Prof. Akitoshi Hayashi, Department of Applied Chemistry, Graduate School of Engineering, Osaka Metropolitan University, with a HORIBA Raman spectrometer, LabRAM HR.
With advancements in science, combustion cars could one day become a relic of outdated technology. Electric cars are penetrating the market worldwide, driving a need for lighter and more compact batteries. Lithium-ion batteries have become a game-changer, especially since Dr. Akira Yoshino received the Nobel Prize in Chemistry in 2019. Now, all-solid-state lithium-ion batteries are gaining traction as we strive for improved performance and safety.
We at HORIBA recently had the opportunity to sit down with Professor Akitoshi Hayashi in October. 2021. Prof. Hayashi is from the Graduate School of Engineering at Osaka Metropolitan University. He is an expert in solid electrolytes, which are key to enhancing battery stability and efficiency. His insights are crucial as we look towards the future of battery technology.
In this segment, he is excited to share the story of his research journey in solid-state batteries. He said, “It all began in the lab of Dr. Tsutomu Minami, a former president of Osaka Prefecture University as well as my mentor. As a student, I joined the lab, which sparked my interest in battery material research.”
One of the key topics in the lab was ion-conductive glasses. Dr. Masahiro Tatsumisago, now the president of Osaka Metropolitan University, also belonged to the lab at that time, and they were making an advanced glass containing silver ions to achieve electrical conductivity. The lab was working on another research project to investigate the chemical structure and the mechanism of another advanced glass based on sulfide to achieve lithium-ion conductivity for a solid-state battery. He was captivated by the breakthrough that advanced glasses could achieve ion conductivity—a challenge because ions traditionally have a hard time moving through solid materials.
Prof. Hayashi said, “I was deeply intrigued by how different materials performed and how they achieved their results.”
His research was particularly inspired by the discovery that sulfide materials exhibited better lithium-ion conductivity compared to oxides. This led him to focus on sulfide-based solid electrolytes for batteries, and it has been an exciting and fulfilling journey ever since.
Typically, glass is made by melting raw materials and then cooling them. However, when it comes to glasses with high lithium-ion content, vitrifying them during the cooling process can be quite challenging. One of the standout sulfide electrolytes for all-solid-state batteries has been Li3PS4 (LPS), a lithium-ion glass material. Traditionally, producing this glass using conventional methods was difficult.
In 2001, Prof. Hayashi’s research team made a significant breakthrough by publishing the first paper on synthesizing Li3PS4 glass using a mechanochemical method. He considers this an innovative leap forward in the field of solid-state batteries, and it continues to influence his work today.
Raman spectra for the mechanochemically prepared Na2S-P2S5 glasses.
When it comes to characterizing glass materials, researchers rely on various analytical tools like infrared absorption, Raman spectroscopy, and solid-state Nuclear Magnetic Resonance (NMR) to explore the local structure. Among these, Raman spectroscopy stands out because it allows for straightforward measurements by simply shining a laser on a powder sample.
While solid-state NMR is great for detailing the structure around specific nuclei, Raman spectroscopy excels at identifying various chemical bonds within the glass structure. Unlike X-ray diffraction, which is better suited for crystalline structures, Raman spectroscopy can effectively analyze both amorphous and crystalline materials. That’s why Prof. Hayashi’s research team uses Raman spectroscopy to confirm whether the glass produced through mechanochemical methods matches the desired composition.
For instance, in his work with Na2S-P2S5 solid-state electrolytes, Raman spectroscopy helps him observe changes in the material’s structure. Specifically, he can detect a PS43- anion peak around 420 cm-1 when the Na2S composition exceeds 75 mol%. This capability makes Raman spectroscopy crucial for analyzing the structural properties of his solid electrolytes.
He particularly values HORIBA's Raman spectrometer for its ability to handle challenging samples that may interfere with conventional Raman systems due to fluorescence. The HORIBA spectrometer’s UV laser option and micro-Raman capabilities allow us to analyze specific regions and detect local degradation or structural changes in complex samples like solid-state batteries. This makes it an invaluable tool in his research toolkit.
It’s been about 20 years since he first started exploring sulfide electrolytes with ionic conductivity and assessing their potential in all-solid-state batteries. Over this time, global perspectives on these batteries have evolved significantly.
He said, “Moving forward, it is important for us to develop battery materials that utilize abundant elements like sodium, sulfur, and iron. Research should aim at making these materials practical for storage batteries, aligning with resource sustainability.”
Japanese suppliers are now becoming proficient in providing sulfide electrolytes, which is a notable strength for Japan. However, Prof. Hayashi thinks researchers must address safety concerns, particularly due to the generation of hydrogen sulfide. He also thinks that another key challenge is to create solid electrolytes that offer a balanced performance—good atmospheric stability, mechanical strength, electrochemical stability, and electrical conductivity.
As a researcher, it would be a significant milestone to achieve commercialization of the all-solid-state battery. Yet, even after reaching this goal, Prof. Hayashi plans to continue exploring new materials and battery processes in collaboration with industry partners to push performance boundaries further. Additionally, as a university professor, he sees it as his mission to pass on the current generation’s research passion and knowledge to the next generation. He hopes his work will contribute to advancing the field of all-solid-state batteries, and that Japan will continue to lead in this research area. He is committed to this journey with his research group and look forward to what the future holds.
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