The boundaryless structure for the electrode particles eliminates reactions that reduce battery life.
Researchers at the US Department of Energy’s (DOE) Argonne National Laboratory have a long history of breakthrough discoveries with lithium-ion batteries. Many of these discoveries focused on a battery cathode known as NMC, a nickel-manganese-cobalt oxide. Batteries with this cathode now power the Chevy Bolt.
Argonne researchers have made another breakthrough with the NMC cathode. The team’s new structure for cathode microparticles could lead to longer-lasting and safer batteries capable of operating at very high voltages and powering vehicles for longer ranges.
“We now have guidelines that battery manufacturers can use to prepare a boundary-free cathode material that operates at high voltages.” — Khalil Amine, Emeritus Member of Argonne
“The current NMC cathode has been a major obstacle to high-voltage operation,” said assistant chemist Guiliang Xu. With charge-discharge cycles, the performance decreases rapidly due to the formation of cracks in the cathode particles. For several decades, battery researchers have been looking for ways to eliminate these cracks.
A past approach involved spherical microscale particles made up of many much smaller particles. Large spherical particles are polycrystalline, with differently oriented crystal regions. As a result, they have what scientists call grain boundaries between the particles, which cause cracks as the battery cycles. To prevent this, Xu and Argonne’s colleagues had previously developed a protective polymer coating around each particle. This coating surrounds the large spherical particles and the smaller ones inside.
A different approach to avoiding this cracking involves single crystal particles. Electron microscopy of these particles indicated that they have no boundaries.
The problem the team faced was that cathodes made of both coated polycrystals and single crystals still formed cracks with cycling. So, they subjected these cathode materials to extensive analysis at the Advanced Photon Source (APS) and Center for Nanoscale Materials (CNM), the user facilities of the DOE Office of Science in Argonne.
Different X-ray analyzes were performed on five APS beamlines (11-BM, 20-BM, 2-ID-D, 11-ID-C and 34-ID-E). It turned out that what scientists thought were single crystals, as evidenced by electron and x-ray microscopy, actually had limits inside. Scanning and transmission electron microscopy at the CNM confirmed the finding.
“When we look at the surface morphology of these particles, they look like single crystals,” said physicist Wenjun Liu. “But when we use a technique called synchrotron X-ray diffraction microscopy and other techniques at APS, we find hidden limits within.”
Importantly, the team developed a method to produce boundary-less single crystals. Small cell tests with such very high voltage single crystal cathodes have shown a 25% increase in energy storage per unit volume, with almost no loss in performance over 100 test cycles. In contrast, over the same life cycle, the capacitance decreased by 60% to 88% in NMC cathodes composed of single crystals with many internal joints or coated polycrystals.
Atomic-scale calculations have revealed the mechanism behind the cathode’s capacitance drop. According to CNM nanoscientist Maria Chan, compared to regions further away from it, the borders are more vulnerable to the loss of oxygen atoms when the battery is charging. This loss of oxygen leads to degradation with the cell cycle.
“Our calculations showed how the limits lead to the release of high-voltage oxygen and, therefore, lower performance,” Chan said.
The elimination of the limits prevents the release of oxygen and thus improves the safety and stability of the cathode with cycling. Oxygen release measurements at the APS and DOE’s Lawrence Berkeley National Laboratory Advanced Light Source supported this finding.
“We now have guidelines that battery manufacturers can use to prepare boundary-free cathode material that operates at high voltage,” said Khalil Amine, an Argonne Distinguished Fellow. “And the guidelines should apply to other cathode materials besides NMC.”
An article on this research appeared in natural energy. In addition to Xu, Amine, Liu, and Chan, Argonne authors include Xiang Liu, Venkata Surya Chaitanya Kolluru, Chen Zhao, Xinwei Zhou, Yuzi Liu, Liang Yin, Amine Daali, Yang Ren, Wenqian Xu, Junjing Deng, Inhui Hwang, Chengjun Sun, Tao Zhou, Ming Du and Zonghai Chen. Scientists from Lawrence Berkeley National Laboratory (Wanli Yang, Qingtian Li and Zengqing Zhuo), Xiamen University (Jing-Jing Fan, Ling Huang and Shi-Gang Sun) and Tsinghua University (Dongsheng Ren, Xuning Feng and Minggao) also contributed to this project. Ouyang).
The research was supported by the DOE Vehicle Technologies Office.
About the Argonne Center for Nanoscale Materials
The Center for Nanoscale Materials is one of five DOE Nanoscale Science Research Centers, the nation’s premier user facilities for interdisciplinary nanoscale research supported by the DOE Office of Science. Together, the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model materials at the nanoscale, and constitute the largest infrastructure investment of the National Initiative on nanotechnology. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia, and Los Alamos National Laboratories. For more information on DOE’s NSRCs, please visit https://science.osti.gov/User-Facilities/U ser-Facilitis-at-a-Glance.
About the Advanced Photon Source
The U.S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the most productive x-ray light source facilities in the world. APS provides high-luminosity X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, life and environmental sciences, and applied research. These X-rays are perfectly suited to the exploration of materials and biological structures; elementary distribution; chemical, magnetic, electronic states; and a wide range of technologically significant engineering systems from Battery to fuel injectors, all of which are the foundations of our nation’s economic, technological and physical well-being. Each year, more than 5,000 researchers use APS to produce more than 2,000 publications detailing impactful discoveries and solving more vital biological protein structures than users of any other X-ray light source research facility. Scientists and APS engineers are innovating in technology that is central to advancing accelerator and light source operations. This includes insertion devices that produce the extremely bright X-rays that are prized by researchers, lenses that focus X-rays down to a few nanometers, instrumentation that maximizes how X-rays interact with samples studied and the software that gathers and manages the massive amount of data resulting from discovery research at APS.
This research utilized resources from the Advanced Photon Source, a United States DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02- 06CH11357.
Article and featured image by Argonne National Laboratory.
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