Tokyo University of Science researchers demonstrated unprecedented control of response speed by over two orders of magnitude in a solid state battery. This is a major steppingstone towards realization of commercial all-solid-state batteries.

Solid-state battery‘s high surface resistance causes these batteries to have low output, limiting their applications. The researchers have employed a novel technique to investigate and modulate electric double layer dynamics at the solid/solid electrolyte interface. The achievement is carrier modulation and improved switching response speed control in these batteries.

The reporting paper detailing their technique was published in Volume 31 of Materials Today Physics.

Electric double layer(EDL) transistor with Li+ solid electrolyte. Investigating the EDL effect at the solid electrolyte interface by a novel technique, researchers tuned switching response time and achieved carrier modulation at the interface between a hydrogen-terminated diamond (H-diamond) and a Li+-conducting solid electrolyte/solid interface. Image Credit: Tokyo University of Science. Click the press release link for the largest and more informative view.

The search for clean energy and carbon neutrality, all-solid-state lithium-ion batteries (ASS-LIBs) offers considerable promise. ASS-LIBs are expected to be used in a wide range of applications including electric vehicles (EVs). However, commercial application of these batteries is currently facing a bottleneck – their output is reduced owing to their high surface resistance. Moreover, the exact mechanism of this surface resistance has been so far, unknown. Researchers have alluded it to a phenomenon called the “electric double layer” (or EDL) effect seen in colloidal substances (which are microscopic dispersions of one kind of particle in another substance).

The EDL effect occurs when colloidal particles gain negative electric charge by adsorbing the negatively charged ions of the dispersion medium on their surface.

Dr. Tohru Higuchi, Associate Professor at Tokyo University of Science (TUS) explained, “This occurs at the solid/solid electrolyte interface, posing a problem in all-solid-state lithium batteries.”

Dr. Higuchi, along with colleagues Dr. Makoto Takayanagi from TUS, and Dr. Takashi Tsuchiya and Dr. Kazuya Terabe from National Institute for Materials Science in Japan, have devised a novel technique to quantitatively evaluate the EDL effect at the solid/solid electrolyte interface.

The researchers employed an all-solid-state hydrogen-terminated diamond (H-diamond)-based EDL transistor (EDLT) to conduct Hall measurements and pulse response measurements that determined EDL charging characteristics. By inserting a nanometer-thick lithium niobate or lithium phosphate interlayer between the H-diamond and lithium solid electrolyte, the team could investigate the electrical response of the EDL effect at the interface between these two layers.

The electrolyte’s composition did, indeed, influence the EDL effect in a small region around the electrode interface. The EDL effect was reduced when a certain electrolyte was introduced as an interlayer between the electrode/solid electrolyte interface. EDL capacitance for the lithium phosphate/H-diamond interface was much higher compared to the lithium niobate/H-diamond interface.

The team’s article also explains how they improved the switching response time for charging ASS-EDLs. Dr. Higuchi noted, “The EDL has been shown to influence switching properties, so we considered that the switching response time for charging ASS-EDLs could be greatly improved by controlling the capacitance of the EDL. We used the non-ion-permeable property of diamond in the electron layer of the field-effect transistor and combined it with various lithium conductors.”

The interlayer accelerated and decelerated the EDL charging speed. The electrical response time of the EDLT was highly variable – it ranged from about 60 milliseconds (low speed switching for lithium phosphate/H-diamond interface) to about 230 microseconds (high speed switching for lithium niobate/H-diamond interface). The team, however, exhibited control over the EDL charging speed for over two orders of magnitude.

To summarize, the researchers were able to achieve carrier modulation in all-solid-state devices and improved their charging characteristics. “These results from our research on the lithium-ion conductive layer are important for improving the interface resistance and may lead to the realization of all solid-state batteries with excellent charge-discharge characteristics in the future,” noted Dr. Higuchi.

All in so far this is a major steppingstone towards controlling the interface resistance of ASS-LIBs that catalyzes their feasibility for many applications. It will also help design better solid-electrolyte-based devices, a class of gadgets which also includes neuromorphic devices.


The solid-state design has some very attractive characteristics. For now the architecture design isn’t cooperating. It sounds like this team has revealed that the electrodes cover themselves thus restricting the flow and in turn the power.

It does seem now that the problems and challenges are getting defined thanks to this team’s efforts. Once defined and explained the solutions can be worked out.

Solid-state might not be imminent, but now one has lots more confident they are inevitable.


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