Researchers Develop New Method for Making High-Energy Lithium-Ion Batteries

Scientists from Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a new method that could help develop lithium nickel manganese cobalt oxide, also known as NMC. NMC is one of the most promising chemistries for developing improved lithium-ion batteries, particularly when it comes to electric vehicle applications. Though it has been a challenge in the past for scientist to gain a higher capacity out of them.

Marca Doeff, Berkeley Lab battery scientist, headed the study that also involved researchers from SLAC National Accelerator Laboratory and Brookhaven National Laboratory. The team observed that the spray pyrolysis technique helps in resolving the issue of surface reactivity, which leads to material degradation, and is one of the biggest problems associated with NMC cathodes. The study titled, “Metal segregation in hierarchically structured cathode materials for high-energy lithium batteries” was published in the premier issue of the journal, Nature Energy.

We made some regular material using this technique, and lo and behold, it performed better than expected. We were at a loss to explain this, and none of our conventional material characterization techniques told us what was going on, so we went to SLAC and Brookhaven to use more advanced imaging techniques and found that there was less nickel on the particle surfaces, which is what led to the improvement. High nickel content is associated with greater surface reactivity.

Marca Doeff, Berkeley Lab Battery Scientist

For the study, Department of Energy (DOE) Office of Science User Facilities – the Center for Functional Nanomaterials (CFN) at Brookhaven and the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC – were used. The results appear to hold potential significance, and could help in designing lithium-ion batteries that not only have improved energy density, but also are affordable.

We still want to increase the nickel content even further, and this gives us a possible avenue for doing that. The nickel is the main electro-active component, plus it’s less expensive than cobalt. The more nickel you have, the more practical capacity you may have at voltages that are practical to use. We want more nickel, but at the same time, there’s the problem with surface reactivity.

Marca Doeff, Berkeley Lab Battery Scientist

In any battery the cathode serves as a positive electrode, and this is why it is essential to develop an improved cathode material to acquire a steady high-voltage cell. Such high-voltage cells have been studied extensively over the years. As a commercially available technique spray pyrolys is primarily used for creating powders and thin films, but this approach has not been extensively used for making materials that can be applied to battery production.

Surface reactivity poses a major challenge to high-voltage cycling, which is needed to reach higher capacities for high-energy devices. This phenomenon has been thoroughly investigated and different strategies have been tried and tested to improve the existing problem, these strategies include subsitituting titanium with colbat, which slightly helped in counteracting the surface reactivity.

X-ray transmission microscopy and spectroscopy was used by SSRL scientists Yijin Liu and Dennis Nordlund to study the material in the tens of nanometers to 10-30 micron range. CFN researcher Huolin Xin also employed 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 both of these scales, Doeff and her Berkeley Lab colleagues—Feng Lin, Yuyi Li, Matthew Quan, and Lei Cheng—working with the scientists at SSRL and CFN were able to make some important findings about the material.

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.

Yijin Lin, Former Berkeley Lab Postdoctoral Researcher, First Author on the Paper

“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.”

The new study could pave the way for more improvements, since spray pyrolysis presents a low-cost option for making materials for battery production.

The reason we like it is that it offers a lot of control over the morphology. You get beautiful spherical morphology which is very good for battery materials. We’re not the first ones who have come up with idea of decreasing nickel on the surface. But we were able to do it in one step using a very simple procedure.

Marca Doeff, Berkeley Lab Battery Scientist

DOE’s Vehicle Technologies Office funded the study.