High-performance lithium-rich layered oxide materials : Effects of chelating agents on microstructure and electrochemical properties

The mechanisms and effects of three typical chelating agents, namely glucose, citric acid and sucrose on the sol-gel synthesis process, electrochemical degradation and structural evolution of 0.5Li₂MnO₃·0.5LiNi_0.5Co_0.2Mn_0.3O₂ (LLMO) materials are systematically compared for the first time. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy analysis indicate that the sample synthesized from sucrose owns well structure, homogenous distribution, low Ni³⁺ concentration and good surface structural stability during cycling, respectively. Electrochemical tests further prove that the LLMO material obtained from sucrose maintains 258.4 mAh g^-1 with 94.8% capacity retention after 100 cycles at 0.2 C. The superior electrochemical performance can be ascribed to the exceptional complexing mechanism of sucrose, compared to those of the glucose and citric acid. Namely, one mole sucrose can be hydrolyzed into two different monosaccharides and further chelates three M (Li, Ni, Co and Mn) ions to form a more uniform ion-chelated matrix during sol-gel process. This discovery is an important step towards understanding the selection criterion of chelating agents for sol-gel method, that chelating agent with excellent complexing capability is beneficial to the distribution, structural stability and electrochemical properties of advanced lithium-rich layered materials.