Impact of synthesis conditions on the structure and performance of Li2FeSiO4

Three different synthesis techniques (hydrothermal synthesis, modified Pechini synthesis and Pechini synthesis) were successfully used for preparation of Li₂FeSiO₄ samples. The obtained samples possess some differences in the morphology and in the particle size, as well as in the presence of in situ formed carbon. The best electrochemical performance has been obtained with the smallest particles embedded into carbon matrix. Such a Li₂FeSiO₄/C composite contains the highest amounts of impurities (Fe₂O₃, SiO₂ and Li₂SiO₃) and only 68.8 at.% of iron is in the form of Fe^II as detected by Mössbauer spectroscopy, respectively. Despite the highest amount of impurities, the sample shows the highest reversible capacity (approximately 100 mAh g⁻¹ based on whole silicate-derived material). With the proper structuring of Li₂FeSiO₄/C composites, utilisation of large part of capacity is also possible at current densities corresponding to C/5 and C/2 cycling rate. A lower amount of impurities was found in the samples that do not contain any in situ carbon after synthesis. Among them, the highest purity is possessed by the sample prepared at 900 °C, as determined using Mössbauer spectroscopy. The results obtained by Mössbauer spectroscopy and XRD analysis indicate on the differences in the crystal structure between the thermally treated samples and the sample prepared by hydrothermal synthesis.