While manganese (blue) fills out this lithium ion battery nanoparticle evenly, nickel (green) clumps in certain regions, interfering with the material’s smooth operation. Image: Chongmin Wang/PNNL |
Anyone who owns an electronic device knows that lithium-ion batteries
could work better and last longer. Now, scientists examining battery materials
on the nanoscale reveal how nickel forms a physical barrier that impedes the
shuttling of lithium ions in the electrode, reducing how fast the materials
charge and discharge. Published in Nano Letters, the research also
suggests a way to improve the materials.
The researchers, led by the U.S. Department of Energy (DOE)’s
Pacific Northwest National Laboratory (PNNL)’s Chongmin Wang, created
high-resolution 3D images of electrode materials made from lithium-nickel-manganese
oxide layered nanoparticles, mapping the individual elements. These maps showed
that nickel formed clumps at certain spots in the nanoparticles. A higher-magnification
view showed the nickel blocking the channels through which lithium ions
normally travel when batteries are charged and discharged.
“We were surprised to see the nickel selectively
segregate like it did. When the moving lithium ions hit the segregated nickel
rich layer, they essentially encounter a barrier that appears to slow them
down,” says Wang, a materials scientist based at EMSL, the Environmental
Molecular Sciences Laboratory, a DOE user facility on PNNL’s campus. “The
block forms in the manufacturing process, and we’d like to find a way to
prevent it.”
Lithium ions are positively charged atoms that move between
negative and positive electrodes when a battery is being charged or is in use.
They essentially catch or release the negatively charged electrons, whose
movement through a device such as a laptop forms the electric current.
In lithium-manganese oxide electrodes, the manganese and oxygen
atoms form rows like a field of cornstalks. In the channels between the stalks,
lithium ions zip towards the electrodes on either end, the direction depending
on whether the battery is being used or being charged.
Researchers have known for a long time that adding nickel
improves how much energy the electrode can hold, battery qualities known as
capacity and voltage. But scientists haven’t understood why the capacity falls
after repeated usage—a situation consumers experience when a dying battery holds
its charge for less and less time.
To find out, Wang, materials scientist Meng Gu and their
collaborators used electron microscopy at EMSL and the National Center for
Electron Microscopy at Lawrence Berkeley National Laboratory to view how the
different atoms are arranged in the electrode materials produced by Argonne
National Laboratory researchers. The electrodes were based on nanoparticles
made with lithium, nickel, and manganese oxides.
First, the team took high-resolution images that clearly
showed rows of atoms separated by channels filled with lithium ions. On the
surface, they saw the accumulation of nickel at the ends of the rows,
essentially blocking lithium from moving in and out.
To find out how the surface layer is distributed on and
within the whole nanoparticle, the team used a technique called 3D composition
mapping. Using a nanoparticle about 200 nm in size, they took 50 images of the
individual elements as they tilted the nanoparticle at various angles. The team
reconstructed a 3D map from the individual elemental maps, revealing spots of
nickel on a background of lithium-manganese oxide.
The 3D distribution of manganese, oxygen, and lithium atoms
along the surface and within the particle was relatively even. The nickel,
however, parked itself in small areas on the surface. Internally, the nickel
clumped on the edges of smaller regions called grains.
To explore why nickel aggregates on certain surfaces, the
team calculated how easily nickel and lithium traveled through the channels.
Nickel moved more easily up and down the channels than lithium. While nickel
normally resides within the manganese oxide cornrows, sometimes it slips out into
the channels. And when it does, this analysis showed that it flows much easier
through the channels to the end of the field, where it accumulates and forms a
block.
The researchers used a variety of methods to make the
nanoparticles. Wang says that the longer the nanoparticles stayed at high
temperature during fabrication, the more nickel segregated and the poorer the
particles performed in charging and discharging tests. They plan on doing more
closely controlled experiments to determine if a particular manufacturing
method produces a better electrode.
