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

Thoroughly revealing their failure mechanisms is crucial for the development of next-generation high-performance lithium-ion battery cathode materials.

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

Dr. Wang Chunyang from the Institute of Metal Research of the Chinese Academy of Sciences (who conducted postdoctoral research at the University of California and Brookhaven National Laboratory from 2019 to 2023) is committed to addressing this significant challenge in the global battery field.

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He and his collaborators have developed a super-resolution transmission electron microscopy (TEM) imaging technique by integrating deep learning with atomic-resolution scanning TEM imaging. Using this technique, they have deeply revealed the complex phase interface structures, phase transition failure mechanisms, and mechanical instability mechanisms in layered oxide cathode materials for lithium-ion batteries.For his significant contributions to the development and application of artificial intelligence in transmission electron microscopy technology, as well as research on the failure mechanisms of layered oxides and the development of new materials, he has been selected as one of the Chinese honorees for the "35 Innovators Under 35" by MIT Technology Review in 2023.

Revealing the failure mechanisms 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. Layered oxide cathode materials play a key role in lithium-ion batteries.

Currently, these materials face significant challenges during the battery charge and discharge cycles, namely, the inevitable occurrence of a series of complex phase transformation degradation and stress-related failure issues.Especially for the existing high-nickel layered oxide cathodes, the higher the initial driving range of electric vehicles, the faster the performance degradation rate.

That is to say, there is an inverse relationship between the energy density and cycle stability of lithium-ion layered oxide cathodes, and the two are "like fish and bear's paws, which cannot be obtained at the same time."

"How to make electric vehicles have a high initial driving range and still maintain 80% or even higher capacity after thousands of charge and discharge cycles of the battery is one of the problems that scientists in the current battery field want to solve most. To overcome this challenge, the first step we need to do is to understand how the existing materials fail or how they break down," he said.

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

They revealed the O3→O1 phase transformation caused by lithium extraction and lattice instability in layered oxides at the atomic scale, and found that the O3→O1 phase transformation during the lithium insertion process is not completely reversible, and the generation of mismatch dislocations at the phase interface provides preferential nucleation sites for the formation of rock salt phase and cracks [1,2].Further, they expanded their research to commercial oxide cathode materials, observing the O1 phase transition induced by lattice shear instability, and successfully resolved the complex atomic configuration of the O1-O3 two-phase interface [3].

"The achievement is the first to reveal the phase interface structure generated by delithiation-lattice shear at the atomic scale in layered oxides," he said.

Around the O1 phase transition, they also combined in-situ electron microscopy and electron tomography three-dimensional reconstruction technology, discovering a new phase transition mechanism of O1 phase → rock salt phase, and pioneered the three-dimensional configuration of cracks in layered oxides and their intrinsic connection with phase transitions [4].

In addition, they also discovered the stress-induced phase transition mechanism in layered oxides, subverting the traditional understanding that multi-scale cracking is the only mode of mechanical instability in layered oxides, thus building a bridge between the mechanical deformation and phase transition of layered oxides [5].

This series of research fully reveals 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 optimized design of the next generation of cathode materials.For example, based on breakthroughs in the aforementioned basic research, Wang Chunyang and his collaborators have respectively designed multi-element doped zero-strain cobalt-free high-nickel layered oxide cathode materials with better performance than the commercial lithium battery cathode NMC-811 [6], as well as cobalt-free layered oxides with medium to low nickel content that outperform the commercial NMC-532 [7].

"NMC-811 is currently the mainstream commercial cathode material widely used in electric vehicle power batteries. The initial capacity of the new high-nickel cathode material we have developed is comparable to that of NMC-811, but after 1000 cycles, its capacity retention rate can still reach more than 85%, far higher than the latter. In other words, we have successfully broken the inverse relationship between capacity and cycle stability in existing high-nickel cathode materials," he said.

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

"Our research has confirmed that the O1 phase is not as insignificant as traditional research has thought. We have found that the O1 phase can both exacerbate structural degradation and mechanical instability, so it is a truly harmful phase. With this new understanding, we now only need to put the cathode material into the battery and run it for one or two cycles. From the amount of O1 phase generated, we can roughly infer the stability of the material, thereby greatly shortening the material performance assessment cycle," he said.

He continued: "What's more important is that, considering the essence of the O1 phase transition is lattice shearing, we have designed a material modification strategy that can suppress lattice shearing and reduce material strain - multi-element doping technology - based on the characteristics of layered oxides, 'adapting to local conditions'. This technology allows us to significantly improve the cycle life of high-nickel layered oxide cathodes without losing initial capacity."Advanced electron microscopy characterization techniques play a vital role in addressing core scientific issues in the energy field and in the development of new materials.

His ability to achieve the aforementioned series of results is attributed to his expertise in electron microscopy, especially in the development and application of super-resolution transmission electron microscopy imaging technology.

"The technology represents the cross-integration of artificial intelligence and advanced transmission electron microscopy characterization techniques, opening a new door for the fundamental research of layered oxide cathode materials," he said.

The uniqueness of layered oxides lies in the fact that once lithium ions are extracted from the lattice, the material undergoes non-uniform volume changes and local phase transitions. The resulting lattice distortion leads to the collected atomic resolution images becoming blurred and indecipherable, posing a fatal challenge for electron microscopists to "see" and reveal the structure of the material.

To this end, Wang Chunyang and his collaborators fully utilized the advantages of convolutional neural networks in image segmentation, combining them with atomic resolution transmission electron microscopy imaging technology, and developed an artificial intelligence-assisted super-resolution imaging technology, achieving high-precision imaging and analysis of the crystal structure and defects of layered oxide cathode materials."Currently, this technology is performing exceptionally well, even exceeding our initial expectations. Next, we hope to utilize artificial intelligence technology to achieve intelligent analysis of material structures at the atomic scale, which is one of the directions of our future efforts," he said.

In addition to this, he and his collaborators have also made significant progress in understanding the atomic-scale failure mechanisms of all-solid-state lithium battery cathode materials.

They found that surface "lattice fragmentation" and shear phase transitions jointly lead to the structural performance degradation of layered oxides [8], a mechanism that is significantly different from that in traditional liquid batteries, and is expected to provide theoretical guidance for the optimization design of the cathode-electrolyte interface in 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 School of Materials Science and Engineering at China University of Mining and Technology from Xiantao Middle School in Hubei Province.In 2014, he was recommended to pursue a Ph.D. 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 on metallic materials and the development and application of three-dimensional imaging technology for transmission electron microscopy (TEM).

After obtaining his Ph.D. in 2019, he joined the University of California, Irvine, and the Brookhaven National Laboratory for postdoctoral research (Co-advisor: Professor Xin Huolin). In this stage, he mainly focused on the development and application of advanced TEM techniques, as well as the failure mechanism and structure-performance relationship of lithium-ion battery materials.

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

Take the field of battery materials he is in as an example, the research on layered oxide cathode materials has exceeded forty years. A more common view in the field is that the framework of phase transformation theory and failure mechanism of layered oxide cathodes has been "completed."

"Perhaps because I happened to be a blank slate when I entered this field, I was not bound by many rules and regulations. Even a question that many scientists think is very foolish, I often have a strong desire to know," said Wang Chunyang."My moments of greatest accomplishment often come in the middle of the night when conducting transmission electron microscopy experiments. In the profound silence, my brain cells and visual cells interact at a high frequency. At that moment, I feel as if I have grasped the truth of the world and feel incredibly happy," he continued.

His intense desire for knowledge, coupled with keen 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 breakthroughs are also inseparable from the scientific research training he has received.

During his doctoral studies at the Institute of Metal, he focused on metal materials, which laid a solid foundation for his in-depth understanding of material structure and defects, as well as the establishment of his knowledge system. This interdisciplinary background and asymmetric advantage are also important driving forces for his innovative breakthroughs in the field of battery materials.

An interesting phenomenon is that, as an electron microscopy researcher with a "ten-year work history," Wang Chunyang's breakthroughs in the field of materials research largely depend on the "super magnifying glass" - the transmission electron microscope. Despite this, he has repeatedly emphasized that scientific research cannot be "equipment-oriented."He believes that ultimately, it is the "person" who decides what scientific questions to study, how to design experiments, analyze data, and write papers, not the equipment. Equipment or experimental techniques are the "cat," while the scientific question is the "mouse." Whether it is a black cat or a white cat, the one that catches the mouse is a good cat.

"During my postdoctoral period, three-quarters of my research work was completed on a spherical aberration-corrected electron microscope, and more than half of the work was off-site research. These equipment or techniques may not seem to have any advantages to many people, but they did not prevent us from solving important scientific questions that everyone in the field is concerned about," he said.

From this perspective, choosing a good scientific question is far more important than endlessly pursuing the "high-end, high-quality, and high-tech" of equipment.

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

In the past six months, he has built a young research team with an average age of only 30 years old and has embarked on a new scientific research journey.In the future, his main focus will be on transmission electron microscopy and the study of the relationship between material structure and properties. He will be committed to leading breakthroughs in basic research to develop the next generation of high-performance cathode materials for lithium-ion batteries.

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