Controllable Preparations And Electrochemical Performances Of LiMn₂O₄with Different Morphologies And Particle Sizes

Li-ion batteries are the new generation of environmental friendly chemical power supplies beacuse of thier high working voltage as well as specific energy, long cycle life, good safty performance. They have been widely used in mobile devices and energy storing devices. It is important to research and develop high performance cathode materials to improve the Li-ion batteries. Spinel LiMn2O4cathode material for lithium ion batteries has an excellent utilization prospect due to its low cost, safety performance and acceptable environmental impact. It is well known that the electrochemical performance of the cathode is strongly influenced by the physical properties such as the particle morphology and size. We have researched the preparation, electrochemical performance and Li+diffusion of the LiMn2O4cathode materials with different morphotogies and particle sizes in the paper.MnCO3precursors with different morphotogies and particle sizes were firstly prepared by precipitation and subsequently transformed to LiMn2O4cathode materials through melt-impregnation and temperature programmed process. The integrated crystal structures of the materials could be detected by X-ray diffraction analysis (XRD), and they belong to the spinel cubic crystal system. From the scanning electron microscopy (SEM) images, it could be found tha the LiMn2O4microparticles were uniform spherical or cubic, while the morphology and size were maintained. CR-2032-type coin cells were assembled with lithium metals as anode materials to test the electrochemical performance of the different LiMn2O4cathode materials.The spherical particle cathode materials had better electrochemical properties than the cubic ones by comparation, which delivered an initial discharge capacity of133.4mAh/g and a capacity retention of88.9%after50cycles at0.1C rate, and remained average discharge capacity of104.3mAh/g at5C rate within the voltage range from3.50to4.30V.The middle-sphere cathode materials had the best electrochemical properties compared with the small-sphere and large-sphere ones, which delivered an initial discharge capacity of132.7mAh/g and a capacity retention of92.9%after50cycles at0.1C rate, and remained average discharge capacity of115.7mAh/g at5C rate within the voltage range from3.50to4.30V.The hollow particles had better electrochemical properties than the solid ones, especially for the3min-oxidated sample, which delivered an initial discharge capacity of132.9mAh/g and a capacity retention of94.1%after50cycles at0.1C rate, and remained average discharge capacity of116.5mAh/g at5C rate within the voltage range from3.50to4.30V.Li+diffusion coefficients of the different cathode materials had been test by potentiostatic intermittent titration technique (PITT) and capacity intermittent titration technique (CITT). The coefficients varied from10-12-10-10cm2/s (by PITT) and10-11-10-9cm2/s (by CITT) within the voltage range from3.80to4.30V. There are two minimum peaks at around4.00V and4.15V in the curves of D-E, and the two peaks made the curves a regular "W" shape. The diffusion coefficients of studied samples decrease in the order: MS>SS>BS>CB at the same voltage value. This is the possible reason for the difference of electrochemical performances of four LiMn2O4samples.