Revolutionary Material Found for Safer Li-ion Solid-State Batteries

According to reports, all-solid-state lithium-ion (Li-ion) batteries equipped with solid electrolytes are safer and have a higher energy density and transference numbers compared to those using liquid electrolytes. However, solid electrolytes typically have lower Li-ion conductivity and may face challenges in establishing proper contact with the electrode. While sulfide-based solid electrolytes are conductive, they can produce harmful hydrogen disulfide when exposed to moisture. As a result, there is a growing demand for non-sulfide solid electrolytes that possess both conductivity and stability in air in order to create secure, high-performance, and fast-charging solid-state Li-ion batteries.

According to the study published in the Chemistry of Materials, a group of scientists, headed by Professor Kenjiro Fujimoto from Tokyo University of Science, have uncovered a stable and highly conductive Li-ion conductor in the pyrochlore-type oxyfluoride form.

Fujimoto states that a team of researchers has identified an oxide solid electrolyte that plays a crucial role in all-solid-state lithium-ion batteries. These batteries are known for their high energy density and safety. According to Fujimoto, the discovered material not only possesses stability in air, but also exhibits a higher level of ionic conductivity compared to previously reported oxide solid electrolytes.

In this study, the pyrochlore-type oxyfluoride was subjected to investigation through various methods such as X-ray diffraction, Rietveld analysis, inductively coupled plasma optical emission spectrometry, and selected-area electron diffraction. The development of Li1.25La0.58Nb2O6F was also demonstrated, showing 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 significantly higher than the lithium-ion conductivity of other known oxide solid electrolytes. The ionic conduction activation energy of this material is low and its ionic conductivity at low temperature is considered to be one of the highest among all known solid electrolytes, including those made of sulfide-based materials.

The new material has equivalent conductivity to traditional oxide-based solid electrolytes at room temperature, even at -10°C. This solid electrolyte can function within a temperature range of -10 to 100°C, as its conductivity has been tested and confirmed above 100°C. Unlike conventional lithium-ion batteries, which are unable to operate in freezing temperatures, this solid electrolyte is suitable for use in a wider range of temperatures, specifically 0 to 45°C, commonly found in mobile phones.

The conduction mechanism of Li-ion in the novel material was studied by the researchers, who discovered that the path of conduction in the pyrochlore-type structure involves F ions situated in the tunnels formed by MO6 octahedra. The mechanism of conduction involves the sequential movement of Li-ions, as they form and break bonds with F ions. The Li-ions move to the closest Li position, passing through unstable positions each time. The presence of immobile La3+ bonded to F ions hinders the conduction of Li-ions by obstructing the path of conduction and eliminating the nearby unstable positions.

Compared to traditional lithium-ion secondary batteries, all-solid-state batteries made of oxides do not pose a risk of electrolyte leakage or the production of toxic gases, which is a common issue with sulfide-based batteries.

According to Fujimoto, the recently uncovered substance is secure and displays superior ionic conduction compared to previously documented oxide-based electrolytes. This material shows potential for creating groundbreaking batteries that can function in both low and high temperatures. It is believed that this material meets the necessary standards for solid electrolyte use in electric vehicles.

The recently developed substance is extremely durable and will not catch fire in the event of any harm. Due to its ability to withstand high temperatures and facilitate quick charging, it is well-suited for use in airplanes and high-capacity purposes like electric cars. Additionally, it shows great potential for downsizing batteries, household appliances, and medical equipment.