SPring-8, the large synchrotron radiation facility

It has been difficult to visualize at the atomic level the structure of materials containing both crystalline and amorphous phases with a disordered crystal structure. In this study, the research group developed a new method of structural analysis based on X-ray diffraction using high-brilliance synchrotron radiation. Applying this method to the structure of Li₂VO₂F cathodes during charge and discharge processes, they succeeded in analyzing precisely the structure of amorphous phases. As a result, the relationship between the property degradation of secondary batteries during charge and discharge processes and the material structure was clarified.

Cathodes of lithium-ion secondary batteries are essential components that determine the performance of batteries. Material development is aimed at further improving the high energy density and the endurance to charge and discharge cycles required for cathode materials. The materials used in cathode components have high endurance to charge and discharge cycles. However, the battery capacity usually decreases as the number of these cycles increases in practical applications. It had been considered that the crystal structure is partly broken as a result of repeated charge and discharge processes, forming a disordered atomic arrangement (amorphous phase). However, the mixed structure of crystalline and amorphous phases had not been analyzed, and technology to extract the structural information of crystalline and amorphous phases had been desired.

The research group newly developed a method to separate the structural information of each component from the X-ray diffraction data for the mixed structure of crystalline and amorphous phases and to analyze that structural information. They applied this analytical method to Li₂VO₂F, a cathode material with a high energy density but low endurance to charge and discharge cycles, and precisely analyzed the X-ray diffraction data obtained using the high-brilliance and high-energy X-rays at SPring-8. As a result, they visualized the atomic arrangement in amorphous phases observed in the cathode during a charge process where Li ions move from the cathode to the anode. The atomic arrangement was found to be of a tetrahedral structure with a V ion at the center and O and F ions at the vertices. Li ions act as the “glue” that binds those tetrahedra into crystals. When Li ions are released from the cathode during the charge process, nearby V, O, and F ions are dissolved from crystals and form amorphous phases. Such a picture of the formation of amorphous phases was revealed for the first time. The tetrahedra revert to crystals when Li ions are supplied to the cathode during a discharge process. However, the spatial distortion between V ions and Li ions disappears in the re-formed crystalline phases. As a result, the disorder of the atomic arrangement is decreased, which seems to result in a decrease in battery capacity.

These findings will enable not only the improvement of lithium-ion battery performance but also the atomic-level understanding of disordered structures of other secondary batteries whose development is becoming increasingly competitive. In the future, these achievements will contribute to the improvement of battery capacity and endurance and the understanding of charge and discharge mechanisms.

This study was conducted by a joint research group that includes Satoshi Hiroi (Postdoctoral Researcher), Koji Ohara (Senior Scientist), and Osami Sakata (Director) of JASRI. The results of this study were published in the online edition of Chemistry of Materials (a US scientific journal) on 16 July 2021.

Reference
Title：Structural Characterization of Delithiated Non-crystalline Phase in Li-rich Li₂VO₂F Cathode Material
Authors：Satoshi Hiroi, Koji Ohara and Osami Sakata
Journal：Chemistry of Materials
Publication Date：16 July 2021
DOI：10.1021/acs.chemmater.1c01466