Fleeting defects caught in the act

“Operando measurements are indispensable here. If we looked at the battery before and after the first charge-discharge cycle, we would see no defects. But during the operation, we see how the defects form and self-heal, leaving detectable ‘scars’ behind.”Andrej Singer, Cornell University

Sodium-ion batteries have been touted as a sustainable alternative to lithium-ion batteries because they are powered by a more abundant natural resource. However, sodium-ion batteries have hit a significant snag: their cathodes degrade quickly with recharging.

A Cornell University-led collaboration has now succeeded in identifying an elusive mechanism that can trigger this degradation – transient crystal defects – by using a unique form of X-ray imaging that allowed the researchers to capture these fleeting defects while the battery was in operation. The researchers report their findings in a paper in Advanced Energy Materials.

The project was led by Andrej Singer, assistant professor in the Department of Materials Science and Engineering at Cornell Engineering. His research group has been investigating nanoscale phenomena in energy and quantum materials, often by utilizing advanced operando X-ray tools. These techniques are especially helpful for exploring the behavior of transient defects, which appear only briefly during ionic transport, meaning much about their life cycle and impact remains unknown.

In partnership with researchers from the University of California, San Diego and the Advanced Photon Source at the US Department of Energy’s Argonne National Laboratory, the team used Bragg Coherent Diffractive Imaging with a highly synchronized X-ray beam to focus on the constituent parts of a charging sodium-ion battery. This allowed them to produce real-time 3D snapshots that revealed the morphology and atomic displacements within cathodes made of sodium, nickel, manganese and oxygen (Na_xNi_1-xMn_yO₂).

“Operando measurements are indispensable here,” Singer said. “If we looked at the battery before and after the first charge-discharge cycle, we would see no defects. But during the operation, we see how the defects form and self-heal, leaving detectable ‘scars’ behind.”

To try to explain what they observed, the team took inspiration from metals, in which defects such as dislocations allow the ductile materials to deform without breaking. By using metallurgical modeling, the researchers tracked the movement of the transient – also known as metastable – defects and made qualitative predictions of the stresses that moved them as the cathode material transformed and self-healed.

“Dislocations are one-dimensional crystal defects,” explained Oleg Gorobstov, a postdoctoral fellow at Cornell University and lead author of the paper. “Their presence in the ceramic cathodes we study is surprising, and the mechanisms for their formation are yet to be understood. We found that the dislocations form at a transiently forming anti-phase domain boundary. This preceding configuration is a new piece of the puzzle that we hope will help us better understand the defect dynamics in this important class of materials.”

The researchers are now turning their attention to the way the defects interact with the ions that shuffle in and out of the cathode as the battery operates – a fundamental mechanism for energy delivery. Singer also noted that the orientation of the dislocations suggests that particle shape plays an important role in the process, so his team and collaborators plan to investigate if this morphology can be tuned to either facilitate or eliminate the dislocations.

“We have yet to understand the role of extended defects in battery materials,” Singer said. “For centuries, blacksmiths used defect engineering in metals to create stronger and more durable materials without even realizing it. Applying a defect-engineering approach to ceramics is much more challenging due to the presence of electrostatic charges. Nevertheless, with the help of new operando measurements and a better understanding of the mechanisms involved, we can now begin to address this challenge.”

This story is adapted from material from Cornell 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.