The three-fold symmetry in this image shows the spinel crystal structure of lithium titanate. Image: Taro Hitosugi (source Nature Communications).
The three-fold symmetry in this image shows the spinel crystal structure of lithium titanate. Image: Taro Hitosugi (source Nature Communications).

Researchers at Tokyo Institute of Technology, Tohoku University and the University of Tokyo, all in Japan, have applied advanced scanning methods to visualize the previously unexplored surface of the superconductor lithium titanate (LiTi2O4).

LiTi2O4 is the only known example of a so-called spinel oxide superconductor. This rarity makes LiTi2O4 of enormous interest to those studying the origins of superconductivity, as it has the highest superconducting transition temperature (of up to 13.7K) within this group of materials.

Although LiTi2O4 in bulk form has been studied for decades, little is known about its surfaces, owing to the difficulty of preparing suitable LiTi2O4 surfaces for further analysis. Now, using a combination of experimental and theoretical methods, a team of researchers including Taro Hitosugi of Tokyo Tech and the Advanced Institute for Materials Research at Tohoku University has obtained visual evidence of superconductivity on ultrathin LiTi2O4 films, marking a milestone in surface science.

The study, which is reported in a paper in Nature Communications, began with the detection of an unexpected energy ‘gap’, hinting at the existence of superconductivity at the surface. The team’s investigations also revealed that the surface superconductivity is in different states from that of the bulk interior.

The researchers used two experimental methods to visualize this finding: pulsed laser deposition (PLD), a technique for producing high-quality LiTi2O4 films under vacuum conditions; and low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) for precisely imaging the surfaces.

"Imaging the atoms for the first time was surprising, as it's usually very difficult to observe the spinel-oxide atoms," says Hitosugi. "We then wanted to know the exact atomic arrangement on the surface, and in order to do that, we compared theory and experiment."

To delve deeper into how the atoms are arranged, the researchers made theoretical calculations that led them to consider four types of surface that could be cut from bulk LiTi2O4. By comparing these four types, the researchers found one – called the TiLi2-terminated surface – that matched their experimental observations.

Hitosugi explains that "knowing the accurate arrangement of atoms is the most important thing," as this knowledge will help to advance understanding of superconductivity at the surface of LiTi2O4. It could also help to reveal the mechanisms behind lithium-ion battery operations. The understanding of electrode surfaces is an essential step for designing next-generation lithium-ion batteries with higher capacity, enhanced life cycles and fast charging capabilities, because lithium ions migrate across electrode surfaces.

Hitosugi now plans to collaborate with Tokyo Tech colleagues working on solid-state electrolytes, specifically to improve understanding of the electrode-electrolyte interface (EEI), one of the hottest topics in battery research. "Many people are interested in solid-state batteries -- the future of lithium-ion batteries," he says. "Now that we know the surface atomic arrangement of this material, we can begin to simulate the operation of solid-state lithium batteries."

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