Safer solid electrolyte hits highest-ever 16.3 mS/cm lithium-ion conductivity

Safer solid electrolyte for lithium-ion batteries has reached a record conductivity of 16.3 millisiemens per centimeter (mS/cm) after scientists uncovered how lithium ions move through the material. Researchers at Nagoya University in Japan found that a fluoride ion inside the material shifts slightly whenever a lithium ion moves to a vacant site. This movement effectively clears a path for lithium ions, lowering the energy barrier needed for them to travel through the electrolyte. The finding could help researchers develop solid-state batteries that combine the safety of oxide-based materials with the high ion conductivity typically associated with sulfide-based electrolytes. The material, known as LLNOF, had already attracted attention in 2024 after researchers reported a conductivity of 7 mS/cm. However, the reason for its unusually fast lithium-ion movement was unclear. Fluoride makes room Solid electrolytes are being studied as alternatives to the liquid electrolytes used in conventional lithium-ion batteries. Liquid electrolytes can catch fire if a battery is damaged, while some high-conductivity sulfide and chloride electrolytes can release toxic gases when exposed to moisture. Oxide and oxyfluoride materials are generally more robust and electrochemically stable, but their lithium-ion conductivity has typically been lower. “At this stage, safety and ionic conductivity are a trade-off,” said Takeshi Yajima, an associate professor at Nagoya University’s Department of Materials Design Innovation Engineering. “Oxyfluorides are safer but have low conductivity, while sulfides have high conductivity but can be dangerous.” To understand LLNOF, Yajima’s team first had to produce high-quality single crystals suitable for detailed structural analysis. The researchers spent more than a year developing millimeter-sized crystals using a technique called the Bridgman method. They then used single-crystal diffraction to examine how atoms are arranged and how their positions change inside the material. The analysis showed that four atomic sites form a tetrahedral structure around a central fluoride ion. The sites can contain lithium, lanthanum, or remain vacant. When a lithium ion jumps into a vacant site, the fluoride ion shifts toward the lithium ion’s previous location. That small movement changes the local structure and reduces the energy barrier for the next lithium-ion jump. The fluoride ion is therefore not simply sitting in place while lithium moves through the material. Its movement actively helps lithium ions travel through the crystal. Composition boosts conductivity The researchers then changed the material’s composition by adjusting the relative amounts of lithium, lanthanum and vacant sites. This variable, represented by x in the material’s chemical formula, had a major effect on conductivity. By lowering x, the team increased the material’s conductivity to 16.3 mS/cm. The researchers say this is the highest reported bulk lithium-ion conductivity among oxide-related solid electrolytes. The result challenges the conventional idea that high conductivity depends mainly on highly deformable negative ions, such as those found in sulfide-based electrolytes. “The general understanding has been that sulfide-based materials are better conductors because of their anion character, but this mechanism challenges that understanding,” Yajima said. The newly identified mechanism could provide another route for designing oxide-based solid electrolytes with faster lithium-ion transport. Such materials could help address one of the key challenges in developing practical solid-state batteries: achieving high conductivity without compromising safety.Get the latest in engineering, tech, space & science - delivered daily to your inbox.With over a decade-long career in journalism, Neetika Walter has worked with The Economic Times, ANI, and Hindustan Times, covering politics, business, technology, and the clean energy sector. Passionate about contemporary culture, books, poetry, and storytelling, she brings depth and insight to her writing. When she isn’t chasing stories, she’s likely lost in a book or enjoying the company of her dogs.