Surface Structure and High-Voltage Charge/Discharge Characteristics of Al-Oxide Coated LiNi_1/3Co_1/3Mn_1/3O₂Cathodes

A sol-gel method was used to fabricate Al-oxide coated LiNi 1/3 Co 1/3 Mn 1/3 O 2 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 LiAlO 2 -LiNi 1/3 Co 1/3 Mn 1/3 O 2 is uniformly formed to a depth of several nanometers from the surface of Al-oxide coated LiNi 1/3 Co 1/3 Mn 1/3 O 2 . Furthermore, the discharge capacity and average discharge voltage of Al-oxide coated LiNi 1/3 Co 1/3 Mn 1/3 O 2 is equal to or higher than uncoated LiNi 1/3 Co 1/3 Mn 1/3 O 2 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 LiNi 1/3 Co 1/3 Mn 1/3 O 2 is attributed to an increase in polarization due to an increased charge transfer resistance ( R ct ), which is indicative of degradation of the interface between the electrode and electrolyte. However, this increase in polarization and R ct is effectively suppressed in the Al-oxide coated LiNi 1/3 Co 1/3 Mn 1/3 O 2 . This can be explained by an inhibition of the formation of a rock-salt-structured phase in the surface region of bare LiNi 1/3 Co 1/3 Mn 1/3 O 2 during cycling, as confirmed by STEM and electron energy loss spectrometry.