In order to achieve a high level of reliability in rechargeable Li-ion batteries, battery cell materials must maintain good mechanical stability over many cycles. Stresses due to intercalation, phase transition, and thermal loading can cause local fractures in the active materials of Li-ion batteries, as has been experimentally observed. The resulting fracture of the cathode materials is one putative degradation mechanism of Li batteries; it inevitably results in a loss of electrical contact and an increase in the surface area for active material dissolution and SEI layer formation. In this work, we investigate the conditions under which initial defects propagate and form larger fractures in the cathode material (LiMn2O4) during the charging and discharging cycles of electrochemical reactions. Fracture analysis based on the extended finite element method (XFEM) is used to evaluate the effects of current density, particle size and particle aspect ratio on the propagation of defects. Both current density and particle size are shown to be positively correlated with fracture propagation, though not monotonically so in the case of aspect ratio. With an aspect ratio of 1.5: 1, a particle with a defect at the center will crack at a low C-rate; this case is one of the most severe among all aspect ratios. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.045204jes] All rights reserved.