Lithium-free vanadium tetraphosphide was synthesized and electrochemical characterizations during cycling were carried out. Based on differential capacity plots (DCP) and X-ray analyses, a four-step reaction mechanism was suggested. The reaction sequences are topotactic lithium insertion into the VP4, phase transformation from monoclinic Li3VP4 to cubic Li6VP4 phase, Li3P and VP formation by decomposition, and another lithium insertion into VP as the potential is lowered. Volume expansion of active material was expected from crystallographic data, and confirmed by scanning electron microscope observation during the second reaction step. In monoclinic sustaining range, VP4 showed an excellent cycling performance with a relatively large reversible capacity. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1938047] All rights reserved.