Kinetics Study of the 5 V Spinel Cathode LiMn1.5Ni0.5O4 Before and After Surface Modifications

The 5 V spinel LiMn1.5Ni0.5O4 has been surface-modified with 2 wt % nanosize Al2O3 and Bi2O3 by an electrostatic self-assembly method and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and electrochemical measurements. Although the modifications do not change the crystallographic structure of the spinel sample, the surface-modified samples exhibit better cycling performance (at both room and elevated temperatures) and rate capability compared to the bare LiMn1.5Ni0.5O4. Electrochemical impedance spectroscopy and large potential step chronoamperometry measurements demonstrate that the discharge process is controlled by phase transformation and lithium ion diffusion in the bulk material in addition to charge-transfer reaction. Surface modification is found to affect only the charge-transfer reaction kinetics. The enhanced electrochemical performance of the surface-modified samples is attributed to the suppression of the thick solid electrolyte interfacial layer and the consequent improvement in charge-transfer kinetics. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3206590] All rights reserved.