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

"AlPO 4"-coated LiCoO 2 was shown to exhibit markedly improved capacity retention relative to bare LiCO 2 upon cycling to 4.7 V, Scanning and transmission electron microscopy imaging showed that the coating thickness of "AlPOV 4"-coated LiCoO 2 varied from ??? 10 to ??? 100 nm. Energy-dispersive X-ray mapping revealed that the coating was not single-phase "AlPO 4", 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 "AlPO 4"-coated LiCoO 2 in comparison to bare LiCoO 2 and various reference compounds such as Li 2CO 3, Li 3PO 4, and AlPO4 indicate that (1) AlPO 4 is absent on the surface; (2) the surface consisted of Li 3PO 4 and heavily Al substituted LiAl yCo 1-yO 2, which may result from AlPO 4 nanoparticles reacting with bare LiCoO 2 during the coating heat treatment at 700??C; and (3) the amount of surface Li 2CO 3 is markedly reduced in the coated sample relative to the bare LiCoO 2. The existence of Li 3PO 4 in "AlPO 4"-coated LiCoO 2 was confirmed with X-ray powder diffraction. The coating microstructure of "AlPO 4"- coated LICoO 2 is proposed, and the mechanisms of enhancement in the cycling and thermal characteristics by particle surface microstructure are discussed in detail.