A recently discovered, stable and highly conductive lithium-ion conductor in the form of a pyrochlore-type oxyfluoride has been found to address the need for non-sulphide solid electrolytes in solid-state batteries, offering higher conductivity and stability and paving the way for advanced all-solid-state lithium-ion batteries with improved performance and safety.
All-solid-state lithium-ion batteries with solid electrolytes are non-flammable and have higher energy density and transference numbers than those with liquid electrolytes. However, solid electrolytes have lower Li-ion conductivity and pose challenges in achieving adequate electrode-solid electrolyte contact.
While sulphide-based solid electrolytes are conductive, they react with moisture to form toxic hydrogen disulphide. Therefore, there’s a need for non-sulphide solid electrolytes that are both conductive and stable in air to make safe, high-performance, and fast-charging solid-state Li-ion batteries.
This is where the discovery comes in.
Published in the journal Chemistry of Materials, the finding resulted from a collaboration between Kenjiro Fujimoto, a professor at Tokyo University of Science, and Shuhei Yoshida from Denso Corp.
“Making all-solid-state lithium-ion secondary batteries has been a long-held dream of many battery researchers. We have discovered an oxide solid electrolyte that is a key component of all-solid-state lithium-ion batteries, which have both high energy density and safety. In addition to being stable in air, the material exhibits higher ionic conductivity than previously reported oxide solid electrolytes,” Fujimoto said.
Suitable for airplanes
The pyrochlore-type oxyfluoride studied in this work underwent structural and compositional analysis using various techniques, including X-ray diffraction, Rietveld analysis, inductively coupled plasma optical emission spectrometry, and selected-area electron diffraction. Specifically, it was developed, demonstrating a bulk ionic conductivity of 7.0 mS cm⁻¹ and a total ionic conductivity of 3.9 mS cm⁻¹ at room temperature. This is higher than the lithium-ion conductivity of known oxide solid electrolytes. The activation energy of ionic conduction of this material is extremely low, and the ionic conductivity of this material at low temperatures is one of the highest among known solid electrolytes, including sulphide-based materials.
Even at –10°C, the new material has the same conductivity as conventional oxide-based solid electrolytes at room temperature. Furthermore, since conductivity above 100 °C has also been verified, the operating range of this solid electrolyte is –10 °C to 100 °C. Conventional lithium-ion batteries cannot be used at temperatures below freezing. Therefore, the operating conditions of lithium-ion batteries for commonly used mobile phones are 0 °C to 45 °C.
Unlike existing lithium-ion secondary batteries, oxide-based all-solid-state batteries have no risk of electrolyte leakage due to damage and no risk of toxic gas generation as with sulphide-based batteries. Therefore, this innovation is anticipated to lead future research.
“The newly discovered material is safe and exhibits higher ionic conductivity than previously reported oxide-based solid electrolytes. The application of this material is promising for the development of revolutionary batteries that can operate in a wide range of temperatures, from low to high,” Fujimoto said. “We believe that the performance required for the application of solid electrolytes for electric vehicles is satisfied.”
Notably, the new material is highly stable and will not catch fire if damaged.
According to the researchers, it is suitable for airplanes and other places where safety is critical. It is also suitable for high-capacity applications, such as electric vehicles, because it can be used under high temperatures and supports rapid recharging. Moreover, it is also a promising material for the miniaturization of batteries, home appliances, and medical devices.