Microstructure of LiCoO2 with and without “AlPO4” Nanoparticle Coating: Combined STEM and XPS Studies

"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.