Crystallographic Effects on High Li+ Transportation Properties of LiCoO2
Authors: Yamada Tetsuya , Teshima Katsuya 
Published: February 2020

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.