Performance improvement of spinel LiMn2O4 cathode material by LaF3 surface modification

LiMn2O4 cathode material synthesized by a sol-gel method was modified by LaF3 via wet coating strategy. The phase structures, components and morphologies of pristine and LaF3-coated LiMn2O4 are investigated by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM) and electron diffraction spectroscopy (EDS). The electrochemical performances are investigated and compared at room temperature (25 degrees C) and elevated temperature (55 degrees C) for pristine and LaF3-coated LiMn2O4. The investigation on their cycling performance demonstrates that 3 wt.% LaF3-coated LiMn2O4 exhibits the best cycling performance, with the capacity losses of only 15.9% after 200 cycles at 25 degrees C and 19.7% after 100 cycles at 55 degrees C, much better than those of pristine materials, 60% and 59.8%. Rate performance was improved after LaF3 surface modification. Chronoamperometry experiments indicate that phase transformation is the rate determining step in 4.0-3.5 V region. Cyclic voltammetry (CV) results confirm that LaF3 surface coating improves structure stability of LiMn2O4 during cycling. Electrochemical impedance spectroscopy (EIS) data further demonstrate that modification layer suppresses the undesirable growth of resistance of surface (R-sf) film. Differential scanning calorimetry (DSC) tests show that LaF3 coating layer helps to improve the thermal stability of pristine electrode. (C) 2013 Elsevier BM. All rights reserved.