Effect of pH-dependent intermediate on the performance of LiFePO4/C cathode material

Ferric phosphate (FePO4), as the precursor for LiFePO4 preparation, is customary to be obtained by coprecipitation for its convenience and low cost. In this study, FePO4 with different morphology were prepared at gradient coprecipitation pH and reduced to form LiFePO4/C cathode materials to quantitatively investigate the pH-dependent intermediate influence. The characterization results showed that the structure and morphology of FePO4 changed markedly from crystalline hydro-micro ball to amorphous and to fusiform intermediate with the increase of coprecipitation basicity, while high concentration of NH4+ would occur in the intermediate composition with a severe particle growth and aggregation at exorbitant pH. The electrochemical analysis manifested that FePO4 morphology and crystallinity would not affect the cyclic stability of cathode materials, however, a more even and porous FePO4 obtained from intermediate at highly acidic solution would equip the LiFePO4/C with an excellent performance. Besides, the hydro-crystalline intermediate-oriented FePO4 could provide LiFePO4/C with an excellent low charging-rate performance and the LiFePO4/C from amorphous spherical intermediate-oriented precursor was endowed with an advantageous high-rate capacity via the excellent crystallinity and charge transfer. Uneven morphology and severe particle growth would bring about obvious reduction to the discharge capacity of LiFePO4/C that the discharge capacity at 1C rate would decrease from the optimum of 157.79 mAh·g−1 to <104.38 mAh·g−1 and 82.79mAh·g−1 for the FePO4 prepared with intermediates precipitated at pH of 3.0 and 2.0, respectively.