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

The surface of LiCoO2 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 723K in air for 10h. The structure and morphology of the surface modified LiCoO2 samples have been characterized with XRD, SEM, EDS, TEM, BET, XPS/ESCA and solid-state 27Al 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 20nm. The corresponding 27Al 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 1wt.%. This provides direct evidence that Al2O3 was on the surface of the core materials. The LiCoO2 coated with 1wt.% 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.