Lithium-ion batteries are well-known for their long lasting lives. However, researchers believe there is still room for discussion.
A group of scientists at the Department of Energy’s Pacific Northwest National Laboratory’s Chongmin Wang claim that when added, Nickel reduces the speed of battery discharge. They mapped the individual elements of electrodes made of Lithium-Nickel-Manganese oxide layered nanoparticles by creating high-resolution 3D images.
Surprisingly for the team, Nickel formed clumps within the nanoparticles, hence blocking the channels through which Lithium travels during discharge.
In a standard Lithium-ion battery, the charged Li atoms bounce between the negative and positive electrodes while the device is in use. When the electrodes are made of Lithium-Manganese oxide, the Oxygen and Manganese atoms form rows, in between which the Lithium-ion moves.
Scientists have proved that adding Nickel to the batteries can improve their capacity, however it is not yet clear why this capacity drops after repeated usage.
The team at Wang, led by materials scientist Meng Gu examined the arrangement of the different atoms within the Lithium-Nickel-Manganese oxide layered nanoparticle electrode materials, using a microscope. They observed the formation of rows of atoms, the channels filled with Lithium ions, as well as the accumulation of Nickel at the end of the rows.
They applied the so-called three-dimensional composition mapping technique in order to establish the distribution of the surface layer. Gathering 50 images of individual elements at various angles, the team was able to reconstruct a three dimensional map showing the distribution of each element. They were surprised to observe that while the Manganese, Oxygen and Lithium atoms are evenly spread across the surface, the Nickel was clustered at particular areas, forming grains.
In order to explain this clustering, the team compared the movement of Nickel and Lithium through the channels. It was found that although Nickel generally resides within the Manganese oxide rows, in cases when it escapes from the channels, it flows much easier and accumulates in the end of the field.
A number of additional experiments were conducted by the team in order to suggest ways to improve the materials. They tested the influence of changing temperature, and intend to continue by examining manufacturing methods for producing better electrodes.
This work is supported by PNNL’s Chemical Imaging Initiative and more details can be read in last week’s issue of Nano Letters.