A Study on Chemical Defect of Li1-XFe0.5-YMn0.5-YPO4 related to the Electrochemical Behavior Using Synchrotron Based X-Ray Techniques

Olivine-type LiMPO₄ (M = Fe, Mn, Co, Ni) has been investigated as promising cathode materials due to their excellent electrochemical performance, thermal stability. Although LiFePO₄ has practical reversible capacity (about 160mAh/g) closed to the theoretical value (170mAh/g), it has lower energy density because of relatively low Fe²⁺/Fe³⁺ redox reaction potential. LiMnPO₄ material generates high energy density due to its high chemical potential (4.1 V), but undergoes poor lithium ion diffusion kinetics due to low electric, ionic conductivity, and large anisotropic distortion of Mn³⁺during cycle. It also has been reported that the capacity of LiMnPO₄ is not achieved without Fe coexisting in the Mn octahedral site.

In the olivine-type, LiFeMnPO₄ mixed system is considered as one of the promising materials due to benefit of energy density and their favored operation voltage. The Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ redox reactions are operated at 3.4V and 4.1V plateau in the LiFeMnPO₄ system. However, many researches show the capacity fading by increasing the Mn content in the binary LiFeMnPO₄ system. To remedy this problem, improvement of Mn redox reaction and understanding of structural and electrochemical properties are necessary to improve the material performance.

 In this study, we try to explain correlation between electrochemical performance and characteristic of the materials on different M³⁺ (M = Fe, Mn) content using the synchrotron based X-ray techniques. Because ionic radius of M³⁺ and Li⁺ is similar, M³⁺ can be located in the Li site, which reflects in electrochemical performance of the mixed LiFeMnPO₄ system. Structural characteristics of stoichiometric LiFe_0.5Mn_0.5PO₄ and non-stoichiometric Li_1-xFe_0.5-yMn_0.5-yPO₄ are investigated using the high resolution powder diffraction (HRPD). Although the unit cell volume and lattice parameter of two samples is very similar, ratio of M³⁺ composition in the Li site is higher for the stoichiometric LiFe_0.5Mn_0.5PO₄ than non-stoichiometric Li_1-xFe_0.5-yMn_0.5-yPO₄ as a result of Rietveld refinement using the fullprof. This chemistry defect induces significant different in the electrochemical performance of Mn redox reaction region around 4.1V. The non-stoichiometric Li_1-xFe_0.5-yMn_0.5-yPO₄ shows the higher capacity than stoichiometric LiFe_0.5Mn_0.5PO₄ as shown in the Fig1a.

X-ray absorption fine structure (XAFS) spectra are measured for different state of charge (every 20mAh/g) to observe the change in oxidation state during charge. The Mn K-edge XANES spectra clearly shows the change of oxidation state in the Mn redox reaction region. Non-stoichiometric Li_1-xFe_0.5-yMn_0.5-yPO₄ goes to the higher energy than stoichiometric LiFe_0.5Mn_0.5PO₄ during the charge as shown in Fig1b. The Fe k-edge XANSE spectra show similar tendency both two samples. This indicates that, additional capacity of non-stoichiometric Li_1-xFe_0.5-yMn_0.5-yPO₄ is caused by Mn oxidation. The Fe k-edge XANSE spectra show similar tendency both two samples. To understand the effect of doping, Cr doped on the non-stoichiometric Li_1-xFe_0.5-yMn_0.5-yPO₄ investigated at the same time.

Decrease of the chemical defect can achieve improvement in electrochemical property of the Li_1-xFe_0.5-yMn_0.5-yPO₄ system. The detailed discussion will be presented in the meeting.