Paramagnetic Resonance: A Rising Star in Battery Research Methods

An interview with Hu Bingwen, researcher at East China Normal University

In recent years, magnetic resonance technology has been widely used in the study of electrochemical energy storage systems including lithium/sodium ion batteries, fuel cells, and supercapacitors. And lithium-ion batteries in the power supply of new energy vehicles and sodium-ion batteries which are expected to be used as large-scale energy storage batteries are the popular research focuses. However, all kinds of battery materials are faced with unavoidable “ceilings”, such as limited lithium reserves, high costs, and technical indicator questions like how to achieve faster charging speed and higher energy density. To solve these problems requires the unremitting efforts of scientific researchers and the support of more precise instruments.

A few days ago, an editor of specially interviewed Hu Bingwen, a researcher at East China Normal University, and invited Mr. Hu to introduce how magnetic resonance technology boosts the scientific research in the battery field. Mr. Hu, first of all, please introduce your current research direction and the reasons for choosing this direction.

Hu Bingwen: At present, the main research direction of our research group is the application of nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) in the battery field, including lithium-ion battery and sodium-ion battery.

While studying for my Ph.D. degree in France, I majored in the development of NMR methodology. After returning to China, I did not change my research direction at the beginning and developed many methods, such as SHANGHAI and SHA+. But I gradually realized a problem in my work: it is difficult for postgraduates to master NMR methodology and make some scientific research achievements in a short time. After investigation, I found that the research on "battery-NMR" is relatively scarce in China, so I began to study and do some battery research, from electrolytes to negative electrodes and then to positive electrodes.

My mentor, JP, likes chatting and has a wide field of vision. He said to me many times that the research directions of five national NMR research centres in France all have their unique priorities. However, in China, which has a larger territory, there are relatively fewer national NMR research centres, and most of them concentrate on the fields of catalysis and biology. At the age of 40, JP made a dramatic career change. He has also been encouraging me to make a dramatic career change to pursue a unique and focused new direction.

After turning to the battery field for some time, I found that NMR technology is far from enough for the research in this field, which also needs support from the results of paramagnetic resonance technology. Therefore, our research group applied for a demo instrument from Bruker in 2016 and later purchased a Bruker E580 Continuous Wave/Pulse Electron Paramagnetic Resonance Spectrometer for exploring the advantages of paramagnetic resonance. By constantly studying and exploring the application of paramagnetic resonance technology in the battery system research, we finally formed a mixed research direction of NMR, paramagnetic resonance, and battery. When was the magnetic resonance technology applied to the battery field? Compared with other analytical instruments, what are the characteristics of magnetic resonance technology?

Hu Bingwen: At present, there are not many research groups applying magnetic resonance technology to the battery field in China. As far as I know, there are about three to five such research groups. Internationally, however, NMR entered the field of battery research around the year 2000. After I returned to China in 2010, I started carrying out related research. The application of paramagnetic resonance technology in the battery field has been neglected for a long time and had sporadic results. Its actual start is around 2015, which is relatively late compared with the application of NMR technology. Our research group entered this field in 2016, basically keeping pace with the international research groups.
Compared with other analytical instruments, NMR and paramagnetic resonance have many unique features in battery research.

NMR is a macroscopic scientific tool, which can obtain more comprehensive element information, while other tools like TEM can only get the local element information, lacking the understanding of the overall situation. NMR mainly studies Li, Na, and O, and its ability to distinguish these elements is also stronger than other analytical technologies. For example, when determining the position of Li ion in NaLiMnO2 battery, NMR can most directly detect whether Li ion is in the sodium layer or other layers.

Paramagnetic resonance is very good at distinguishing the valence states of elements. Synchrotron radiation is the most widely used technology in the testing of V systems. It can obviously observe the changes of V4+ and V5+, but can hardly distinguish minor changes of V3+. However, in the paramagnetic resonance spectrum, you can clearly identify V3+, which is exactly the “strength” of paramagnetic resonance. What do you think of the application prospect of magnetic resonance technology in the field of battery energy?

Hu Bingwen: We should say that magnetic resonance technology has a very bright application prospect in the battery field. NMR and paramagnetic resonance technologies can provide complementary information, which can present complete information about the battery material.

In fact, the battery has a unique behaviour called "partial amorphization". There are some disordered places in the cationic battery materials, and this disordered structure can not be well explained in most other technologies. However, NMR and paramagnetic resonance can well explain this phenomenon. In addition, the internal and external structures of batteries are very different. So, no matter from the point of view of phase transition or disordering, magnetic resonance technology is irreplaceable. It is worth pointing out that the use of MRI does not exclude the use of synchrotron radiation, TEM, and other technologies, because we can gain some complementary information by using different technologies.
Continuous exploration of EPR for more potential. How many magnetic resonance instruments are there in your laboratory at present? When did you purchase them, and which instrument is mostly used now for your scientific research?

Hu Bingwen: Nuclear magnetic instruments in our laboratory are all solid-state nuclear magnetic spectrometers, including one 300MHz spectrometer, one 400MHz spectrometer, and two 600MHz spectrometers, which were all purchased around 2010-2014. A paramagnetic resonance spectrometer was purchased in 2018 and currently serves as the main instrument in our laboratory. With the help of this Bruker E580 paramagnetic resonance spectrometer, what research work have you carried out and what remarkable results have you produced?

Hu Bingwen: The article we just published on JPCL is about the unique application of paramagnetic resonance in NaCrO2 system. By using paramagnetic technology, we can observe Cr5+ ions that are difficult to be observed by other technologies. By using the charging and discharging equipment, we can know that the stability of the battery is very good when the voltage is below 3.7V; when the voltage is above 3.7V, the signal will soon disappear. In fact, once the voltage is above 3.7V, Cr3+ will transform into Cr5+, and Cr5+ will dissolve in the electrolyte, resulting in a sharp decline in the battery’s performance. And if you want to get this information, the most direct and effective tool is paramagnetic resonance. With the paramagnetic resonance imaging tool, we can see that Cr ions are located on the membrane in the electrolyte, which directly demonstrates the great potential of paramagnetic resonance imaging technology.

Figure 1. (a) the in situ EPR diagram of NaCrO2 system below 3.6V; (b) the in situ EPR diagram of NaCrO2 system above 3.9V;

I would like to disclose a few more upcoming research results: In situ paramagnetic resonance is an effective analytical method to study the sedimentation process of Li ions on copper sheets, and its resolution is far higher than that of magnetic resonance imaging (MRI), which makes me very excited. In addition, by observing the changes of O in lithium-air battery through the in situ EPR diagram, we found that oxides (like Co3O4) have a unique effect on O2, which is helpful to understand why oxides can enhance the cycle performance of lithium-air battery. How do you feel about this E580 paramagnetic resonance spectrometer? Why did you choose this instrument in the first place?

Hu Bingwen: Generally speaking, the E580 is easy to operate and has strong performance, which allows it to meet the needs of advanced scientific research. The after-sales service is also quite good. The configurability of instruments is important for scientific research. Bruker also equipped my E580 with L band, X band, Q band, and the imaging system according to my needs.

As for the reasons for choosing this instrument: On the one hand, we have rented Bruker’s E580 Demo machine for related research since 2016, and it has been five years so far. On the other hand, the Q band and imaging system I need, as far as I know, are not available in the spectrometers of other brands.

Magnetic Resonance, a technology that strives for progress and continues to break through Do you think the current development of magnetic resonance technology can meet the needs of battery research? For the sake of scientific research, what are your expectations for the development of magnetic resonance instruments in the future?

Hu Bingwen: I explain that with an idiom called "cutting feet to fit shoes", which means that the conditions are limited, and relevant research can only be done according to the existing functions of the instrument.

At present, magnetic resonance technology can basically meet the needs of battery research. I am most looking forward to the development of fast imaging function, which can realize faster imaging while maintaining greater sensitivity. The imaging speed is relatively slow now. In the past, it took 24 hours for the battery to charge and discharge, so it was no problem to collect a spectrogram in half an hour. However, now the battery may only take one hour to charge and discharge, which requires the collecting results to be presented in a few minutes. Therefore, if we want to study this high-speed charging and discharging problem, we must have faster imaging technology and spectroscopy technology.
With regard to the development of magnetic resonance instruments in the future, in fact, I have been continuously improving the instruments. The original hardware design must be general, not dedicated to the research in the battery field. We make some local changes based on the existing architecture, and then we will independently design a tool that is more suitable for battery research. The progress of these tasks may be relatively slow, but I have been doing them all the time. My goal is to optimize the magnetic resonance instrument and related technology for the battery system, and maintain our unique advantages in this field. At present, China is making great efforts to develop the new energy field, and the battery is also a very popular industry. Could you share your future work plan with us?

Hu Bingwen: In the future, I hope to find the source of pains and difficulties in the battery field through the combination of NMR and paramagnetic resonance technologies. For example, by using paramagnetic resonance technology to study the cationic disordered cathode materials we are using now, we found that the internal manganese ion aggregation is the core factor of performance degradation. After understanding the reason for performance degradation, it will be easy to do some modification work.

Although the application of NMR technology is relatively mature, our research group is still exploring more applications. The application of paramagnetic resonance does not have a long time, and many technologies have not been applied. So, in recent years, I’d like to spend my time studying paramagnetic resonance technology and applying it to battery systems. Even on a global scale, the applications of paramagnetic resonance and paramagnetic resonance imaging are very scarce, so they will be part of my priorities in the future.

Hu Bingwen, researcher at East China Normal University

Hu Bingwen is currently a researcher at the School of Physics and Electronic Science, East China Normal University, deputy director of Shanghai Key Laboratory of Magnetic Resonance, winner of the NSFC Excellent Young Scientist (EYS) Scheme, member of Zijiang Outstanding Young Scholars, and young editor of Chinese Journal of Magnetic Resonance. He mainly studies magnetic resonance and its application in the battery field, and has developed solid-state NMR pulse sequences including SHANGHAI, SHA+, and RFDF-XY8-4-1, and the methods of in situ NMR, in situ EPR, and in situ EPR Imaging for the lithium battery system.

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