Microstructure of LiCoO2 with and without AIPO4 nanoparticle coating:: Combined STEM and XPS studies

AIPO(4)-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 AIPO(4)-coated LiCoO2 varied from similar to 10 to similar to 100 nm. Energy-dispersive X-ray mapping revealed that the coating was not single-phase AIPO(4), rather consisting of P-rich thick regions (similar to 100 nm) and A1-rich thin regions (similar to 10 nm). Detailed X-ray photoelectron spectroscopy (XPS) studies of the AIPO(4)-coated LiCoO2 in comparison to bare LiCoO2 and various reference compounds such as Li2CO3, Li3PO4, and AIPO(4) indicate that (1) AIPO(4) is absent on the surface; (2) the surface consisted of Li3PO4 and heavily A1 substituted LiA1(y)Co(1-y)O(2), which may result from AIPO(4) nanoparticles reacting with bare LiCoO2 during the coating heat treatment at 700 degrees 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 AIPO(4)-coated LiCoO2, was confirmed with X-ray powder diffraction. The coating inicrostructure of AIPO(4)-coated LiCoO2 is proposed, and the mechanisms of enhancement in the cycling and thermal characteristics by particle surface microstructure are discussed in detail.