Electrochemical Behaviors of Binary and Ternary Manganese Phosphides

The present work provides a comparative study of the electrochemical behaviors of binary and ternary manganese phosphides toward lithium. The redox mechanism involved in these two electrodes is elucidated by means of galvanostatic and in situ X-ray diffraction measurements and first-principles calculations. Compared to the previous study reported on MnP4, two additional processes are here clearly identified: after a first conversion process that transforms the binary MnP4 electrode into Li7MnP4, a partial and irreversible decomposition of Li7MnP4 into Li3P + Mn0 is achieved, responsible for the poor electrodes capacity retention. In charge, the formation of an amorphous low-lithiated Li∼1 MnP4 phase, very similar to the charged LixVP4 and LixTiP4 electrodes ever reported, is shown to compete with the kinetically limited MnP4 reconstruction. This suggests that the high performances of ternary transition metal phosphides could be combined with the technological advantage of using more suitable nonlithiated MP4 starting anodes.