(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 FETs can be used to modulate the hole density at the diamond channel by applying a voltage, which in turn allows the EDL effect to be measured. (c, d) Two diamond-based FETs made using different Li-based solid electrolytes. The oxidation-reduction of titanium (Ti) atoms leads to charge neutralization within the Li-La-Ti-O electrolyte, greatly suppressing the EDL effect. Image: Tohru Higuchi from Tokyo University of Science.
(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 FETs can be used to modulate the hole density at the diamond channel by applying a voltage, which in turn allows the EDL effect to be measured. (c, d) Two diamond-based FETs made using different Li-based solid electrolytes. The oxidation-reduction of titanium (Ti) atoms leads to charge neutralization within the Li-La-Ti-O electrolyte, greatly suppressing the EDL effect. Image: Tohru Higuchi from Tokyo University of Science.

Lithium-ion (Li-ion) batteries have made all sorts of portable devices feasible and fueled the growth of electronics. But conventional Li-ion batteries with a liquid electrolyte have intrinsic limitations that 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 toxicity and carbon footprint.

Fortunately, scientists are now focusing on a technology that will help solve all these problems: all-solid-state batteries. Using a solid electrolyte produces batteries that are safer with a greater power density.

However, a key issue with all-solid-state batteries is the high resistance found at the electrolyte–electrode interface, which reduces their output and prevents them from being charged rapidly. One proposed cause of this high interface resistance is the electric double layer (EDL) effect, which involves the gathering of charged ions from the electrolyte at the interface with the electrode. This produces a layer of positive or negative charges, which in turn causes charge of the opposite sign to accumulate throughout the electrode at an equal density, creating a double layer of charges. Unfortunately, detecting and measuring the EDL in all-solid-state batteries has proved tricky, because conventional electrochemical analysis methods don’t make the cut.

Now, a team of researchers led by Tohru Higuchi at the Tokyo University of Science in Japan has solved this conundrum by using a completely new method for assessing the EDL effect in the solid electrolytes of all-solid-state batteries. Higuchi and his team report their findings in a paper in Communications Chemistry.

The new method is based on using field-effect transistors (FETs) made with hydrogenated diamond to study solid Li-based electrolytes. An FET is a three-terminal transistor in which the current between the source and drain electrodes can be controlled by applying a voltage to the gate electrode. Thanks to the electric field generated in the semiconductor region of the FET, this voltage controls the density of electrons or holes (‘electron vacancies’ with a positive charge). By exploiting these characteristics and using chemically inert diamond channels, the researchers were able to prevent oxidation-reduction reactions affecting the conductivity of the channels, meaning only the electrostatic charges accumulating due to the EDL were able to affect conductivity.

This allowed the researchers to perform Hall effect measurements, which are sensitive to charged carriers only on the surface of materials, on the diamond channels. 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 5nm in thickness). This means the EDL effect can be suppressed by several orders of magnitude if the electrolyte material allows for oxidation-reduction 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,” says Higuchi.

The team now plans to use their method to analyze the EDL effect in other electrolyte materials, hoping to find clues about 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,” says Higuchi. Moreover, understanding the EDL better will also aid in the development of capacitors, sensors, and memory and communication devices.

This story is adapted from material from the Tokyo University of Science, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.