Synthesis and characterization of LiMPO[4] (M=Mn, Ni and Fe) olivine materials / Azlin Sanusi

Sanusi, Azlin (2010) Synthesis and characterization of LiMPO[4] (M=Mn, Ni and Fe) olivine materials / Azlin Sanusi. PhD thesis, University of Malaya.

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Abstract

Lithium transition metal phosphate, LiMPO4 compounds with olivine structure namely LiMnPO4, LiNiPO4 and LiFePO4 were successfully synthesized in the present work. LiMnPO4 and LiNiPO4 were synthesized by the sol-gel method with tartaric acid as the chelating agent. LiFePO4 was synthesized via the chemical precipitation and low temperature sterilizer heating method. TGA was carried out on LiMnPO4 and LiNiPO4 samples to determine the firing temperature of the gel precursor and the thermal stability of the prepared samples. LiMnPO4 was heated for 15 hours at various temperatures and LiNiPO4 was formed at 1000 oC with firing time of 24 hours. Firing of samples in this work was carried out in air. The absence of inert gas and reducing agents made the synthesis simple and low cost which is one of the objectives of this work. Reactions mechanisms for both LiMnPO4 and LiNiPO4 were deduced from the TGA curves. The weight loss predicted by these reaction mechanisms agree with that obtained experimentally for the two samples. XRD reveals that both LiMnPO4 and LiNiPO4 samples are pure while LiFePO4 was formed with minor impurities. Crystallite size of samples were calculated from Scherrer equation and both methods have proven to produce nano-size crystallites. The crystallite size was found to be in the range of 31 nm to 46 nm for LiMnPO4 produced at sintering temperatures from 350 oC to 1000 oC for 15 hours, 66 nm for LiNiPO4 fired at 1000 oC for 24 hours and 58 nm for LiFePO4 produced at 121 oC for 5 hours. Thus, tartaric acid as chelating agent is successful in inhibiting the growth of the particle as expected since nanosized samples are obtained. Samples were also characterized by FTIR. All spectra show the presence of peaks at 1135-1138 cm-1,1088-1096 cm-1,1051-1071 cm-1 and 984-1040 cm-1 and in addition asymmetric PO bending bands in the region 550-700 cm-1 were also observed. These observations indicate the presence of PO43- ions in all samples v prepared. EDX analysis confirmed the percentage composition of the samples and that Mn2+, Ni2+ and Fe2+ oxidation state remain unchanged although fired in air. Cyclic voltammetry measurements gave redox peaks for all samples prepared in this work indicating lithiation and delithiation of Li+ ion has taken place. Oxidation peak at ~ 4.4 V and reduction peak at ~ 4.0 V was observed for LiMnPO4 and as for LiNiPO4 anodic and cathodic peaks are observed at ~ 4.9V and ~ 4.8V respectively. Oxidation peak at ~3.8 V and reduction peak at ~3.2 V are observed in the cyclic voltammogram of LiFePO4. Diffusion coefficient calculated using CV and Randles-Sevcik equation are 3.50 x 10-11 cm2s-1 for LiNiPO4 and 3.09 x 10-13 cm2 s-1 for LiFePO4. The diffusion coefficient for LiMnPO4 are 9.37 x 10-13 cm2 s-1, 8.57 x 10-13cm2 s-1 and 2.97 x 10-14 cm2 s-1 for sample A3, A4 and A5 respectively indicating that smaller particles at lower heating temperature has shortened the path length and enhances intercalation.

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