The effect of Mg-substitution in Li(Ni,Co,Al)O-2 during overcharge was studied by scanning electron microscopy (SEM) observations and in situ synchrotron X-ray Diffraction (XRD) measurements. The higher Mg-substituted samples showed higher electrochemical stability during the overcharge cycling. After overcharge, the non Mg-substituted sample featured many cracks, not only at the grain boundaries in secondary particles but also inside the primary particles. This morphological change is assigned to the drastic shrinking of the c-axis during overcharge. The Mg-substitution suppressed the c-axis shrinking and consequently avoided the cracking of the particles. This effect of Mg-substitution in Li(Ni,Co,Al)O-2 correlates with the electrochemical stability during the overcharge cycling. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.025111jes] All rights reserved.