On the reactivity of Li8-yMnyP4 toward lithium

The Li5.5Mn2.5P4 and Li7MnP4 phases having a tetragonal structure (P-4/nmm, a = b = 4.176(1) angstrom and c = 5.978(1) angstrom) and a cubic structure (Fm3m a = 5.968(3) angstrom) were synthesized by a two-step process consisting in ball milling Li, Mn, and P powders followed by annealing at 800 degrees C under argon in a sealed stainless steel tube. The resulting materials were tested on Li cells between 0.01 and 2 V. Both of them can reversibly react with Li, leading to reversible capacities as high as 1000 mAh/g, but only the Li5.5Mn2.5P4 electrode can nicely maintain its initial capacity upon cycling. The origin of such a difference was found in the partial decomposition of Li7MnP4 in Li3P and Mn nanoparticles at low potential, as deduced by in situ X-ray diffraction measurements. In contrast, no decomposition was shown to occur for the Li5.5Mn2.5P4 electrode that shows the best capacity retention among of ternary LixMyP4 (M = Ti, V) so far studied.