FeP:  Another Attractive Anode for the Li-Ion Battery Enlisting a Reversible Two-Step Insertion/Conversion Process

FePy (y = 1, 2, 4) anodes all react with lithium through a conversion reaction FePy + 3yLi → yLi3P + Fe0 in their first discharge, leading to nanocomposite discharged electrodes described by nanosized Fe0 particles embedded in yLi3P matrixes. From electrochemical and complementary in situ X-ray diffraction and high-resolution transmission electron microscopy studies, we deduce that the conversion reaction occurring during the first discharge is followed by two successive insertion and conversion processes in further cycles for the FeP electrode. The insertion process is highly reversible, leading to a capacity retention of 300 mA h g-1 and 1900 mA h cm-3 after 100 cycles, and corresponds to the formation of an intermediate tetragonal LiFeP phase as deduced from first-principles T = 0 K phase diagram calculations and preliminary Mössbauer analyses. We expect the kinetics of this reaction to be strongly limited by the increase in y, thus leading to an increasing capacity fading when increasing the y P/Fe ratio.