Scientists have discovered a stable and highly conductive lithium-ion conductor for use as solid electrolytes for solid-state lithium ion battery. All-solid-state lithium ion battery with solid electrolytes are non-flammable and have higher energy density and transference numbers than those with liquid electrolytes. They are expected to take a share of the market for conventional liquid electrolyte Li-ion batteries, such as electric vehicles.
However, despite these advantages, solid electrolytes have lower Li-ion conductivity and pose challenges in achieving adequate electrode-solid electrolyte contact. While sulfide-based solid electrolytes are conductive, they react with moisture to form toxic hydrogen disulfide. Therefore, there’s 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.
In a recent study published in Chemistry of Materials on 28 March 2024, a research team led by Professor Kenjiro Fujimoto, Professor Akihisa Aimi from Tokyo University of Science, and Dr. Shuhei Yoshida from Denso Corporation, discovered a stable and highly conductive Li-ion conductor in the form of a pyrochlore-type oxyfluoride.
According to Prof. Fujimoto, “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.”

The pyrochlore-type oxyfluoride studied in this work can be denoted as Li2-xLa(1+x)/3M2O6F (M = Nb, Ta). It 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 extremely low, and the ionic conductivity of this material at low temperature is one of the highest among known solid electrolytes, including sulfide-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.
The Li-ion conduction mechanism in this material was investigated. The conduction path of pyrochlore-type structure cover 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. 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. Therefore, this new innovation is anticipated to propel 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,” says Prof. Fujimoto. “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 ignite if damaged. 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 miniaturization of batteries, home appliances, and medical devices.
In summary, researchers have not only discovered a Li-ion conductor with high conductivity and air stability but also introduced a new type of superionic conductor with a pyrochlore-type oxyfluoride. Exploring the local structure around lithium, their dynamic changes during conduction, and their potential as solid electrolytes for all-solid-state batteries are important areas for future research.
More information: Akihisa Aimi et al, High Li-Ion Conductivity in Pyrochlore-Type Solid Electrolyte Li2–xLa(1+x)/3M2O6F (M = Nb, Ta), Chemistry of Materials (2024). DOI: 10.1021/acs.chemmater.3c03288
Journal information: Chemistry of Materials

In our technologically dependent society, the mobility, dependability, and safety of our devices—including phones and laptops—are critical. Just as important is our ability to easily charge and recharge these devices so they are available when we need them. To do this, we use rechargeable batteries, specifically lithium batterie.

They give us the freedom of movement and connectivity we need. As society’s needs evolve, so too does our tech, and so too must the batteries that allow us to use this tech. One of the most urgent concerns regarding lithium-ion batteries is their safety. Though rare, there are issues with explosions and fires caused by electrochemical system instability.

“Consequently, there is an urgent need to develop LIBs that can provide higher energy density, longer cycle life, and improved safety,” said Ying Bai, corresponding author of new research on this topic and a professor at the Beijing Institute of Technology in China.

Beijing scientists have been researching the use of additives in the sulfone-based electrolyte of  lithium batterie to improve their performance. They found that by adding triphenylphosphine oxide (TPPO), “the TPPO improves the thermal stability of the electrolyte, which has important industrial value and foundational significance of TPPO as an additive for advancing the development of LIB’s,” said Chuan Wu, co-corresponding author on the research and a professor at Beijing Institute of Technology.

The team’s paper is published in Energy Materials and Devices.

When lithium batterie is discharging lithium-ions, they move from an anode, which is an electrode where current enters the battery, through an electrolyte that passes through a separator to a cathode, which is where the current leaves the storage battery to energize a device. The path is reversed when recharging.

“In the composition of the battery, the non-aqueous electrolyte used in LIBs plays a crucial role in determining key performance parameters such as cycle life, power density, and efficiency,” said Ying Bai. Power density is a measure of stored power per volume, and cycle life is the number of charge/discharge cycles that a battery can undergo before it starts to decrease the percentage of charge it can hold.

The electrolyte solutions in use now have some issues with cycle stability, thermal stability, and safety. Rather than completely changing the electrolyte solution, the team chose to test the use of an additive, TPPO, in the electrolyte to improve the performance of the overall battery.

When tested, TPPO was found to have several important properties.

“Firstly, it reduces the flame point of the sulfone electrolyte; Secondly, it selectively forms a stable passivation film, enhancing the interface stability between the sulfone electrolyte and the electrode material,” said Chuan Wu. The passivation film forms as the TPPO decomposes and coats the cathode, rendering it more resistant to wear and tear, similarly reducing the electrolyte’s breakdown while enhancing the lithium ions’ movement across the electrolyte.

Using theoretical calculations, electrochemical characterization, and flammability tests, the researchers found “that the addition of 2 wt.% TPPO to the sulfone-based electrolyte significantly enhances the ionic conductivity within the temperature range of 20–60°C.”

“Additionally, it increases the discharge capacity of LIBs in the range of 2–4.8 V while maintaining excellent rate performance and cycling stability. Flammability tests and thermal gravimetric analysis (TGA) results indicate the excellent non-flammability and thermal stability of the electrolyte,” said Ying Bai.

In short, the new electrolyte that they have developed is safer as it is non-flammable, is thermally stable and has an increased energy discharge capacity.

More information: Qiaojun Li et al, Enhanced safety of sulfone-based electrolytes for lithium batterie: broadening electrochemical window and enhancing thermal stability, Energy Materials and Devices (2024). DOI: 10.26599/EMD.2023.9370022

Provided by Tsinghua University Press

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