Study On Hydrothermal Synthesis And Growth Mechanism Of LiNi_0.5Mn_1.5O₄ Cathode Material

Spinel LiNi_0.5Mn_1.5O₄cathode material has been widely studied for its high voltage platform,high energy density,low cost and environment friendliness.However,its practical process has been hindered due to its poor cycling performance at high temperature.In order to improve the cycling performance at no cost of the rate performance,a spinel LiNi_0.5Mn_1.5O₄cathode material with a hollow porous architecture was successfully prepared by a urea-assisted hydrothermal method followed by a high-temperature calcination process.The growth mechanism of carbonate precursor,the effect of metal ion concentration and volume ratio of ethylene glycol（EG）to H₂O on the structure,morphology and electrochemical properties of the carbonate precursor and corresponding LiNi_0.5Mn_1.5O₄product has been intensively investigated,respectively.The growth mechanism of precursor was studied by sampling at different hydrothermal times.The analysis shows that the particle morphology changes from"peanut"to"capsule"as the hydrothermal time increases.The carbonate precursor is actually composed of three parts:the core of Mn CO₃,the middle layer of Ni CO₃+Mn CO₃,and the shell of Ni CO₃.The growth mechanism of precursors can be summarized as a nucleation--growth--self-assembly Ostwald ripening mechanism of Mn CO₃,and a dissolution-reaction-crystallization mechanism of Ni CO₃.The effect of metal ion concentration on the structure,morphology and electrochemical properties of carbonate precursors and LiNi_0.5Mn_1.5O₄material were studied.SEM observation shows that the particle morphology and size of carbonate precursor can be tailored by changing metal ion concentration.The particle size gradually decreases and the morphology tends to be spherical with the increase of metal ions concentration.The LiNi_0.5Mn_1.5O₄sample obtained from the carbonate precursor hydrothermally synthesized at a metal ion concentration of 0.3mol·L^-1exhibits the optimal electrochemical properties,with a capacity retention rate of 96.8%after 100 cycles at 1 C rate and 10 C discharge capacity of 124.1 m Ah·g^-1,accounting for 99.3%of that at 0.2C rate.The effect of EG/H₂O volume ratio in the solvent on the structure,morphology,and electrochemical properties of carbonate precursors and LiNi_0.5Mn_1.5O₄products was systematically investigated.SEM observation illustrates that by increasing the volume ratio of EG,the morphology of carbonate precursor can be tailored from olive-like to peanut-like then further to sphere-like,and the particle size shows a gradual decreasing trend.The electrochemical results show that the LiNi_0.5Mn_1.5O₄product synthesized at EG/H₂O volume ratio of 1:3 has the most homogeneous particle size distribution and exhibits optimal electrochemical performance,with a discharge capacity of 128.4 m Ah·g^-1at 10 C and capacity retention rate of 98.93%after 100 cycles at 1 C.