Reversible lithium uptake by CoP3 at low potential:: role of the anion

Although various transition metal compounds in Group V (nitrides and antimonides) have been reported to act as low potential Li insertion hosts, the phosphides have remained unexplored to date. We show here that lithium uptake and extraction in the metal phosphide, CoP3, provides a reversible capacity of 400 mAh/g at an average potential of 0.9 V vs Li/Li+, via a novel mechanism. The latter was revealed using a combination of X-ray diffraction, electron microscopy, and potentiodynamic and galvanostatic intermittent measurements coupled with X-ray photoelectron spectroscopy (XPS). Initial uptake of Li forms highly dispersed cobalt clusters embedded in a matrix of Li3P; extraction of Li from this ion-conductive matrix on charge yields nano-particles of LiP, with little change evident in the oxidation state of the Co site. This shows that contrary to the case of metal oxides here the anion plays the major role in reduction and oxidation. We expect this is a general phenomenon for phosphides, and to be of fundamental interest and future importance in the search for new negative electrode materials. (C) 2002 Published by Elsevier Science B.V.