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

LiCoO₂ 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 1^st and 20^thcycles, 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).