Researchers identify mechanism causing cathode degradation in sodium-ion batteries

Sodium-ion batteries have been considered a sustainable substitute for lithium-ion batteries due to their reliance on a more abundant natural resource. However, sodium-ion batteries have encountered a significant obstacle: their cathodes degrade rapidly during recharging.

A collaboration led by Cornell University has successfully identified a previously elusive mechanism that can trigger this degradation - transient crystal defects - through the use of a unique form of X-ray imaging that allows researchers to capture these fleeting defects while the battery is in operation.

The team’s findings were published in the journal Advanced Energy Materials in a paper titled “Operando Interaction and Transformation of Metastable Defects in Layered Oxides for Na-Ion Batteries,” with postdoctoral fellow Oleg Gorobstov as the lead author.

Andrej Singer, an assistant professor and David Croll Sesquicentennial Faculty Fellow in the Department of Materials Science and Engineering at Cornell Engineering, led the project. His research group focuses on nanoscale phenomena in energy and quantum materials, frequently employing advanced operando X-ray tools. These techniques are especially valuable for investigating the behavior of transient defects, which only appear briefly during ionic transport, leaving much about their life cycle and impact unknown.

Together with researchers from the University of California, San Diego, led by Professor Shirley Meng, and the Advanced Photon Source at the US Department of Energy’s Argonne National Laboratory, the team employed Bragg Coherent Diffractive Imaging with a highly synchronized X-ray beam to focus on the constituent parts of a charging sodium-ion battery. They created real-time 3D snapshots that revealed the morphology and atomic displacements within NaxNi1-yMnyO2 cathodes.

“Operando measurements are essential here,” Singer emphasized. “If we observed the battery before and after the initial charge-discharge cycle, we would not observe any defects. However, during operation, we can see how the defects form and self-heal, leaving detectable ‘scars’ behind.”

To explain their observations, the team drew inspiration from metals, which contain defects like dislocations that allow 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 material transformed and self-healed.

“Ceramic cathodes containing dislocations, which are one-dimensional crystal defects, are unexpected. The mechanisms responsible for their formation remain unclear,” Gorobtsov stated. “We discovered 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 shifting their focus towards understanding how the defects interact with the ions that move in and out of the battery, i.e., ionic diffusion, as it operates—a crucial mechanism for energy delivery. Singer also pointed out that the orientation of the dislocations suggests that particle shape plays a vital role in the process. Therefore, his team and collaborators aim to investigate whether this morphology can be adjusted to either facilitate or eliminate the dislocations.

“The role of extended defects in battery materials is still not fully comprehended,” Singer noted. “For centuries, blacksmiths have utilized 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.”

Source: Cornell University

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