An ORNL-led research team has found the key to fast ion conduction in a solid electrolyte: tiny features (represented by red and green in the image) that maximize ion transport pathways. Image: Oak Ridge National Laboratory, U.S. Dept. of Energy.
An ORNL-led research team has found the key to fast ion conduction in a solid electrolyte: tiny features (represented by red and green in the image) that maximize ion transport pathways. Image: Oak Ridge National Laboratory, U.S. Dept. of Energy.

In a lithium-ion rechargeable battery, the electrolyte transports lithium ions from the negative to the positive electrode during discharging; the path of ionic flow then reverses during recharging. The organic liquid electrolytes in commercial lithium-ion batteries are flammable and subject to leakage, making their large-scale application potentially problematic. Solid electrolytes, in contrast, overcome these challenges, but their ionic conductivity is typically low.

Now, a team led by the US Department of Energy's Oak Ridge National Laboratory (ORNL) has used a state-of-the-art microscopy technique to identify a previously undetected feature, about 5nm wide, in a solid electrolyte. The work experimentally verifies the importance of this feature for fast ion transport, and corroborates the observations with theory. As the researchers report in a paper in Advanced Energy Materials, this work could point the way to a novel strategy for the design of highly-conductive solid electrolytes.

"The solid electrolyte is one of the most important factors in enabling safe, high-power, high-energy, solid-state batteries," said first author Cheng Ma of ORNL, who conducted most of the study's experiments. "But currently the low conductivity has limited its applications."

"Our work is basic science focused on how we can facilitate ion transport in solids," said Miaofang Chi of ORNL and senior author of the paper. "It is important to the design of fast ion conductors, not only for batteries, but also for other energy devices." These other devices include supercapacitors and fuel cells.

To directly observe the atomic arrangement in the solid electrolyte, the researchers used aberration-corrected scanning transmission electron microscopy to send electrons through a sample of the electrolyte. In order to observe an extremely small feature in a three-dimensional (3D) material with a method that essentially produces a two-dimensional (2D) image, they needed a sample of extraordinary thinness. To prepare one, they relied on the comprehensive materials processing and characterization capabilities of the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL.

"Usually the transmission electron microscopy specimen is 20nm thick, but Ma developed a method to make the specimen ultra-thin (approximately 5nm)," Chi said. "That was the key because such a thickness is comparable to the size of the hidden feature we finally resolved."

The researchers examined a prototype electrolyte called LLTO, named for its lithium, lanthanum, titanium and oxygen building blocks. LLTO possesses the highest bulk conductivity among oxide systems.

In this material, lithium ions move fastest in the planar 2D pathways that form between alternating stacks of atomic layers rich in either lanthanum or lithium. The ORNL-led team was the first to see tiny domains or features, approximately 5–10nm wide, throughout the 3D material that provided more directions in which the lithium ions could move, but without hurting the superior 2D transport. These domains looked like sets of shelves stacked at right angles to each other. The smaller the shelves, the easier it was for ions to flow in the direction of an applied current.

ORNL's Yongqiang Cheng and Bobby Sumpter performed molecular dynamics simulations that corroborated these experimental findings.

Previously, scientists tended to look at the atomic structure of the simplest repeating unit of a crystal – called a unit cell and typically less than 1nm wide – and rearranged its atoms or introduced different elements to see how they could facilitate ion transport. In the material that the ORNL scientists studied for this paper, the unit cell is nearly half a nanometer. The team's unexpected finding – that fine features of only a few nanometers in size and traversing a few unit cells can maximize the number of ionic transport pathways – offers a new perspective.

"The finding adds a new criterion," Chi said. "This largely overlooked length scale could be the key to fast ionic conduction." This means researchers will need to consider phenomena on the order of several nanometers when designing materials for fast ion conduction.

"The prototype material has high ionic conductivity because not only does it maintain unit-cell structure, but also it adds this fine feature, which underpins 3D pathways," Ma said. "We're not saying that we shouldn't be looking at the unit-cell scale. We're saying that in addition to the unit cell scale, we should also be looking at the scale of several unit cells. Sometimes that outweighs the importance of one unit cell."

For several decades, when researchers had no explanation for certain material behaviors, they speculated that phenomena transcending one unit cell could be at play, but they never saw any experimental evidence. "This is the first time we proved it experimentally," Ma said. "This is a direct observation, so it is the most solid evidence."

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