Urea-assisted hydrothermal synthesis of a hollow hierarchical LiNi_0.5Mn_1.5O₄ cathode material with tunable morphology characteristics

A hollow hierarchical LiNi_0.5Mn_1.5O₄ cathode material has been synthesized via a urea-assisted hydrothermal method followed by a high-temperature calcination process. The effect of reactant concentration on the structure, morphology and electrochemical properties of the carbonate precursor and corresponding LiNi_0.5Mn_1.5O₄ product has been intensively investigated. The as-prepared samples were characterized by XRD, FT-IR, SEM, CV, EIS, GITT and constant-current charge/discharge tests. The results show that all samples belong to a cubic spinel structure with mainly Fd3m space group, and the Mn³⁺ content and impurity content initially decrease and then increase slightly with the reactant concentration increasing. SEM observation shows that the particle morphology and size of carbonate precursor can be tailored by changing reactant concentration. The LiNi_0.5Mn_1.5O₄ sample obtained from the carbonate precursor hydrothermally synthesized at a reactant concentration of 0.3 mol L^-1 exhibits the optimal overall electrochemical properties, with capacity retention rate of 96.8% after 100 cycles at 1C rate and 10C discharge capacity of 124.9 mA h g^-1, accounting for 99.9% of that at 0.2C rate. The excellent electrochemical performance can be mainly attributed to morphological characteristics, that is, smaller particle size with homogeneous distribution, in spite of lower Mn³⁺ content.