Crystallographic structure of LiFe_(1-x)Mn_xPO_4 solid solutions studied by neutron powder diffraction

摘要

High-resolution neutron powder diffraction (NPD) data were recorded on a series of cathode material LiFe_(1-x)Mn_xPO_4 (x = 0, 0.2, 0.5, 0.8, and 1.0) solid solutions using the HRPD machine at SINQ/PSI, Switzerland. Ab initio crystal structure solution via program FOX indicates demonstrably that the space group of LiFePO_4 is Pnma with Li~(1+) occupying octahedral (4a) sites and Fe~(2+) octahedral (4c) sites, respectively, in the olivine structure. Rietveld refinement (program FullProf suite version July-2011), complementary with X-ray diffraction data, shows that Fe~(2+) may partially (about 2%) distribute over Li~(1+) sites. NPD data for LiFe_(1-x)Mn_xPO_4 (x = 0,0.2,0.5,0.8, and 1.0) reveal that the Mn~(2+) replaces Fe~(2+) at the octahedral (4c) sites. The cell parameters a, b, and c increase linearly and the interatomic distances (in A) of Li-O(2) and Li-O(1) increase, while the interatomic distances (in A) of Li-O(3) decrease on the addition of Mn, respectively, partially explaining a higher potential plateau of ～4.1 eV in LiMnPO_4 compared to ～3.5 eV in LiFePO_4.