Oxyfluoride Crystal Advances Solid Batteries To 16.3 mS/cm

Researchers at Nagoya University have grown an oxyfluoride crystal with a lithium-ion conductivity of 16.3 millisiemens per centimetre, the highest reported for oxide-related solid electrolytes.

A research team at Nagoya University has grown millimetre-sized crystals of a lithium lanthanum niobium oxyfluoride solid electrolyte, known as LLNOF. At room temperature, the material reached a bulk lithium-ion conductivity of 16.3 millisiemens per centimetre (mS/cm), reported as the highest yet achieved among oxide-related solid electrolytes. The work was led by Associate Professor Takeshi Yajima.

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“At this stage, safety and ionic conductivity are a trade-off,” said Yajima, 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.”

The same LLNOF material family was first reported in 2024 with an ionic conductivity of around 7 mS/cm. The new crystals reach 16.3 mS/cm, more than doubling that figure. According to the research team, this is the highest bulk lithium-ion conductivity reported for an oxide-related solid electrolyte.

The gain comes from the material’s fluoride-ion relaxation mechanism. In LLNOF, fluoride ions do not remain completely fixed as lithium ions move through the crystal. When a lithium ion hops to a vacant site, nearby fluoride ions shift locally, lowering the energy barrier for lithium-ion movement. The researchers describe this coupled motion as a key reason for LLNOF’s unusually high ionic conductivity.

This differs from mechanisms commonly associated with fast sulfide electrolytes, where highly polarizable sulfur ions help create favourable pathways for lithium-ion transport. LLNOF demonstrates that high conductivity can also be achieved through local anion relaxation in an oxyfluoride structure.

This is laboratory-stage materials research, not a complete battery demonstration. The team grew millimetre-sized LLNOF single crystals and measured their intrinsic bulk lithium-ion conductivity, reaching 16.3 mS/cm at 25°C. Single crystals are useful for studying fundamental transport properties because they eliminate grain boundaries, but they are not directly equivalent to a practical battery electrolyte.

A commercial solid-state battery would require the material to be manufactured as a thin, dense electrolyte layer and integrated with suitable electrodes. The study does not demonstrate a complete battery cell or report cell-level cycling performance, so significant work would still be needed before LLNOF could become a practical battery technology.

For India, an air-stable solid electrolyte could eventually be attractive because some sulfide electrolytes require careful moisture-controlled handling. However, LLNOF is still a laboratory material, and the manufacturing requirements for a practical LLNOF-based solid-state battery have not yet been established. It is therefore too early to claim that the material would eliminate dry-room infrastructure or substantially reduce factory costs.

There is also a supply-chain trade-off. A future LLNOF battery chemistry would depend on materials including niobium and lanthanum, so domestic cell manufacturing would not necessarily mean complete independence from imported raw materials. India’s battery manufacturing programme also faces broader challenges involving critical-mineral supply chains and domestic materials processing.

For now, this is a materials result rather than a battery product. But it pushes oxide-related solid electrolytes into a conductivity range that could make them more relevant for future solid-state batteries.

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