Structure studies on LiMn_(0.25)Fe_(0.75)PO_4 by in-situ synchrotron X-ray diffraction analysis

摘要

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.