A porous hierarchical micro/nano LiNi0.5Mn1.5O4 cathode material for Li-ion batteries synthesized by a urea-assisted hydrothermal method.

A spinel LiNi0.5Mn1.5O4 cathode material was synthesized by a urea-assisted hydrothermal method followed by high-temperature calcination with a lithium source. The effects of the molar ratio of urea to transition metal ions (U/TM ratio) on the structure, morphology and electrochemical properties of the carbonate precursor and LiNi0.5Mn1.5O4 product were systematically investigated. The as-synthesized samples were characterized by XRD, FT-IR, SEM, TEM, a constant-current charge/discharge test, CV and EIS. XRD and FT-IR results show that the LiNi0.5Mn1.5O4 samples synthesized at U/TM ratios of 1.0 : 1 to 4.0 : 1 have mainly a disordered structure with the Fd3m space group, and the sample synthesized at a U/TM ratio of 2.0 : 1 has the lowest cation disordering degree (Mn3+ content). SEM observations show that the U/TM ratio has a significant influence on the phase composition, particle morphology and size of the carbonate precursor, thus leading to different electrochemical properties of the LiNi0.5Mn1.5O4 material. Among them, the carbonate precursor synthesized at the U/TM ratio of 2.0 : 1 shows the smallest particle size with the most homogeneous distribution, thus leading to an optimal electrochemical performance of the derived LiNi0.5Mn1.5O4 material in spite of its lowest Mn3+ content, whose discharge capacity at 10C rate can reach 120.2 mA h g-1, accounting for 98.6% of that at 0.2C rate, and capacity retention rate after 100 cycles at 1C rate can reach 95.3%.