Effect of Iron-based Impurities on the performance of nanostructured C-LiFePO$_{4}$ cathode materials for Li ion Batteries

We report synthesis of pure and C-LiFePO$_{4}$ nanoparticles in 20-30 nm size by sol-gel method. Three samples of C-LiFePO$_{4}$ were prepared by mixing 0.25M, 0.50M, and 1M lauric acid in the precursor solutions for carbon coating the particles. The samples were~characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), IR spectroscopy, SQUID magnetometery, Raman spectroscopy, and Fe$^{57}$ M\"{o}ssbauer spectroscopy. All the samples were thoroughly investigated for their electrochemical properties. The XRD measurements showed all the samples are single phase materials with no impurity phase. However, we identified at least three residual non crystalline impurity phases simultaneously using Fe$^{57 }$M\"{o}ssbauer spectroscopy, XPS, and the magnetic measurements. The elemental chemical states for Fe 2p, P 2p, and O 1s are analyzed using XPS for LiFePO$_{4}$ and compared with those of C-LiFePO$_{4}$ materials. SQUID magnetometery measurements suggest an antiferromagnetic transition $\sim $50 K in both pure LiFePO$_{4 }$and C-LiFePO$_{4}$ samples. The role of various phases, such as FeP, Fe$_{2}$P, $\alpha $-Fe and Fe$_{2}$O$_{3 }$identified and analyzed by Fe$^{57}$ M\"{o}ssbauer spectroscopy and XPS, will be discussed in relationship to the electrochemical properties of the cathode materials.