Dr. Kira Welter, Senior Associate Editor of ChemPhysChem, talked to Dr. Masaru Yao, National Institute of Advanced Industrial Science and Technology (AIST), Osaka, Japan, about his work on anthraquinone-based oligomers in batteries that was recently published in ChemPhysChem.
Dr. Yao, could you explain the key findings of your study?
We have described a way to effectively use organic compounds as active electrode materials for rechargeable lithium-ion batteries (LIBs), replacing the minor heavy metals that are widely used in current LIBs for mobile phones, electric vehicles, and other applications.
Solving the resource problem is a big challenge in battery research. The electrodes of current LIBs contain minor heavy metals such as cobalt and nickel. The ever-increasing concerns about resource depletion and environmental issues strongly call for a reduction in the use of such materials. Our research group has focused on redox-active organic compounds as an alternative to the current minor-metal-based electrodes.
Which implications could this work have for the development of better rechargeable batteries?
Our results might shed some light on the use of organic compounds as electrode materials for such devices. Many organic compounds have already been reported in this field, but it has not been easy to fulfill the requirements of high capacity and long cycle life simultaneously. Organic compounds showing a high capacity often end up having a short cycle life, whereas others showing longevity usually suffer from a low capacity.
To confront this apparent trade-off, we developed the idea that oligomerization should extend the cycle life of organic materials. Our findings provide a new guide to designing high-performance organic electrode materials and could trigger the development of low-cost and light-weight organic batteries.
Why did you focus on organic electrode materials? Why did you study anthraquinone, in particular?
Many efforts have been made to reduce the minor-heavy-metal load in LIBs by partially substituting such metals with less problematic ones in conventional inorganic electrodes. But I am fascinated by the idea that organic compounds—consisting only of benign elements such as C, H, and O—could completely eliminate the need for minor-heavy-metal raw materials in the electrodes. Furthermore, the light elements should lead to a high gravimetric capacity that rivals—or even exceeds—that of conventional inorganic materials if we succeed in using the multi-electron-transfer reactions specific to organic compounds.
Regarding the anthraquinone moiety, although its theoretical capacity is not very high, it can be modified with a variety of substituents. The resulting derivatives can serve as scientific model compounds to systematically investigate the relationship between structure and electrochemical properties.
You were able to improve the cycling performance of anthraquinone-based electrodes without affecting their discharge capacity. Could you explain how you did this?
The key is the oligomerization step. While polymerization has been a way to pin down organic molecules in electrodes to extend their cycle stability, this method often results in a low utilization ratio, i.e., only a fraction of the active points functions as an electrode. We experimentally demonstrated that connecting only a couple of redox-active organic units—a process known as oligomerization—significantly extends the cycle life of the organic electrode without impairing the utilization ratio of the active units. We confirmed this by comparing the anthraquinone monomer with its newly synthesized dimer and trimer.
Quantum chemistry calculations corroborated our experiments, indicating that the attractive intermolecular π–π interactions in the oligomers are strong enough to stabilize the molecules in the crystal. In general, such π–π interactions are considered to be very weak, but we have shown that they can become stronger when they are combined, reaching a strength comparable to that of covalent bonds.
Which part of the work proved the most challenging?
The synthesized oligomers are almost insoluble in ordinary organic solvents, so collecting convincing characterization data for these compounds was the most time-consuming and difficult part. However, finding a good scheme for the quantum chemistry calculation, which gave us a quantitative insight into the intermolecular interactions, seems to have been the most exciting one! We think that this calculation technique and our results could serve as a guide for designing good materials for reliable molecule-based electronic devices.
What are your plans for future work in this area?
Based on the results reported in this study, we have already developed an organic electrode material with a high energy density (>1000 mWh/g) and a long cycle life. We are now considering a new molecular design that could lead to a much higher energy density and hope to be able to publish those results in the near future.
The organic electrodes could change the image of current batteries; for instance, we are thinking about a printable flexible battery for which, I believe, organic compounds are a better technological fit than their inorganic counterparts. Last but not least, aside from the new molecular design and synthesis, we need to continue our research studies to produce practical, high-performance organic batteries, for example, by optimizing the electrode composition, electrolyte solution, separators, and so on.
The article they talked about
- Anthraquinone-based Oligomer as a Long Cycle-life Organic Electrode Material for Use in Rechargeable Batteries,
Masaru Yao, Hikaru Sano, Hisanori Ando, Tetsu Kiyobayashi, Nobuhiko Takeichi,
ChemPhysChem 2019.
https://doi.org/10.1002/cphc.201900012
