Improve the structure and electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ cathode material by nano-Al₂O₃ ultrasonic coating

Highlights: • Al₂O₃ nano-particles, dispersed well by ultrasonic treatment, were used to coat on the surface of LiNi_0.6Co_0.2Mn_0.2O₂ powder. • Appropriate amount of Al₂O₃-coating could facilitate hexagonal ordering of the material. • The LiNi_0.6Co_0.2Mn_0.2O₂ has initial discharge capacity of 197.1 mA h g⁻¹ at 0.2 C over 2.8–4.5 V and capacity retention of 91.0% after 30 cycles at 1 C. • The 1.0 wt.% Al₂O₃-coated sample shows a high discharge capacity of 153.5 mA h g⁻¹ after 5 cycles under a 10 C rate. - Abstract: The LiNi_0.6Co_0.2Mn_0.2O₂ powder was coated with nano-Al₂O₃ particles via ultrasonic coating, and its electrochemical performances were improved greatly. The X-ray diffraction, scanning electron microscopy, transmission electron microscope and energy dispersive spectroscopy were employed to research the structure variation, surface appearance and coating status. The results show that the nano-Al₂O₃ particles are successfully coated and the thickness of coating layer is about 20–25 nm. The XRD test results indicate that appropriate amount of Al₂O₃ coated is beneficial to form a better layered structure with smaller cation disorder. The charge–discharge results show that LiNi_0.6Co_0.2Mn_0.2O₂ coated with Al₂O₃ of 1.0 wt.% has initial discharge capacity of 197.1 mA h g⁻¹, initial coulombic efficiency of 91.2% at 0.2 C over 2.8–4.5 V and capacity retention of 91.0% much more than the pristine one of 82.9.0% after 30 cycles at 1 C. Especially, the rate capability of 1.0 wt.% Al₂O₃-coated sample is improved evidently, it shows a high discharge capacity of 153.5 mA h g⁻¹ after 5 cycles under a 10 C rate, whereas the pristine one is only 126.3 mA h g⁻¹ under the same conditions. The electrochemical impedance spectroscopy tests results also show that the impedance of LiNi_0.6Co_0.2Mn_0.2O₂ is significantly reduced by Al₂O₃ coating during the cycling