Mg-doped LiCoO2 is synthesized by a solid-state reaction as a positive electrode material for lithium-ion batteries. The uniphase solid solution is confirmed by X-ray diffraction (XRD) and Rietveld refinements. The structural changes during charging are calculated theoretically by the general gradient approximation (GGA) ab initio method. A blue-pouched single half-cell was used to collect in-situ XRD patterns while the cell charged and discharged in the range of 3.0 similar to 4.5 V. Good structural reversibility and symmetric cell parameter changes were observed during charging and discharging. The 'W' and inverse 'W' shapes of the a-and c-cell parameter changes, respectively, were also observed. The cell volume changes show an inverse 'W' shape. No large hysteresis occurs during the changes in the cell parameters during the charging-discharging process. The thermally stable Mg-doped LiCoO2 is confirmed by differential scanning calorimetry (DSC) and evolved gas analysis-mass spectrometry (EGA-MS). c 2012 The Electrochemical Society. [DOI: 10.1149/2.006203jes] All rights reserved.