Lithium-ion batteries are widely used in home electronics and are now being used to power electric vehicles and store energy for the power grid. But their limited number of recharge cycles and tendency to degrade in capacity over their lifetime have spurred a great deal of research into improving the technology.
An international team led by researchers from the Molecular Foundry used advanced techniques in electron microscopy to show how the ratio of materials that make up a lithium-ion battery electrode affects its structure at the atomic level, and how the surface is very different from the rest of the material. The work was published in the journal Energy & Environmental Science.
Knowing how the internal and surface structure of a battery material changes over a wide range of chemical compositions will aid future studies on cathode transformations and could also lead to the development of new battery materials.
Using a suite of electron microscopes both at the National Center for Electron Microscopy (NCEM), a Molecular Foundry facility, and at SuperSTEM, the National Research Facility for Advanced Electron Microscopy in Daresbury, U.K., the research team found that while the atoms throughout the interior of the cathode material remained in the same structural pattern across all compositions, decreasing the amount of lithium caused an increase in randomness in the position of certain atoms within the structure.
By comparing different compositions of cathode material to battery performance, the researchers also demonstrated it was possible to optimize battery performance in relation to capacity by using a lower ratio of lithium to other metals.
The most surprising finding was that the surface structure of an unused cathode is very different from the interior of the cathode. A thin layer of material on the surface possessing a different structure, called the “spinel” phase, was found in all of their experiments. Several previous studies had overlooked that this layer might be present on both new and used cathodes.