LiNi(0.8)Co(0.2)O(2)00 and Ca-doped LiNi0.8Co0.2O2 cathode materials were synthesized via a theological phase reaction method. It is found that the Ca doping significantly improves reversible capacity, cycling performance, thermal stability and rate capability. The Ca-doped LiNi0.8Co0.2O2 cathode material maintains nearly its initial discharge capacity up to 100 cycles at room temperature. It also delivers an initial discharge capacity of 183 mA h g(-1) and still keeps 131 mA h g(-1) even after 120 cycles at 60 degrees C. These results' together with the X-ray diffraction and electrochemical impedance spectroscopy analysis, reveal that Ca2+ ions occupy Li+ ion sites to form Ca-Li defects and lithium vacancies (V-Li(')), which reduce the resistance and increases conductivity of LiNi0.8Co0.2O2. (c) 2006 Elsevier B.V. All rights reserved.