Brookhaven Lab physicist
Huolin Xin in front of an aberration-corrected scanning
transmission
electron microscope at the Center for Functional Nanomaterials
(January 11, 2016) Controlling
surface chemistry could lead to higher-capacity, faster-charging batteries for
electronics, vehicles, and energy-storage applications
Building a better battery is a delicate balancing act.
Increasing the amounts of chemicals whose reactions power the battery can lead
to instability. Similarly, smaller particles can improve reactivity but expose
more material to degradation. Now a team of scientists from the U.S. Department
of Energy's (DOE) Brookhaven National Laboratory, Lawrence Berkeley National
Laboratory, and SLAC National Accelerator Laboratory say they've found a way to
strike a balance—by making a battery cathode with a hierarchical structure
where the reactive material is abundant yet protected.
3D elemental
association maps of the micron-scale spherical structures,
generated using
transmission x-ray tomography, reveal higher levels
of manganese and
cobalt (darker blue, red, and purple) on the exterior
of the sphere and
higher levels of nickel-containing materials
(green, light
blue, yellow and white) on the interior. (Credit: SLAC)
Test batteries incorporating this cathode material exhibited
improved high-voltage cycling behavior—the kind you'd want for fast-charging
electric vehicles and other applications that require high-capacity storage.
The scientists describe the micro-to-nanoscale details of the cathode material
in a paper published in the journal Nature Energy January 11, 2016.
"Our colleagues at Berkeley Lab were able to make 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).
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).
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 millionths of meter, or 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.
The lithium ion
shuttle
Chemistry is at the heart of all lithium-ion rechargeable
batteries, which power portable electronics and electric cars 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.