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The cathode is the positive electrode in a battery, and development of an improved cathode material is considered essential to achieving a stable high-voltage cell, the subject of intense research. Spray pyrolysis is a commercially available technique used for making thin films and powders but has not been widely used to make materials for battery production.
The surface reactivity is a particular problem for high-voltage cycling, which is necessary to achieve higher capacities needed for high-energy devices. The phenomenon has been studied and various strategies have been tried to ameliorate the issue over the years, including using partial titanium substitution for cobalt, which counteracts the reactivity of the surfaces to some extent.
At SSRL researchers Dennis Nordlund and Yijin Liu used x-ray transmission microscopy and spectroscopy to examine the material in the tens of nanometers to 10-30 micron range. At CFN researcher Huolin Xin used a technique called electron energy loss spectroscopy (EELS) with a scanning transmission electron microscope (STEM), which was able to zoom in on details down to the nanoscale.
At these two scales, Doeff and her Berkeley Lab colleagues—Feng Lin, Yuyi Li, Matthew Quan, and Lei Cheng—working with the scientists at SSRL and CFN made some important findings about the material.
Lin, a former Berkeley Lab postdoctoral researcher working with Doeff and first author on the paper, said: “Our previous studies revealed that engineering the surface of cathode particles could be the key to stabilizing battery performance. After some deep effort to understand the stability challenges of NMC cathodes, we are now getting one step closer to improving NMC cathodes by tuning surface metal distribution.”
The research results point the way to further refinements. “This research suggests a path forward to getting these materials to cycle with higher capacities—that is to design materials that are graded, with less nickel on the surface,” Doeff said. “I think our next step will be to try to make these materials with a larger compositional gradient and combine some other things to make them work together, such as titanium substitution, so we can utilize more capacity and thereby increase the energy density in a lithium ion battery.”
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