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 Moessbauer 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 x¼ 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 <LieO> and <FeeO> interatomic distances in the M1 and M2 sites are almost constant, whereas the <PeO> distance slightly decreases (1.4% of the initial value). Interestingly, O2eO1eO2 bond angle along the [001] direction decreases with increasing Zncontent, thus resulting in a marked decrease of the c0 axis length although the <M1-O> distance remains essentially constant. M€ossbauer analyses show the majority of Fe to be divalent and located in an octahedral site similar to available literature data for LiFePO4. The Fe3þ/(Fe3þþFe2þ) 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.