Invited Presentation: Enhanced Cycleability of LiNi1/3Co1/3Mn1/3O2 Coated with Metal Oxides Under High Voltage Charge/Discharge Cycles

Introduction

In order to enhance cycleability of positive electrode materials, coating of metal oxides [1], phosphates [2], and fluorides [3] have been reported to be effective on depressing degradation of positive electrode materials.  However, so far, mechanisms of surface modification by coatings have not been clarified well.  In recent, we have reported that Al oxides coating significantly suppressed the increase of cracks in LiNi_1/3Co_1/3Mn_1/3O₂ particle, and that both capacity and area specific impedance retentions of LiNi_1/3Co_1/3Mn_1/3O₂during charge-discharge cycles were improved by the coating [4]. 

In this paper, effect of coating of various metal oxides on electrochemical characteristics and cycleability enhancement of LiNi_1/3Co_1/3Mn_1/3O₂ particles during high voltage charge higher than 4.5 V (vs. Li⁺/Li) have been examined.  Furthermore, effects of coating of various metal oxides on crystal structure deformation at the surface of LiNi_1/3Co_1/3Mn_1/3O₂during the charge-discharge cycles were investigated by s-TEM/EELS. 

 

Experiments

Precursors of metal oxides (Al oxides, Zr oxides, etc.) were coated on LiNi_1/3Co_1/3Mn_1/3O₂ particles by sol-gel methods.  The precursors coated LiNi_1/3Co_1/3Mn_1/3O₂ particles were calcined at 500°C to obtain metal oxides coated LiNi_1/3Co_1/3Mn_1/3O₂.  Coat layers of metal oxides and surface area of LiNi_1/3Co_1/3Mn_1/3O₂ were observed by high resolution s-TEM.  Electrochemical characteristics of bare and metal oxides coated LiNi_1/3Co_1/3Mn_1/3O₂ were examined in pouch cells with using Li metal counter electrode.  The electrolyte used was 1M-LiPF₆/EC+DMC +EMC (1:1:1 by volume).  The cells were cycled between discharge cutoff voltage of 2.5 V and charge cutoff voltages of 4.5, 4.6 and 4.7 V. 

 

Results and discussion

Figure 1 shows an s-TEM image and EDX mappings (Al, Ni, and Co) of cross section of Al oxides coated LiNi_1/3Co_1/3Mn_1/3O₂.  The high resolution s-TEM/EDX observation revealed that coated Al oxides exist as a solid solution (LiTMO₂-LiAlO₂) from the surface to depth of several nm.  Figure 2 shows effect of Al oxides coating on capacity retention of LiNi_1/3Co_1/3Mn_1/3O₂ during the charge-discharge cycles.  Bare LiNi_1/3Co_1/3Mn_1/3O₂ electrodes displayed clear capacity fading during the charge-discharge cycles under the high voltage charge conditions.  On the other hand, LiNi_1/3Co_1/3Mn_1/3O₂ coated by Al oxides of 0.5 w/o showed enhanced cycleability even in charge cutoff voltages of 4.5 and 4.6 V, while capacity fading of the Al coated LiNi_1/3Co_1/3Mn_1/3O₂was not negligible in charge cutoff voltage of 4.7 V.  AC impedance analysis showed that capacity fading of LiNi_1/3Co_1/3Mn_1/3O₂ was attributed to increases of polarization resistance of LiNi_1/3Co_1/3Mn_1/3O₂ electrodes.  Since crystal structure deformation was observed at the surface of degraded LiNi_1/3Co_1/3Mn_1/3O₂ particles by s-TEM/EELS, increase of polarization is thought to be attributed to this crystal structure deformation.  Al oxides coating effects on the crystal structure deformation at the surface of LiNi_1/3Co_1/3Mn_1/3O₂will be discussed in comparison with Zr oxides coating effects.

 

Acknowledgements

This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) project of NEDO, Japan.

 

References

1) L. A. Riley et al., J. Power Sources, 196, 3317 (2011).

2) J. Y. Shi et al., J. Power Sources, 195, 6860 (2010).

3) S.-U. Woo et al., J. Electrochem. Soc., 154, A1005 (2007).

4) K. Araki et al., J. Power Sources, to be submitted.