Electrochemical and solid-state NMR studies on LiCoO2 coated with Al2O3 derived from carboxylate-alumoxane

The surface of LiCOo(2) cathodes was coated with various wt.% of Al2O3 derived from methoxyethoxy acetate-alumoxane (MEA-alumoxane) by a mechano-thermal coating procedure, followed by calcination at 723 K in air for 10 h. The structure and morphology of the surface modified LiCoO2 samples have been characterized with XRD, SEM, EDS, TEM, BET, XPS/ESCA and solid-state 27 Al magic angle spinning (MAS) NMR techniques. The Al2O3 coating forms a thin layer on the surface of the core material with an average thickness of 20 nm. The corresponding Al-27 MAS NMR spectrum basically exhibited the same characteristics as the spectrum for pristine Al2O3 derived from MEA-alumoxane, indicating that the local environment of aluminum atoms was not significantly changed at coating levels below I wt.%. This provides direct evidence that Al2O3 was on the surface of the core materials. The LiCoO2 coated with 1 wt.% Al2O3 sustained continuous cycle stability 13 times longer than pristine LiCoO2. A comparison of the electrochemical impedance behavior of the pristine and coated materials revealed that the failure of pristine cathode performance is associated with an increase in the particle-particle resistance upon continuous cycling. Coating improved the cathode performance by suppressing the characteristic structural phase transitions (hexagonal to monoclinic to hexagonal) that occur in pristine LiCoO2 during the charge-discharge processes. (c) 2006 Elsevier B.V. All rights reserved.