Abstract
In this work, we substituted Mn 2+ at the 4c site of LiFePO 4 to prepare LiMn 0.25 Fe 0.75 PO 4 in order to raise the working voltage. In order to study the phase transformation of lithium bi-metal phosphate (LiM′M′′PO 4 ) during the lithiation/delithiation, in-situ synchrotron X-ray diffraction has been used. At 0.05C charge/discharge, X-ray patterns reveal that LiMn 0.25 Fe 0.75 PO 4 undergoes two two-phase transformations during the delithiation, resulting from Fe 2+ /Fe 3+ and then Mn 2+ /Mn 3+ redox reactions. However, the phase transformation for lithiation is different, becoming a two-phase (Mn 2+ /Mn 3+ ) reaction and single-phase (Fe 2+ /Fe 3+ ) reaction. Even at a higher charge/discharge rate (0.5C), the results are the same. LiMn 0.25 Fe 0.75 PO 4 also has a good cyclability, since there is no significant capacity fading during the cycling test. The X-ray patterns reveal that LiMn 0.25 Fe 0.75 PO 4 still maintains a good crystal structure after 40 cycles because of its stable olivine structure. © 2008 Elsevier B.V. All rights reserved.