Temperature dependent study of impurities in LiFePO$_{4}$/C nanoparticles and their impact on electrochemical performance

We have synthesized LiFePO$_{4}$/C nanoparticles using a simple sol-gel method followed by calcination at various temperatures from 600 $^{\circ}$C to 900 $^{\circ}$C. X-ray diffraction shows that samples annealed at 600$^{\circ}$ C are phase pure while those treated at higher temperatures contain Fe$_{2}$P and Li$_{3}$PO$_{4}$ impurity phases, which increase with increasing annealing temperature. Mossbauer spectroscopy and magnetic measurements were used to quantify the amount of Fe$_{2}$P impurity phase. Scanning electron microscopy measurement reveals a noticeable increase in particle size as the annealing temperature increases from 700 $^{\circ}$C to 900 $^{\circ}$C. Optimal results are obtained in LiFePO$_{4}$/C samples annealed at 700 $^{\circ}$C, which show the lowest charge transfer resistance, highest Li-ion diffusion coefficient, the highest specific capacity of 166 mAh/g at a rate of 1C and the best rate capability and cycling stability among all samples.