Development of high-power density battery is one of important issues for managing high-performance energetic applications including electric vehicles, rescue robots, elevating machines, and so on. Lithium cobalt oxide, LiCoO₂, is one of the most popular active materials in lithium ion secondary batteries. LiCoO₂ consists of LiO₂ layers and CoO₂ layers stacking along to c axis, resulting into a rock-salt layered structure with hexagonal system. The layer structured LiCoO₂ potentially provides efficient Li⁺ transport along a,b-axes, exhibiting high-output power as electrodes. For application of LiCoO₂ as high-output batteries, there are still some issues to be solved. Usually, LiCoO₂ was prepared by solid-state reaction, which provided a few micron-size polycrystals with inhomogeneous distribution in shapes. At high-speed Li⁺ transportations of the usual LiCoO₂ particles, it is presumed that the LiCoO₂ undergoes irreversible phase transition and cracks of the particles, which should be caused by local overcurrent and volume changes during lithiation / delithiation. Since these electrochemical overloads to the usual LiCoO₂ particles lead serious degradation of the cycle abilities, improvements of LiCoO₂ are inevitable.

There are two kinds of approaches for the electrochemical improvement. The first one is modification of chemical compositions, including doping with other elements, coating with inactive materials, and the second one is control of crystallographic characteristics, such as crystal habits, particle dispersibility, and sizes. Combining them, synergetic improvement of LiCoO₂ toward high-output battery would be possible. Recently, we have grown LiCoO₂ single crystals by using flux method, which is one of liquid-phase crystal growth techniques.[1] Exhibiting submicron-size, low-aspect ratio, and high crystalline natures, the LiCoO₂ single crystals would show less amounts of grain boundaries, shorter diffusion pathways, and more homogeneous lithiation / delithiation at the interface between LiCoO₂ and conductive materials, compared to the usual particles. It is expected that these advantages of the LiCoO₂ single crystals inhibit the unfavorable electrochemical changes.

In this study, we examined crystallographic effects on Li⁺ transportation properties of LiCoO₂ at fast electrochemical load. The flux grown LiCoO₂ single crystals were applied as an active materials for high-rate batteries. The high-rate cycle abilities up to 20C rate were examined. Their electrochemical degradation characteristics would be discussed by investing macroscopic and microscopic particle features and resistence at each electrochemical cycling. These results would be summalized to suggest dominant crystallographic factors for developing active materials applicable to high electrochemical load.

References

[1] Teshima, S. Lee, Y. Mizuno, H. Inagaki, M. Hozumi, K. Kohama, K. Yubuta, T. Shishido, S. Oishi, Cryst. Growth Des., 2010, 10, 4471-4475.

Acknowledgement

This work was supported by JST CREST Grant Number JPMJCR1322 in Japan and Program for Building Regional Innovation Ecosystems of MEXT.