laboratory has found that using a simple technique called spray pyrolysis can help to overcome one of the biggest problems associated with NMC cathodes—surface reactivity, which leads to material degradation. An open-access paper on their work is published in the journal Nature Energy.
The team was led Berkeley Lab battery scientist Marca Doeff, who has been studying NMC cathodes for about seven years.
The facilities used were the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC and the Center for Functional Nanomaterials (CFN) at Brookhaven, both DOE Office of Science User Facilities.
The Berkeley Lab researchers made a particle structure that has two levels of complexity where the material is assembled in a way that it protects itself from degradation, explained Brookhaven Lab physicist and Stony Brook University adjunct assistant professor Huolin Xin, who helped characterize the nanoscale details of the cathode material at Brookhaven Lab’s Center for Functional Nanomaterials (CFN).
X-ray imaging performed by scientists at the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC along with Xin’s electron microscopy at CFN revealed spherical particles of the cathode material measuring microns in diameter made up of lots of smaller, faceted nanoscale particles stacked together like bricks in a wall. The characterization techniques revealed important structural and chemical details that explain why these particles perform so well.
Lithium-ion rechargeable batteries work by shuttling lithium ions between positive and negative electrodes bathed in an electrolyte solution. As lithium moves into the cathode, chemical reactions generate electrons that can be routed to an external circuit for use. Recharging requires an external current to run the reactions in reverse, pulling the lithium ions out of the cathode and sending them to the anode.
Reactive metals such as nickel have the potential to make great cathode materials—except that they are unstable and tend to undergo destructive side reactions with the electrolyte. So the team experimented with ways to incorporate nickel but protect it from these destructive side reactions.
They sprayed a solution of lithium, nickel, manganese, and cobalt mixed at a certain ratio through an atomizer nozzle to form tiny droplets, which then decomposed to form a powder. Repeatedly heating and cooling the powder triggered the formation of tiny nanosized particles and the self-assembly of these particles into the larger spherical, sometimes hollow, structures.
X-ray spectroscopy revealed that the outer surface of the spheres was relatively low in nickel and high in unreactive manganese, while the interior was rich in nickel. The manganese layer forms an effective barrier, like paint on a wall, protecting the inner structure of the nickel-rich bricks from the electrolyte.
To determine how Li ions were still able to enter the material to react with the nickel, Xin’s group at the CFN ground up the larger particles to form a powder composed of much smaller clumps of the nanoscale primary particles with some of the interfaces between them still intact.
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 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 nthe 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.
The smaller lithium ions can move along these highways to reach the interior structure of the wall and react with the nickel, but much larger 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.
The researchers do not know yet if this is functionally significant, but they think it could be beneficial and 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. Such a combination might provide a longer lifetime for the battery along with the higher charging capacity of the nickel.
In future experiments, the researchers plan to probe the NMC cathode with X-rays while it’s charging and discharging to see how its structure and chemistry change. They also hope to improve the material’s safety: As a metal oxide, it could release oxygen during operation and potentially cause a fire.