Web Release Date: October 13,
Microstructure of LiCoO2 with and without "AlPO4" Nanoparticle Coating: Combined STEM and XPS Studies
and
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Naval Surface Warfare Center, Carderock Division, West Bethesda, Maryland 20817-5700, and Kumoh National Institute of Technology, Gumi, Republic of Korea
Received June 8, 2007
Revised Manuscript Received August 1, 2007
Abstract:
"AlPO4"-coated LiCoO2 was shown to exhibit markedly improved capacity retention relative to bare
LiCoO2 upon cycling to 4.7 V. Scanning and transmission electron microscopy imaging showed that the
coating thickness of "AlPO4"-coated LiCoO2 varied from ~10 to ~100 nm. Energy-dispersive X-ray
mapping revealed that the coating was not single-phase "AlPO4", rather consisting of P-rich thick regions
(~100 nm) and Al-rich thin regions (~10 nm). Detailed X-ray photoelectron spectroscopy (XPS) studies
of the "AlPO4"-coated LiCoO2 in comparison to bare LiCoO2 and various reference compounds such as
Li2CO3, Li3PO4, and AlPO4 indicate that (1) AlPO4 is absent on the surface; (2) the surface consisted of
Li3PO4 and heavily Al substituted LiAlyCo1-yO2, which may result from AlPO4 nanoparticles reacting
with bare LiCoO2 during the coating heat treatment at 700 
C; and (3) the amount of surface Li2CO3 is
markedly reduced in the coated sample relative to the bare LiCoO2. The existence of Li3PO4 in "AlPO4"-coated LiCoO2 was confirmed with X-ray powder diffraction. The coating microstructure of "AlPO4"-coated LiCoO2 is proposed, and the mechanisms of enhancement in the cycling and thermal characteristics
by particle surface microstructure are discussed in detail.
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