All-solid-state lithium-ion (Li-ion) batteries with solid electrolytes are non-flammable and have higher energy density and transference numbers than those with liquid electrolytes. These advancements position them to compete in the market traditionally dominated by conventional liquid electrolyte Li-ion batteries, especially in sectors such as electric vehicles.
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However, despite these advantages, solid electrolytes have lower Li-ion conductivity and pose challenges in achieving adequate electrode-solid electrolyte contact.
Solid electrolytes based on sulfides are conductive, but when they react with moisture, they generate toxic hydrogen disulfide. As a result, non-sulfide solid electrolytes that are both conductive and air-stable are required to create safe, high-performance, and fast-charging solid-state Li-ion batteries.
A research team headed by Professor Kenjiro Fujimoto, Professor Akihisa Aimi from Tokyo University of Science, and Dr. Shuhei Yoshida from DENSO CORPORATION observed a stable and highly conductive Li-ion conductor in the shape of a pyrochlore-type oxyfluoride in a recent study that was published in Chemistry of Materials on March 28th, 2024.
Making all-solid-state lithium-ion secondary batteries have 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.
Kenjiro Fujimoto, Professor, Tokyo University of Science
The pyrochlore-type oxyfluoride investigated in this study is Li2–xLa(1+x)/3M2O6F (M = Nb, Ta). It was structurally and compositionally analyzed using a variety of methods, including X-Ray diffraction, Rietveld analysis, inductively coupled plasma optical emission spectrometry, and selected-area electron diffraction.
Li1.25La0.58Nb2O6F was produced with a bulk ionic conductivity of 7.0 mS cm⁻¹ and a total ionic conductivity of 3.9 mS cm⁻¹ at ambient 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 extremely low, and the ionic conductivity of this material at low temperatures is one of the highest among known solid electrolytes, including sulfide-based materials.
Even at –10 °C, the novel material has the same conductivity as standard oxide-based solid electrolytes at normal temperatures. Furthermore, since conductivity above 100 °C has been established, this solid electrolyte’s operational temperature range is –10 °C to 100 °C. Conventional lithium-ion batteries cannot operate at temperatures below freezing. As a result, lithium-ion batteries for frequently used mobile phones operate at temperatures ranging from 0 to 45 °C.
The Li-ion conduction mechanism in this material was analyzed. The conduction pathway within the pyrochlore-type structure covers the fluoride ions positioned in the tunnels created by MO6 octahedra. The conduction mechanism involves the sequential movement of lithium ions, exchanging bonds with fluoride ions as they progress.
Li ions migrate to the closest Li position, always passing through metastable positions. Immobile La3+ bonded to F ion impedes the Li-ion conduction by obstructing the conduction pathway and eliminating the nearby metastable positions.
Unlike conventional lithium-ion secondary batteries, oxide-based all-solid-state batteries eliminate the risk of electrolyte leakage from damage and the potential generation of toxic gases found in sulfide-based batteries. This innovation is poised to spearhead future research in battery technology.
“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 added.
The new material boasts remarkable stability, ensuring it remains non-flammable even under damage, making it particularly well-suited for critical safety applications, such as aviation. Its resilience to high temperatures and ability to support rapid recharging also make it a good choice for high-capacity uses like electric vehicles.
Moreover, its potential for miniaturization opens up exciting possibilities for advancements in various sectors, including home appliances and medical devices.
In conclusion, researchers have not only developed a Li-ion conductor with excellent conductivity and air stability but also introduced a new form of superionic conductor using pyrochlore-type oxyfluoride.
Future research should prioritize delving into the intricacies of the local structure surrounding lithium, comprehending its dynamic changes during conduction, and evaluating its viability as solid electrolytes for all-solid-state batteries.
Journal Reference:
Aimi, A., et. al. (2024) High Li-Ion Conductivity in Pyrochlore-Type Solid Electrolyte Li2–xLa(1+x)/3M2O6F (M = Nb, Ta). Chemistry of Materials. doi:10.1021/acs.chemmater.3c03288
Source: https://www.tus.ac.jp/en/