This paper presents a study on electrochemical lithiation of CoMnSb with a half-Heusler structure as anode material for Li-ion batteries. During the first electrochemical lithiation, the CoMnSb electrode showed 300 mAh g(-1), which corresponds to a large volumetric capacity of 1930 mAh/cm(3). Combined first principles and X-ray diffraction studies revealed that Mn atoms were extruded from the host structure, resulting in the formation of a cubic phase of Li2CoSb during the first electrochemical lithiation. The volume expansion for the reaction from CoMnSb to Li2CoSb (per Sb atom) is 21%. Although this first lithium insertion process is irreversible, following charge-discharge cycles showed a steady capacity of similar to 220 mAh g(-1) within the voltage range of 0.01 to 1.5 V. (c) 2007 The Electrochemical Society.