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2021.08.26 Thursday

A Solid Favor for Researchers: A New Way to Investigate the Electric Double Layer Effect

Scientists develop methodology to explore an elusive interfacial phenomenon in all-solid-state batteries

All-solid-state batteries are expected to replace conventional batteries with a liquid electrolyte thanks to their improved safety, durability, and capacity. However, the electric double layer (EDL) effect is a phenomenon that is suspected to be a hindrance to battery performance and is difficult to measure. To tackle this issue, scientists from Japan have developed a new methodology to explore the EDL using diamond-based field-effect transistors, paving the way to better solid-state ionic devices and batteries.

Progress in lithium-ion (Li-ion) batteries have made all sorts of portable devices feasible and fueled the growth of electronics. However, the intrinsic disadvantages of conventional Li-ion batteries, whose cells use a liquid electrolyte solution, render them not entirely suitable for much-anticipated applications like electric vehicles. These limitations include limited durability, low capacity, safety issues, and environmental concerns about their toxicity and carbon footprint. Fortunately, scientists are now focusing on the next-generation solution to all these problems: all-solid-state batteries. The use of a solid electrolyte makes this type of batteries safer and capable of holding a greater power density.

However, a key issue of these batteries is the high resistance found at the electrolyte–electrode interface, which reduces the output of all-solid-state batteries and prevents them from being charged rapidly. One discussed mechanism behind this high interface resistance is the electric double layer (EDL) effect, which involves the gathering of charged ions from an electrolyte at the interface with an electrode. This produces a layer of positive or negative charge, which in turns causes charge of the opposite sign to accumulate throughout the electrode at an equal density, creating a double layer of charges. The problem with detecting and measuring the EDL in all-solid-state batteries is that conventional electrochemical analysis methods don't make the cut.

At Tokyo University of Science, Japan, scientists led by Associate Professor Tohru Higuchi have solved this conundrum using a completely new methodology for assessing the EDL effect in solid electrolytes of all-solid-state batteries. This study, published online in Nature's Communications Chemistry, was conducted in collaboration with Takashi Tsuchiya, Principal Researcher at the International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Japan, and Kazuya Terabe, MANA Principal Investigator at the same organization.

The new method revolves around field-effect transistors (FETs) made using hydrogenated diamond and a solid Li-based electrolyte. FETs are a three-terminal transistor in which the current between the source and drain electrodes can be controlled by applying a voltage at the gate electrode. This voltage, thanks to the electric field generated in the semiconductor region of the FET, controls the density of electrons or holes ('electron vacancies' with a positive charge). By exploiting these characteristics and using chemically inert diamond channels, the scientists ruled out chemical reduction–oxidation effects affecting the conductivity of the channel, leaving only the electrostatic charges accumulated thanks to the EDL effect as the necessary cause.

Accordingly, the scientists performed Hall effect measurements, which are sensitive to charged carriers only on the surface of materials, on the diamond electrodes. They used different types of Li-based electrolytes and investigated how their composition affected the EDL. Through their analyses, they revealed an important aspect of the EDL effect: it is dominated by the electrolyte's composition in the immediate vicinity of the interface (about five nanometers in thickness). The EDL effect can be suppressed by several orders of magnitude if the electrolyte material allows for reduction–oxidation reactions that give way to charge compensation. "Our novel technique proved useful for revealing aspects of EDL behavior at the vicinity of solid electrolyte interfaces and helped clarify the effects of interface characteristics on the performance of all-solid-state Li-ion batteries and other ionic devices," highlights Dr. Higuchi.

The team now plans to use their method to analyze the EDL effect in other electrolyte materials, hoping to find clues on how to reduce the interfacial resistance in next-generation batteries. "We hope that our approach will lead to the development of all-solid-state batteries with very high performance in the future," concludes Dr. Higuchi. Moreover, understanding the EDL better will also aid in the development of capacitors, sensors, and memory and communication devices. Let us hope exploring this complex phenomenon becomes easier for other scientists so that the field of solid-state ionic devices keeps advancing.

A Solid Favor for Researchers: A New Way to Investigate the Electric Double Layer Effect

Figure 1. Probing the electric double layer (EDL) effect using diamond-based FETs

(a) Diagram of an all-solid-state Li-ion battery suffering from high interface resistance, suspected to be due to the EDL effect. (b) Diamond-based field effect transistors (FETs) let us modulate the hole density at the diamond channel by applying voltage, which in turns lets us measure the contributions of the EDL effect. (c, d) Two diamond-based FETs made using different Li-based solid electrolytes. The reduction–oxidation of Ti atoms enables charge neutralization within the Li-La-Ti-O electrolyte, greatly suppressing the EDL effect.

Image courtesy: Tohru Higuchi from Tokyo University of Science

Reference
Title of original papers  : The electric double layer effect and its strong suppression at Li+ solid electrolyte/hydrogenated diamond interfaces
Journal  : Communications Chemistry
DOI  : 10.1038/s42004-021-00554-7
About The Tokyo University of Science

Tokyo University of Science (TUS) is a well-known and respected university, and the largest science-specialized private research university in Japan, with four campuses in central Tokyo and its suburbs and in Hokkaido. Established in 1881, the university has continually contributed to Japan's development in science through inculcating the love for science in researchers, technicians, and educators.

With a mission of "Creating science and technology for the harmonious development of nature, human beings, and society", TUS has undertaken a wide range of research from basic to applied science. TUS has embraced a multidisciplinary approach to research and undertaken intensive study in some of today's most vital fields. TUS is a meritocracy where the best in science is recognized and nurtured. It is the only private university in Japan that has produced a Nobel Prize winner and the only private university in Asia to produce Nobel Prize winners within the natural sciences field.

■Tokyo University of Science(About TUS) : https://www.tus.ac.jp/en/about/
■Research List: https://www.tus.ac.jp/en/news/03/
About Associate Professor Tohru Higuchi from Tokyo University of Science

Tohru Higuchi is a member of the Department of Applied Physics in the Tokyo University of Science. He graduated in Applied Physics from the Tokyo University of Science in 1995, where he then proceeded to obtain Master's and PhD degrees. His research mainly focuses on functional material science specializing in thin film/surface and interfacial physical properties, as well as inorganic industrial materials. He has authored over 200 papers and received several awards, such as those for his contributions in the GREEN-2019 conference and the 2019 International Symposium on Advanced Material Research.
https://www.tus.ac.jp/en/fac/p/index.php?3402
https://www.rs.kagu.tus.ac.jp/higuchi/

Funding information

This study was supported by JSPS Grants-in-Aid for Scientific Research on Innovative Areas "Interface Ionics" A04 (JP20H05301), Grant-in-Aid for Scientific Research (JP19K05279), and Research Fellowship (JP19J22244). STEM-EELS measurements were supported by the NIMS Storage Battery Platform.

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