January 11, 2016

Unique Two-Level Cathode Structure Improves Battery Performance

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.

journal reference (Open Access)  >>