Unlocking the mystery behind the performance decline in a promising cathode material

(Press-News.org) The first generation of lithium-ion batteries for electric vehicles has been a remarkable success story. Yet, the question arises: What changes to battery materials will spur further advances to extend driving range and lower costs?

A better positive electrode, or cathode, for lithium-ion batteries has been the focus of intense past research. The cathode is one of the main components in batteries. Several candidates for cathode materials offer the prospect of batteries with much higher energy storage, leading to longer driving range. However, the capacity, or amount of current flowing out within a given time, tends to decline rapidly with charge-discharge cycling for reasons unknown.

“Our method should also be useful to understanding failure mechanisms in other battery types than present-day lithium-ion.” — Tongchao Liu, assistant chemist 

Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have discovered the main reason why and how one of the more promising cathode materials degrades with use. That material is a lithium nickel manganese cobalt (NMC) oxide rich in nickel and in the form of single nanosized crystals. In single crystals, all the atoms are arranged in the same highly ordered pattern.

“Nickel-rich NMC is especially appealing because it uses 70-80% nickel, a high-capacity material, and requires much less cobalt,” said Assistant Chemist Tongchao Liu. Cobalt is expensive and considered a critical mineral because of supply issues.

Typically, the nickel-rich NMC cathode consists of particles of multiple crystalline forms, or polycrystals, randomly oriented with respect to each other. With charge-discharge cycling, however, these clusters crack at the boundaries among the crystals, and the cathode capacity rapidly drops.

It had been hypothesized that fabricating the cathode with single crystals instead of polycrystals would solve the cracking problem, as the boundaries would be eliminated. However, even single-crystal cathodes failed prematurely, leaving scientists perplexed.

To uncover the mechanism, the team devised a pioneering method that combines multiscale X-ray diffraction and high-resolution electron microscopy. These materials analyses were done at the Advanced Photon Source (APS) at Argonne, the National Synchrotron Light Source at DOE’s Brookhaven National Laboratory and Argonne’s Center for Nanoscale Materials (CNM). All three are DOE Office of Science user facilities.

“The problem with electron microscopy alone is that it only provides a snapshot of a small area on a single crystal,” said Materials Scientist Tao Zhou in CNM. ​“And while X-ray diffraction offers insights into internal structures of many particles, it lacks surface-level information. Our method bridges this gap, offering a comprehensive understanding at the scale of one, 10 to 50, and 1,000 particles.”

The atoms in single crystals are arranged in neatly ordered rows and columns called lattices. The team’s multifaceted analyses of single-crystal cathodes provided crucial information about changes in the lattice on charge and discharge.

As Liu and Zhou explained, introduction of a charge triggers a strain on the lattice that causes it to expand and rotate, disrupting the neatly ordered pattern of atoms. Upon discharge, the lattice contracts to its original state, but the rotation remains. With repeated charge-discharge cycles, the rotation becomes more pronounced. This change in the cathode structure causes a steep performance drop.

Critical to gaining these insights were measurements with the Hard X-ray Nanoprobe operated jointly by CNM and APS.

“The team’s new method was instrumental in understanding the burning issue of why nickel-rich NMC cathodes with single crystals fail so rapidly,” said Khalil Amine, an Argonne Distinguished Fellow. ​“This newfound understanding will give us ammunition to fix this issue and enable lower-cost electric vehicles with longer driving range.”

“Our method should also be useful to understanding failure mechanisms in other battery types than present-day lithium-ion,” added Liu.

This research appeared in Science. In addition to Liu, Zhou and Amine, authors include Weiyuan Huang, Lei Yu, Jing Wang, Junxiang Liu, Tianyi Li, Rachid Amine, Xianghui Xiao, Mingyuan Ge, Lu Ma, Steven N. Ehrlich, Martin V. Holt and Jianguo Wen.

This work was supported by support by the DOE Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.

About Argonne’s Center for Nanoscale Materials
The Center for Nanoscale Materials is one of the five DOE Nanoscale Science Research Centers, premier national user facilities for interdisciplinary research at the nanoscale 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 nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge, Sandia and Los Alamos National Laboratories. For more information about the DOE NSRCs, please visit https://​sci​ence​.osti​.gov/​U​s​e​r​-​F​a​c​i​l​i​t​i​e​s​/​U​s​e​r​-​F​a​c​i​l​i​t​i​e​s​-​a​t​-​a​-​G​lance.

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 world’s most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.

This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://​ener​gy​.gov/​s​c​ience.

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