Improvement of structural and electrochemical properties of NMC layered cathode material by combined doping and coating

Prospective cathode materials, pristine and Mg-/Zr-modified LiNi0.33Mn0.33Co0.33O2 (NMC333), are successfully synthesized through a synthetic method including a citric acid-assisted sol-gel processing followed by drying and calcination at different temperatures. The combined results from structural, chemical, and morphological investigations reveal that the modified NMC, where the modification is obtained by doping and a uniform coating layer with an optimal thickness, show a better-organized layered structure and improved interfacial properties compared to the pristine NMC. The modified NMC material shows a remarkable improvement in terms of capacity retention, especially when setting the charge cut-off voltage at 4.2 V, which results in improved cycling stability after more than 200 cycles with capacity retention of around 95%. In order to shed light on the kinetics of redox processes, and its impact on charge/discharge behavior, electrochemical impedance spectroscopy is carried out at different states of charge during oxidation and reduction for both pristine and modified NMC-based electrodes. The promising results obtained with this synthesis open possibilities for performance improvements of intercalating layered oxides by structure doping and surface enhancement.