Composition and Conductivity Evaluation of Cathode Active Materials/Solid Electrolytes for Lithium-Ion Batteries
Authors: Teranishi Riku , Tanaka Atsushi , Yamada Tetsuya , Hayashi Fumitaka , Teshima Katsuya 
Published: December 2024

Solid-state batteries have attracted considerable attention as innovative secondary batteries for future electric energy societies due to their high safety and energy density. In particular, sulphide-based solid electrolytes are positioned as promising materials for solid-state batteries due to their excellent ion conductivity properties. However, a challenge exists with sulfide-based solid electrolytes, as high lithium-ion conductivity resistance occurs upon contact with the positive electrode active material due to the formation of a lithium-ion deficiency layer. Previous studies have shown that reducing interface resistance can be reduced by coating the active material with lithium-ion conductors such as LiNbO3. It is believed that by structuring the active material and solid electrolyte to conduct ions through a buffer layer, the lithium ion deficiency can be reduced, and the electrical potential barrier of lithium ions near the interface can be suppressed. However, the ion conduction mechanism at the interface between the active material and solid electrolyte formed by uniformly coating of the active material has not been sufficiently elucidated.

This study aims to elucidate the ion conduction mechanism at the interface between the positive electrode active material crystal and the solid electrolyte by achieving uniform coating of a buffering layer. We aim to achieve uniform coating driven by the evaporation of a mixed solution of LiTaO3 dispersed in water and LiCoO2 crystals. Furthermore, by preparing electrodes using only binders without the use of conductive agents via a paste method, we aim to reduce factors related to resistance and elucidate the ion conduction mechanism at specific interfaces.

For the coating of the buffer layer on the active material crystal, a LiTaO3 water solution obtained from an external source was prepared, and a LiCoO2 crystal dispersion solution was prepared, and the influence on coating under different conditions of LiTaO3 concentration and heat treatment temperature for LiCoO2 crystals was investigated.

Scanning electron microscope (SEM) observations showed no change in the coating morphology under temperature conditions of 300 to 500°C. Additionally, X-ray photoelectron spectroscopy (XPS) showed no change in the dependence of the Ta bonding state on temperature conditions. However, it was confirmed that the weight ratio of Ta coated on the surface increased with the increasing concentration of LiTaO3. Subsequently, electrodes were prepared using slurry from synthesized active material/buffering layer composites, and ion conduction resistance was measured using impedance measurements. Ion conduction resistance dependent on the amount of coating was observed in composites synthesized at 400°C and 500°C. In composites synthesized at 300°C, a tendency for higher ion conduction resistance with less coating and an overall high resistance value were observed. Moreover, differential relaxation time (DRT) analysis revealed that the relaxation frequency of the main resistance component changed with increasing heat treatment temperature. However, no shift in relaxation frequency of resistance components was observed with coating amount.

From these results, it was revealed that coating crystal morphology changes depending on the temperature conditions and coating amount changes depending on the concentration. Furthermore, synthesis of composites at high temperatures reduced Li-ion conduction resistance. This is believed to be due to atomic diffusion occurring at high temperatures during heat treatment, resulting in Ta bonding to the CoO2 structure on the surface of LiCoO2, continuously joining the interfaces. Additionally, it is believed that the reduction in grain boundary resistance occurred due to particle densification. In the future, solid-state batteries using sulfide-based solid electrolytes will be fabricated to investigate the suppression of lithium-ion conduction resistance and its conduction mechanism.

Acknowledgement

This work was supported in part by Grant-in-Aid for Scientific Research (22H01847), Aichi Priority Research Program, the Strategic Innovation Program (SIP) of the Cabinet Office and by joint researches with industries.