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Layered oxides are one of the most widely used and most promising commercial cathode materials in lithium-ion batteries.

In-depth understanding of its failure mechanism is crucial for the development of next-generation high-performance lithium-ion battery positive electrode materials.

However, to date, the relevant field still lacks an in-depth understanding at the atomic scale of the harmful phase transitions and mechanical failure mechanisms of such materials and their impact on battery performance.

Researcher Chunyang Wang from the Institute of Metal Research, Chinese Academy of Sciences (who will conduct postdoctoral research at the University of California and Brookhaven National Laboratory from 2019 to 2023) is committed to solving this major challenge in the global battery field.

He and his collaborators developed super-resolution transmission electron microscopy imaging technology by integrating deep learning with atomic resolution scanning transmission electron microscopy imaging. They used this technology to deeply reveal the complex phase interface structure, phase change failure mechanism and mechanical instability mechanism in layered oxide positive electrode materials for lithium-ion batteries.

Due to his important contributions to the development and application of artificial intelligence transmission electron microscopy technology, as well as the research on failure mechanisms of layered oxides and the development of new materials, he became one of the Chinese candidates for the 2023 MIT Technology Review's "35 Innovators Under 35".

Revealing the failure mechanism of lithium-ion layered oxide cathodes to guide the development of next-generation battery cathode materials

Lithium-ion batteries are one of the most commonly used energy storage solutions in electric vehicles today, and layered oxide cathode materials play a key role in lithium-ion batteries.

Currently, this type of material faces huge challenges during the battery charge and discharge cycle process, namely that the layered oxides will inevitably undergo a series of complex phase transition degradation and stress failure problems.

Especially for the existing high-nickel layered oxide positive electrode, the higher the initial range of the electric vehicle, the faster its performance decays.

In other words, there is an inverted relationship between the energy density and cycle stability of the lithium battery layered oxide positive electrode, and you cannot have your cake and eat it too.

"How to make electric vehicles have a long initial range while maintaining 80% or even higher capacity after the battery is charged and discharged thousands of times is one of the problems that scientists in the battery field most want to solve. To overcome this challenge, the first step we need to do is to figure out how existing materials fail, or how they break down," he said.

To this end, he and his collaborators conducted a systematic and in-depth study of the phase change degradation and mechanical failure mechanism of layered oxides at the nano-atomic scale based on super-resolution transmission electron microscopy imaging technology.

They revealed at the atomic scale the O3→O1 phase transition caused by lithium removal and lattice instability in layered oxides, and found that the O3→O1 phase transition is not completely reversible during lithium insertion, and the formation of misfit dislocations at the phase interface provides preferential nucleation sites for the initiation of rock salt phases and cracks [1,2].

Furthermore, they extended their research to commercial oxide cathode materials, observed the O1 phase transition induced by lattice shear instability, and successfully analyzed the complex atomic configuration of the O1-O3 two-phase interface[3].

"This result is the first to reveal the phase interface structure generated by lithium desorption and lattice shearing in layered oxides at the atomic scale," he said.

Focusing on the O1 phase transition, they also combined in-situ electron microscopy and electron tomography three-dimensional reconstruction technology to discover a new phase transition mechanism from O1 phase to rock salt phase, and were the first to analyze the three-dimensional configuration of cracks in layered oxides and its intrinsic relationship with phase transition [4].

In addition, they also discovered the stress-induced phase transformation mechanism in layered oxides, which overturned the traditional understanding that multiscale cracking is the only mode of mechanical instability in layered oxides, thus establishing a bridge between the mechanical deformation and phase transformation of layered oxides[5].

This series of studies comprehensively revealed the O3→O1 phase transition mechanism, interface structure and its impact on the structural performance degradation of materials in layered oxides, providing important theoretical support for the optimal design of the next generation of positive electrode materials.

For example, based on the above-mentioned breakthroughs in basic research, Wang Chunyang and his collaborators designed a multi-component doped zero-strain cobalt-free high-nickel layered oxide cathode material with better performance than the commercial lithium battery cathode NMC-811[6], and a medium-low nickel-content cobalt-free layered oxide with better performance than the commercial NMC-532[7].

"NMC-811 is the mainstream commercial cathode material widely used in electric vehicle power batteries. The initial capacity of the new high-nickel cathode material we developed is comparable to that of NMC-811, but after 1,000 cycles, its capacity retention rate can still reach over 85%, which is much higher than the latter. In other words, we have successfully broken the inverted relationship between the capacity and cycle stability of existing high-nickel cathode materials," he said.

Thanks to the new understanding of the failure mechanism of layered oxides, the research and development cycle of new layered oxide positive electrode materials has been greatly shortened.

"Our research has confirmed that the O1 phase is not as insignificant as traditional research has believed. We found that the O1 phase can both aggravate structural degradation and mechanical instability, so it is an absolutely harmful phase. With this new understanding, we now only need to put the positive electrode material into the battery and run it for one or two cycles. The amount of O1 phase generated can roughly infer the stability of the material, thereby greatly shortening the evaluation cycle of material performance." He said.

He continued: "More importantly, considering that the essence of the O1 phase transition is lattice shear, we started from the characteristics of layered oxides and designed a material modification strategy that can inhibit lattice shear and reduce material strain - multi-component doping technology. This technology allows us to significantly improve the cycle life of high-nickel layered oxide positive electrodes without losing initial capacity."

Advanced electron microscopy characterization technology plays an important role in solving core scientific problems in the energy field and developing new materials.

He was able to achieve the above-mentioned series of results thanks to his expertise in electron microscopy, especially the development and application of super-resolution transmission electron microscopy imaging technology.

"This technology is a cross-integration of artificial intelligence and advanced transmission electron microscopy characterization technology, opening a new door for basic research on layered oxide positive electrode materials," he said.

The peculiarity of layered oxides is that once lithium ions are pulled out of the crystal lattice, the material undergoes non-uniform volume changes and local phase transitions. The resulting lattice distortion causes the collected atomic resolution images to become blurred and uninterpretable, which poses a fatal challenge to electron microscopists in "seeing clearly" and revealing the structure of the material.

To this end, Wang Chunyang and his collaborators fully utilized the advantages of convolutional neural networks in image segmentation, combined them with atomic resolution transmission electron microscopy imaging technology, and developed artificial intelligence-assisted super-resolution imaging technology, achieving high-precision imaging and analysis of the crystal structure and defects of layered oxide positive electrode materials.

"The performance of this technology is currently very good, even beyond our initial expectations. In the next step, we hope to use artificial intelligence technology to achieve intelligent atomic-scale analysis of material structure, which is one of our future directions of effort," he said.

In addition, he and his collaborators have also made important progress in the atomic-scale failure mechanism of all-solid-state lithium battery positive electrode materials.

They found that surface “lattice fragmentation” and shear phase transition jointly led to the degradation of the structural properties of layered oxides[8]. This mechanism is significantly different from that in traditional liquid batteries and is expected to provide theoretical guidance for the optimal design of the cathode-electrolyte interface of all-solid-state batteries.

Choosing a good scientific question is far more important than blindly pursuing the "high-end" equipment

In 2010, he was admitted to the Materials Science and Engineering major of China University of Mining and Technology from Xiantao Middle School in Hubei Province.

In 2014, he was recommended to study for a doctorate at the Institute of Metal Research, Chinese Academy of Sciences (supervisor: Researcher Du Kui). During this period, he mainly engaged in in-situ quantitative electron microscopy research of metal materials and the development and application of transmission electron microscope three-dimensional imaging technology.

After receiving his Ph.D. in 2019, he entered the University of California, Irvine and Brookhaven National Laboratory for postdoctoral research (co-supervisor: Professor Xin Huolin). During this period, he mainly engaged in the development and application of advanced transmission electron microscopy technology and the study of the failure mechanism and structure-activity relationship of lithium-ion battery materials.

When talking about the biggest challenge he encountered in the scientific research process, he said that it did not come from the technical level, but how to find good scientific problems.

Take the field of battery materials where he works for example. The research on layered oxide cathode materials has been going on for more than 40 years. A common view in the field is that the framework of the phase change theory and failure mechanism of layered oxide cathodes has been "completed".

"Maybe it's because I had a blank slate when I entered this field, so I'm not bound by many rules and regulations. Even if a question is very stupid in the eyes of many scientists, I often have a strong desire for knowledge," said Wang Chunyang.

"The times when I feel most accomplished are often when I'm doing transmission electron microscopy experiments late at night. In the silence, my brain cells and visual cells interact with each other at a high frequency. For a moment, I feel that I have grasped the truth of this world and feel extremely happy," he continued.

A strong desire for knowledge, coupled with sharp intuition and critical thinking, may be the core driving force behind his discovery of a series of new failure mechanisms in layered oxides.

Of course, his breakthrough is also closely related to the scientific research training he received.

During his doctoral studies at the Institute of Metal Research, he studied metal materials, which laid a solid foundation for his in-depth understanding of material structure and defects and the establishment of a knowledge system. This cross-disciplinary background and asymmetric advantages are also important driving forces for his innovative breakthroughs in the field of battery materials.

An interesting phenomenon is that as a researcher in electron microscopy with a "ten-year working experience", Wang Chunyang's breakthroughs in the field of material research rely heavily on the "super magnifying glass" - the transmission electron microscope. Despite this, he has repeatedly emphasized that scientific research cannot be "equipment-only".

He believes that it is "people" rather than equipment that ultimately decide what scientific problems to study, how to design experiments, analyze data, and write papers. Equipment or experimental techniques are "cats" and scientific problems are "mice." It doesn't matter if the cat is black or white, as long as it catches the mouse, it is a good cat.

"Three-quarters of my research work during my postdoctoral period was done on aspheric aberration-corrected electron microscopes, and more than half of the work was off-site research. These devices or technologies do not have any advantages in the eyes of many people, but they have not prevented us from solving important scientific problems that everyone in the field is concerned about," he said.

From this perspective, choosing a good scientific problem is far more important than endlessly pursuing "high-end" equipment.

It is understood that in January 2024, he returned to the Institute of Metal Research, Chinese Academy of Sciences, and served as a researcher and doctoral supervisor at the Shenyang National Research Center for Materials Science.

In the past six months, he has formed a young scientific research team with an average age of only 30 years old and started a new scientific research journey.

In the future, his main research direction will be transmission electron microscopy and material structure-activity relationship research. He will be committed to developing the next generation of high-performance lithium-ion battery positive electrode materials based on breakthroughs in basic research.

"Ten years ago, from the moment I stepped into the Institute of Metal Research, the transmission electron microscope opened the door to understanding materials for me. Like my predecessors, I gradually learned to use electron microscopy to understand the microstructure of materials and explore the intrinsic connection between the structure and performance of materials. Understanding the material world and conducting materials science research is not only my current career, but also the career I will do for the rest of my life," said Wang Chunyang.

References:

1. C.Y. Wang, R. Zhang, K. Kisslinger, H.L. Xin. Atomic-Scale Observation of O1 Faulted Phase-Induced Deactivation of LiNiO2 at High Voltage. Nano Letters, 21(8), 3657-3663 (2021). https://doi.org/10.1021/acs.nanolett.1c00862

2. C.Y. Wang, R. Zhang, C. Siu, M.Y. Ge, K. Kisslinger, Y. Shin, H.L. Xin. Chemomechanically Stable Ultrahigh-Ni Single-Crystalline Cathodes with Improved Oxygen Retention and Delayed Phase Degradations. Nano Letters, 21(22), 9797-9804 (2021). https://doi.org/10.1021/acs.nanolett.1c03852

3. C.Y. Wang, X.L. Wang, R. Zhang, T. Lei, K. Kisslinger, H.L. Xin. Resolving Complex Intralayer Transition Motifs in High-Ni-Content Layered Cathode Materials for Lithium-Ion Batteries. Nature Materials, 22, 235-241 (2023). https://doi.org/10.1038/s41563-022-01461-5

4. C.Y. Wang, L.L. Han, R. Zhang, et al. Resolving Atomic-Scale Phase Transformation and Oxygen Loss Mechanism in Ultrahigh-Nickel Layered

Cathodes for Cobalt-Free Lithium-Ion Batteries. Matter, 4(6), 2013-2026 (2021). https://doi.org/10.1016/j.matt.2021.03.012

5. C.Y. Wang, X.L. Wang, P.C. Zou, R. Zhang, S.F. Wang, B.H. Song, K.B. Low, H.L. Xin. Direct Observation of Chemomechanical Stress-Induced Phase Transformation in High-Ni Layered Cathodes for Lithium-Ion Batteries. Matter, 6(4), 1265-1277 (2023). https://doi.org/10.1016/j.matt.2023.02.001

6. R. Zhang#, C.Y. Wang#, et al. Compositionally complex doping for zero-strain zero-cobalt layered cathodes. Nature, 610, 67–73 (2022). https://doi.org/10.1038/s41586-022-05115-z

7. R. Zhang#, C.Y. Wang#, et al. Long-life lithium-ion batteries realized by low-Ni, Co-free cathode chemistry. Nature Energy, 8, 695–702 (2023). https://doi.org/10.1038/s41560-023-01267-y

8. C.Y. Wang, Y.Q. Jing, D. Zhu, H.L. Xin. Atomic Origin of Chemomechanical Failure of Layered Cathodes in All-Solid-State Batteries. Journal of the American Chemical Society, 146 (26), 17712–17718 (2024). https://doi.org/10.1021/jacs.4c02198

Operation/Layout: He Chenlong

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