Solid-state ionics: Review article reveals structural blueprint for superionic conductors

A central question in solid-state ionics is what structural and chemical factors enable certain solids to conduct ions as efficiently as liquids. Understanding this phenomenon is essential for developing new all-solid-state batteries that can surpass conventional lithium-ion systems in both energy density and operational safety.

However, despite decades of progress, researchers have struggled to identify consistent design principles for fast ion conductors.

"Ionic conductivity can vary widely even among materials with similar structures," explains Kartik Sau, a member of the AIMR research team. "Subtle differences in crystal framework, bottleneck geometry, and ion–ion interactions can drastically alter ion mobility. This complexity has made it difficult to move beyond empirical discovery toward predictive, theory-driven design."

In a 2024 review article, the team led by Kartik Sau and Shin-ichi Orimo examined recent advances across multiple classes of solid fast-ion conductors—layered oxides, polyhedral frameworks, and cluster-anion materials¹. Synthesizing experimental observations and theoretical insights, their review sought to clarify the fundamental relationships between structure and ion transport.

What sets this review apart is its structural perspective. Rather than classifying materials by their chemical composition, the authors organized them according to the connectivity of their anion frameworks—the structural scaffolds that define ion migration pathways.

This framework-centered view revealed universal design principles, including three-dimensional interstitial networks, polarizable anion lattices, and cooperative ion motion, that underpin fast ionic conduction across diverse material families.

"Our structural perspective allowed us to identify broader trends," says Sau. "For instance, materials with flexible, polarizable frameworks—particularly halide- and hydride-based conductors—can achieve room-temperature ionic conductivities of 25–70 mS cm^-1 and activation energies as low as 0.1–0.3 eV. Examples such as Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 (25 mS cm^-1) and Na₂(CB₉H₁₀)(CB₁₁H₁₂) (70 mS cm^-1) rival the performance of liquid electrolytes."

By consolidating structure-property relationships across diverse systems, this review provides a clear blueprint for designing next-generation solid electrolytes. Its framework-centered perspective equips researchers to engineer materials with predictable, high ionic conductivities—accelerating progress toward safer and more powerful all-solid-state batteries.

Building on these insights, the team is now focusing on anion rotation mechanisms, material alloying strategies, and machine learning approaches to accelerate the discovery of new fast-ion conductors that balance high performance with the electrochemical stability and safety required for commercial applications.

A personal insight from Dr. Kartik Sau

What part of this research gave you the greatest sense of accomplishment, and what potential societal impact do you foresee?

The most satisfying aspect was transforming decades of scattered research into a clear, usable framework for materials design. After more than a decade working in fast-ion conduction, I recognized several underappreciated directions: the role of rotating polyhedral anions, distinct design guidelines for lithium versus sodium systems, and principles for multivalent ions like Mg²⁺ and Ca²⁺. This review provided a platform to turn these insights into simple design rules. Looking ahead, I envision these guidelines accelerating safer, higher-capacity batteries for electric vehicles and grid-scale renewable storage, with machine learning opening new chemical spaces and opportunities in barocaloric cooling and advanced sensing applications.

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

About the World Premier International Research Center Initiative (WPI)

The WPI program was launched in 2007 by Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT) to foster globally visible research centers boasting the highest standards and outstanding research environments. Numbering more than a dozen and operating at institutions throughout the country, these centers are given a high degree of autonomy, allowing them to engage in innovative modes of management and research. The program is administered by the Japan Society for the Promotion of Science (JSPS).

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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.