Abstract
The Li5.5Mn2.5P4 and Li7MnP4 phases having a tetragonal structure (P4/nmm, a ) b ) 4.176(1) Å andc ) 5.978(1) Å) and a cubic structure (Fm3hm a ) 5.968(3) Å) were synthesized by a two-step process consisting in ball milling Li, Mn, and P powders followed by annealing at 800 °C under argon in a sealed stainless steel tube. The resulting materials were tested on Li cells between 0.01 and 2 V. Bothof 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