Reduction of Surface Residual Lithium Compounds for Single-Crystal LiNi0.6Mn0.2Co0.2O2 via Al2O3 Atomic Layer Deposition and Post-Annealing

Surface residual lithium compounds of Ni-rich cathodes are tremendous obstacles to electrochemical performance due to blocking ion/electron transfer and arousing surface instability. Herein, ultrathin and uniform Al2O3 coating via atomic layer deposition (ALD) coupled with the post-annealing process is reported to reduce residual lithium compounds on single-crystal LiNi0.6Mn0.2Co0.2O2 (NCM622). Surface composition characterizations indicate that LiOH is obviously reduced after Al2O3 growth on NCM622. Subsequent post-annealing treatment causes the consumption of Li2CO3 along with the diffusion of Al atoms into the surface layer of NCM622. The NCM622 modified by Al2O3 coating and post-annealing exhibits excellent cycling stability, the capacity retention of which reaches 92.2% after 300 cycles at 1 C, much higher than that of pristine NCM622 (34.8%). Reduced residual lithium compounds on NCM622 can greatly decrease the formation of LiF and the degree of Li+/Ni2+ cation mixing after discharge–charge cycling, which is the key to the improvement of cycling stability.

Corrosion Behavior of the CoNiCrAlY-Al2O3 Composite Coating Based on Core-Shell Structured Powder Design

The oxidation of the metal powder during the thermal spraying process usually leads to significant deterioration of the microstructure and performance of the coating. In order to isolate the metal powder from oxygen during the spraying process, the CoNiCrAlY-Al2O3 core-shell structured powder with Al2O3 as the shell was designed in this study. The influence of the core-shell structured powder on the microstructure and corrosion resistance of the HVOF coating has been studied in detail. The results show that the temperature field of the molten CoNiCrAlY powder during the spraying process is significantly changed by the Al2O3 shell. The poor deformability of the CoNiCrAlY-Al2O3 droplets leads to an increase in the porosity and unmelted particles of the coating. In addition, the significant difference is that the coating also maintains a high content of β-NiAl phase. The lower oxide content in the CoNiCrAlY-Al2O3 coating indicates that the core-shell structured powder significantly inhibits the oxidation of the CoNiCrAlY core powder during the spraying process. The CoNiCrAlY-Al2O3 coating exhibits high corrosion potential, passive film resistance, charge transfer resistance, and low corrosion current density in 3.5 wt.% NaCl solution, indicating that the coating has excellent corrosion resistance.