The durability of lithium-ion batteries depends highly on the performance of the cathode, the battery's positive electrode. An important type of lithium-ion cathode is the LMR (Lithium-and -Manganese-Rich) design for high-energy density. The well-known NMC (nickel-manganese-cobalt) cathode belongs to this group. These cathodes, which also incorporate lithium metal and oxygen, form layered crystalline structures that allow lithium ions to move in and out as the battery charges and discharges. Though LMR cathodes initially provide high voltage and power, this output steadily declines with repeated use, resulting in premature degradation of battery performance. Rapid voltage decline has been tied to oxygen loss from layered oxide cathodes, including the LMR type. To gain new insights into this oxygen release, researchers for the first time examined cathode materials at the nanoscale as well as micro- and macroscale under changing voltage conditions. These nanoscale x-ray experiments, carried out at the U.S. Department of Energy’s Advanced Photon Source (APS), revealed an ongoing buildup of strain at the smallest levels in the material's lattice. Ultimately, the nanoscale changes drive the observed oxygen release and cathode degradation. These experimental insights, published in the journal Nature, are already helping scientists develop strategies to mitigate LMR cathode voltage fade, which will result in longer-lasting lithium-ion batteries for electric vehicles and other power storage applications.

LMR cathodes have inherently high power densities that are ideal for advanced lithium-ion batteries. The LMR architecture employs two distinct compounds. For this investigation, lithium manganese oxide (Li₂MnO₃) and LiTMO₂ were used. “TM” indicates the transition metals cobalt, nickel, and manganese.

Li₂MnO₃ and LiTMO₂ possess very similar crystalline structures, allowing them to enmesh in alternating layers. The two compounds remain functionally distinct because their respective electrochemical activity occurs at different voltages: when one compound is lithiating or delithiating (absorbing or releasing lithium ions), the other compound is more-or-less dormant. Since Li₂MnO₃ and LiTMO₂ absorb and release lithium ions at different voltages, internal displacements and strains are expected to occur within their combined lattice. Precisely characterizing this nanoscale strain is vital to determining its relation to the observed oxygen release associated with cathode deterioration.

The LMR cathode created for this research consisted of innumerable tiny particles packed together, each composed of alternating Li₂MnO₃ and LiTMO₂ domains. Figure 1 is derived from x-ray measurements of one of those particles. The images depict varying levels of lattice displacement, and therefore strain, at different voltages. Particle nano-strain was determined using Bragg coherent x-ray diffraction imaging (BCDI) performed at the X-ray Science Division (XSD) Microscopy Group’s beamline 34-ID-C at the APS, an Office of Science user facility at Argonne National Laboratory (Fig. 1). The release of oxygen from the particle was measured via differential electrochemical mass spectrometry. By correlating strain evolution with oxygen levels, the researchers endeavored to pinpoint the exact cause of oxygen loss within the particle.

The voltages indicated in Fig. 1 form two stages. Stage 1 begins with the lowest voltage (upper left corner), corresponding to no lithium-ion movement. An increased voltage (3.75 and 4.25 volts) heralds the release of lithium ions from the LiTMO₂ domains, which expand as a result. By contrast, the Li₂MnO₃ layers hold onto their lithium ions at these voltages and resist expansion. The tension between the Li₂MnO₃ and LiTMO₂ domains dramatically increase lattice strain. At 4.46 volts the second stage begins and the Li₂MnO₃ domains begin to release their lithium ions. Strain then begins to subside, and oxygen release is detected shortly thereafter.

The detailed nanoscale mapping of strain evolution indicates that oxygen release has its origins in the opposition between the swelling LiTMO₂ layers and comparatively inactive Li₂MnO₃ layers during the first voltage stage. When this built-up stress is released in stage 2, the Li₂MnO₃ domains become unstable and begin to decompose, leading to oxygen release. This process results in permanent lattice distortions that eventually produce large-scale cathode deterioration.

The nanoscale BCDI results were buttressed by x-ray measurements probing the average of multiple particles in a macroscopic sample. For instance, powder diffraction measurements using high-energy x-ray diffraction (HEXRD) were performed at the XSD Structural Science Group’s beamline 11-ID-C and spectroscopic experiments at the absorption edges were carried out at the XSD Spectroscopy Group’s beamline 9-BM-B at the APS.

In response to these findings, the researchers created a modified LiTMO₂/Li₂MnO₃ system in which the two compounds activated at around the same voltage. This modification greatly reduced the particles' internal nano-strains, thereby limiting lattice distortion and oxygen release. Research has also shown that nano-strain within a cathode particle can be inhibited by reducing the number of sub-lattices that typically appear in such particles. These innovative techniques demonstrate that addressing nano-strain is key to improved LMR cathodes.  ― Philip Koth

See: Tongchao Liu¹, Jiajie Liu², Luxi Li¹, Lei Yu¹, Jiecheng Diao³, Tao Zhou¹, Shunning Li², Alvin Dai¹, Wenguang Zhao², Shenyang Xu², Yang Ren^1‡, Liguang Wang¹, Tianpin Wu¹, Rui Qi², Yinguo Xiao², Jiaxin Zheng², Wonsuk Cha¹, Ross Harder¹, Ian Robinson^3,4, Jianguo Wen¹, Jun Lu¹*, Feng Pan²**, and Khalil Amine^1,5,6***, “Origin of structural degradation in Li-rich layered oxide cathode,” Nature 606, 305 (9 June 2022). DOI: 10.1038/s41586-022-04689-y

Author affiliations: ¹Argonne National Laboratory ²Peking University, ³University College London, ⁴Brookhaven National Laboratory, ⁵Mohammed VI Polytechnic University (UM6P), ⁶Stanford University ^‡Present address: City University of Hong Kong

Correspondence: * [email protected], ** [email protected], *** [email protected]

The authors gratefully acknowledge support from the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office. This work was also supported by the Clean Vehicles, U.S.–China Clean Energy Research Centre (CERC-CVC2) under U.S. DOE EERE Vehicle Technologies Office. This research was also supported by the National Key R&D Program of China (2016YFB0700600), Soft Science Research Project of Guangdong Province (no. 2017B030301013) and the Shenzhen Science and Technology Research Grants (no. ZDSYS201707281026184). Work at Brookhaven National Laboratory was supported by the U.S. DOE Office of Science-Basic Energy Sciences, under contract no. DE-SC0012704. Electron microscopy was carried out at the Argonne Center for Nanoscale Materials, an Office of Science user facility supported by the U.S. Department of Energy Office of Science-Basic Energy Sciences, under contract no. DE-AC02-06CH11357. 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.

The U.S. Department of Energy's APS at Argonne National Laboratory is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS 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.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC, for the U.S. DOE 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 the Office of Science website.

The U.S. Department of Energy's APS at Argonne National Laboratory is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS 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.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC, for the U.S. DOE 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 the Office of Science website.