Surface structures of the bare and AlPO4-coated LiNi0.8Co0.2O2 particles in two electrolytes after 90 degrees C for 4 h storage were investigated using transmission electron microscope (TEM). The structure of bare LiNi0.8Co0.2O2 particles in common electrolyte has been destructed from the layered structure with space group R-3m at interior region to a rock-salt phase (Fm-3m) at edge of the surface layer of the cycled particles, while AlPO4-coated LiNi0.8Co0.2O2 particles in common electrolyte has been transformed into a spinel phase (Fd-3m) on the surfaces of the cycled particles. However, the surface structure of bare LiNi0.8Co0.2O2 particles in functional electrolyte has not been changed. The results showed that functional. electrolyte can more effectively improve thermal stability of LiNi0.8Co0.2O2 cathode cells than the AlPO4 coating. (c) 2008 Elsevier B.V. All rights reserved.