LiCoO2 degradation Behavior in the High-Voltage Phase-Transition Region and Improved Reversibility with Surface Coating
Authors: Yano Akira , Shikano Masahiro , Ueda Atsushi , Sakaebe Hikari , Ogumi Zempachi 
Published: February 2020

Introduction
Layered LiCoO2, which is the most common cathode material used for Li-ion batteries, exhibits phase transitions from the O3 phase to the H1-3 and O1 phases at charge voltages exceeding 4.5 V (vs. Li+/Li) [1]. These phase transitions were believed to result in poor charge/discharge reversibility in the high-voltage region [2]; however, the detailed degradation mechanism has not been elucidated. Additionally, the charge/discharge performance of active materials can be improved with surface coatings. Certain types of coating have been reported to suppress the capacity fade exhibited by LiCoO2, even above 4.5 V [3]. In this study, we examined in detail the degradation behavior of LiCoO2and the suppression effect of a coating at a charge voltage of 4.7 V.
Experimental
LiCoO
2
powder with a secondary particle diameter of 7 μm (Nippon Chemical Industrial) was used as the active material. An Al-oxide coating was formed using a sol-gel method [4]. The surface structures of bare and coated LiCoO2 samples were evaluated using field emission scanning electron microscopy and Cs-corrected scanning transmission electron microscopy (Cs-STEM) combined with energy-dispersive X-ray spectroscopy. The electrochemical characteristics of the samples were examined in pouch cells with a Li-metal counter electrode. A 1.0 mol dm-3 of LiPF6 in ethylene carbonate+diethyl carbonate (EC+DEC; 1:2 by volume) was used as the electrolyte. The cells were cycled between 2.5 and 4.7 V. Initial charge/discharge and cycle performance were evaluated at current densities of 0.05 and 0.2 C, respectively. Here, 1 C was defined as 274 mAh g-1, which is the theoretical capacity of LiCoO2.
Results and Discussion
Figure 1 shows the charge/discharge curves of bare and coated LiCoO2 during the cycle test. The bare LiCoO2 exhibits considerably decreased capacity and increased polarization, whereas these changes are remarkably suppressed in coated LiCoO2. Because coated LiCoO2 shows voltage plateaus in the relatively high-voltage region between the 1st and 20thcycles, phase transitions in this region can be assumed to be fairly reversible.
Figure 2 shows a cross-sectional transmission electron microscopy (TEM) image of bare LiCoO2 after the cycle test. The entire surface of the bare LiCoO2 sample is covered by an altered layer with a thickness of several nanometers. High-resolution STEM revealed that this altered layer has a spinel-like (Co3O4-like) crystal structure, which should exhibit poor Li-ion conductivity. This structure could explain why the formation of this layer substantially increases the polarization of bare LiCoO2. Furthermore, instances of pitting corrosion (approximately 10 nm in size) occur at the grain-boundary surfaces of bare LiCoO2 (Fig. 2). These result from Co dissolution into the electrolyte and lead to intrinsic capacity fade. In contrast, formation of the spinel-like layer and pitting corrosion were suppressed in the coated LiCoO2. The LiCoO2 degradation mechanism in the high-voltage phase-transition region will be discussed by comparing the results for bare and coated LiCoO2samples.
Acknowledgements
This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries (RISING) project of NEDO, Japan.
References
[1] A. Van der Ven et al., Physical Review B. 58(6), 2975 (1998).
[2] Z. H. Chen et al., Electrochim. Acta , 49(7), 1079 (2004).
[3] A. T. Appapillai et al., Chem. Mater., 19(23), 5748(2007).
[4] A. Yano et al., J. Electrochem. Soc. 162(2), A3137 (2015).

Figure 1