Synthesis and characterization of Zn-doped LiFePO4 cathode materials for Li-ion battery

Synthetic LiFe1-xZnxPO4 (0 ≤ x ≤ 0.26) samples have been prepared using wet chemistry method. The samples have been characterized by means of Rietveld structure refinement of Powder-XRD data and by Mössbauer spectroscopy. Impurities such as Li3PO4 and Fe2P have been detected in small amount and quantified by Rietveld analysis. No Zn-bearing impurity has been detected up to x = 0.15, but significant amounts of LiZnPO4 have been observed in the samples with x ≥ 0.2. The unit cell volume of LiFePO4 decreases anisotropically with increasing Zn content (−0.7% of initial volume of 291.343 Å3). Cell parameter (orthorhombic Pbnm space group, #62) b0 and c0 decrease with increasing Zn content, whereas a0 parameter is almost constant. The average and interatomic distances in the M1 and M2 sites are almost constant, whereas the distance slightly decreases (−1.4% of the initial value). Interestingly, O2-O1-O2 bond angle along the [001] direction decreases with increasing Zn-content, thus resulting in a marked decrease of the c0 axis length although the distance remains essentially constant. Mössbauer analyses show the majority of Fe to be divalent and located in an octahedral site similar to available literature data for LiFePO4. The Fe3+/(Fe3++Fe2+) ratio ranges from 0.10 to 0.17, and is not related to the Zn content. Preliminary electrochemical analyses of Zn doped samples in comparison to pristine LiFePO4 are reported.