NASICON solid electrolytes: Relative density links strength and conductivity

Sodium superionic conductors with NASICON-type structures—such as Na₁₊ₓZr₂Si_xP₃₋ₓO₁₂ (NZSP)—are promising solid electrolytes for solid-state sodium batteries due to their high ionic conductivity, wide electrochemical window, and compatibility with sodium metal. Yet, translating these materials into practical applications requires balancing two fundamentally different properties: ionic conductivity, which depends on atomic-scale ion mobility, and mechanical strength, which is shaped by microstructural features like porosity and grain bonding.

However, because these properties arise from distinct mechanisms, their relationship is not straightforward. In fact, past efforts to improve one often compromised the other. As a result, establishing a general, statistically robust correlation between the two properties has remained a key challenge in the design of high-performance solid electrolytes.

In a 2024 viewpoint article, researchers at Tohoku University’s Advanced Institute for Materials Research (AIMR) conducted a comprehensive meta-analysis of experimental data from the literature¹. The team focused on sintered polycrystalline NZSP, a widely studied NASICON solid electrolyte, and applied statistical methods to evaluate the relationship between its mechanical properties—particularly hardness—and ionic conductivity across various compositions.

“We took a novel approach by emphasizing the role of relative density—a measure of how dense a material is compared to its theoretical maximum,” explains Eric Jianfeng Cheng, first author of the article. “We found that this microstructural parameter consistently influences both hardness and ionic conductivity across the dataset, more reliably than factors like doping or grain size.”

One key finding of the article is that hardness, a readily measurable mechanical property, can serve as a reliable predictor of ionic conductivity in oxide solid electrolytes. This insight provides researchers with a practical and efficient screening metric, and points to relative density optimization—through techniques like spark plasma sintering or sintering aid selection—as a unified strategy to improve both durability and performance in next-generation sodium-ion batteries.

“Following these results, one of our future directions is to further investigate the interplay between mechanical and electrochemical properties in solid electrolytes,” says Cheng. “This includes exploring potential approaches such as atomic-scale analysis to deepen the understanding of the underlying correlations.”

A personal insight from Dr. Eric Jianfeng Cheng

What part of this research gave you the greatest sense of accomplishment, and why?

The most rewarding part of this work was proposing—and then finding preliminary evidence for—a positive correlation between hardness and ionic conductivity in NASICON-type solid electrolytes. These two properties are typically treated as separate, even competing, so it was exciting to uncover a potential link between them. Because this connection wasn’t obvious at first, seeing it emerge through careful data analysis gave me a real sense of accomplishment. It suggests a path forward that could help guide future materials research in a more integrated way.

This article was written by Patrick Han, Ph.D. (patrick@sayedit.com).

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Advanced Institute for Materials Research (AIMR)
Tohoku University
Establishing a World-Leading Research Center for Materials Science
AIMR aims to contribute to society through its actions as a world-leading research center for materials science and push the boundaries of research frontiers. To this end, the institute gathers excellent researchers in the fields of physics, chemistry, materials science, engineering, and mathematics and provides a world-class research environment.