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

Abstract

A spinel LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄ 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 LiNi_0.5Mn_1.5O₄ 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%.