Nickelblock: an element's love-hate relationship with battery electrodes

Anyone who owns an electronic device knows that lithium ion batteries could work better and last longer. Now, scientists examining battery materials on the nano-scale 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 last week in Nano Letters, the research also suggests a way to improve the materials.

The researchers, led by the Department of Energy's Pacific Northwest National Laboratory'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," said 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.