New material discovered for safer solid-state Li-ion batteries

All-solid-state lithium-ion (Li-ion) batteries with solid electrolytes are reportedly non-flammable and have higher energy density and transference numbers than those with liquid electrolytes. However, despite these advantages, solid electrolytes have lower Li-ion conductivity and can struggle to achieve adequate electrode-solid electrolyte contact. While sulfide-based solid electrolytes are conductive, they react with moisture to form toxic hydrogen disulfide. Therefore, there is a need for non-sulfide solid electrolytes that are both conductive and stable in air to make safe, high-performance and fast-charging solid-state Li-ion batteries.

A team of researchers, led by Professor Kenjiro Fujimoto from the Tokyo University of Science, have discovered a stable and highly conductive Li-ion conductor in the form of a pyrochlore-type oxyfluoride. Their research findings have been published in the journal Chemistry of Materials.

According to Fujimoto, the researchers 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.

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, Li1.25La0.58Nb2O6F 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. It was found to be higher than the lithium-ion conductivity of known oxide solid electrolytes. The activation energy of ionic conduction of this material is low and the ionic conductivity of this material at low temperature is reportedly one of the highest among known solid electrolytes, including sulfide-based materials.

At -10°C, the new material has the same conductivity as conventional oxide-based solid electrolytes at room temperature. Since conductivity above 100°C has also been verified, the operating range of this solid electrolyte is -10 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 to 45°C.

The researchers investigated the Li-ion conduction mechanism in the new material and found that the conduction path of pyrochlore-type structure covers the F ions located in the tunnels created by MO₆ octahedra. The conduction mechanism is the sequential movement of Li-ions while changing bonds with F ions. Li-ions move to the nearest Li position, always passing through metastable positions. The immobile La³⁺ bonded to F ion inhibits the Li-ion conduction by blocking the conduction path and vanishing the surrounding metastable positions.

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 sulfide-based batteries.

“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. We believe that the performance required for the application of solid electrolytes for electric vehicles is satisfied,” Fujimoto said.

The new material is highly stable and will not ignite if damaged. It is suitable for airplanes and high-capacity applications such as electric vehicles, because it can be used under high temperatures and supports rapid recharging. It is also a promising material for the miniaturisation of batteries, home appliances and medical devices.

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