Hongming Yu

Double-shelled hollow microspheres of LiMn2O4 for high-performance lithium ion batteries

Double-shelled hollow microspheres of LiMn2O4 were prepared by a facile self-template method. The inner and outer shells of the obtained microspheres are composed of nanoparticles with diameters ranging between 200–400 nm. Galvanostatic charge/discharge cycling shows that this material delivers a discharge capacity of 127 mAh g−1 at C/10 rate and a capacity retainability of 80% after 800 cycles at 5 C rate, revealing a high reversible capacity, superior rate capability and excellent cycling stability under high rates. The improved performance is attributed to the short Li+ ion diffusion lengths in the nanobuilding blocks and the void core and space between the inner and outer shells to accommodate the volume expansion/contraction during Li+ ions insertion/extraction processes.

Preparation of Nano-structured LiFexMn1–xPO4 (x=0, 0.2, 0.4) by Reflux Method and Research on the Influences of Fe(II) Substitution

Nano structured LiFexMn1–xPO4 (x=0, 0.2, 0.4) materials were successfully prepared by one-step reflux method in a water/PEG400 mixed solvent, and were coated by carbon using glucose as the precursor. The materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The electrochemical properties of the materials were investigated by galvanostatic cycling, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It was found that the materials consisted of nanorods with a diameter of 50 nm and a length of 500 nm. Galvanostatic cycling showed that the capacity of LiMnPO4 could be largely increased by Fe2+ substitution. At a current rate of C/20, the capacity of the three samples (x=0, 0.2, 0.4) were 47, 107 and 150 mA·h·g−1 respectively. CV result showed that the Fe2+ substitution could decrease the polarization during charging/discharging, accelerating the electrochemical process. EIS result showed that the Fe2+ substitution could decrease the charge transfer resistance between the electrode and electrolyte, as well as increase the Li-ion diffusion coefficient in the bulk material, resulting in an improved electrochemical performance.

Preparation and characterization of LiFePO 4 /graphene-oxide composites

LiFePO 4 /graphene-oxide (GNO) composites were prepared by co-precipitation method. Their structure and morphology were investigated by X-ray diffraction, Fourier transform infrared spectra, field emission scanning electron microscopy, and transmission electron microscopy. A low content of GNO can be uniformly dispersed in the matrix of LiFePO 4 nano particles, while at a higher content, GNO will aggregate severely and has a negative effect on the electrochemical performance of LiFePO 4 . Further heat treatment can improve the crystallinity of LiFePO 4 , and improve the electrochemical performance of LiFePO 4 with a relatively low content of GNO.