Surface Structure and High-Voltage Charge/Discharge Characteristics of Al-Oxide Coated LiNi 1/3 Co 1/3 Mn 1/3 O 2 Cathodes

Abstract

A sol-gel method was used to fabricate Al-oxide coated LiNi1/3Co1/3Mn1/3O2 cathodes and their surface structure and high-voltage charge/discharge characteristics were evaluated with a view to their potential use in Li-ion batteries. Scanning transmission electron microscopy (STEM) revealed that a solid solution of LiAlO2-LiNi1/3Co1/3Mn1/3O2 is uniformly formed to a depth of several nanometers from the surface of Al-oxide coated LiNi1/3Co1/3Mn1/3O2. Furthermore, the discharge capacity and average discharge voltage of Al-oxide coated LiNi1/3Co1/3Mn1/3O2 is equal to or higher than uncoated LiNi1/3Co1/3Mn1/3O2 at a charge voltage of 4.5 to 4.9 V. It was also found that the Al-oxide coating significantly improves the cycle performance at a charge voltage of 4.5 to 4.7 V. A substantial fade in capacity observed during the cycling of bare LiNi1/3Co1/3Mn1/3O2 is attributed to an increase in polarization due to an increased charge transfer resistance (Rct), which is indicative of degradation of the interface between the electrode and electrolyte. However, this increase in polarization and Rct is effectively suppressed in the Al-oxide coated LiNi1/3Co1/3Mn1/3O2. This can be explained by an inhibition of the formation of a rock-salt-structured phase in the surface region of bare LiNi1/3Co1/3Mn1/3O2 during cycling, as confirmed by STEM and electron energy loss spectrometry.