How to synthesize pure Li2−xFeSi1−xPxO4/C (x = 0.03–0.15) easily from low-cost Fe3+ as cathode materials for Li-ion batteries

Abstract

Li₂FeSiO₄ is a low-cost, environmentally friendly electrode material with high theoretical capacity. However, obtaining pure-phase Li₂FeSiO₄ on a large scale is difficult. In this study, pure Li_2−xFeSi_1−xP_xO₄/C is prepared easily by using the low cost compound Fe(NO₃)₃·9H₂O, with the help of citric acid and appropriate ratios of NH₄H₂PO₄ (x = 0.03–0.15). The possible mechanism of the system with NH₄H₂PO₄ to synthesize Li_2−xFeSi_1−xP_xO₄/C is that there is a catalysis process in the system, which helps to produce H₂, providing a reducing environment in every particle of the reactants guaranteeing a complete change from Fe³⁺ to Fe²⁺. The produced H₂ is verified by the gas chromatography of the collected gas produced in the calcination process. The ratios of NH₄H₂PO₄ in this system could adjust the valence of element Fe in the products. Without NH₄H₂PO₄, an Fe₂O₃ impurity is formed accompanying the Li₂FeSiO₄. With the addition of 1 at% NH₄H₂PO₄, the Li₄SiO₄ impurity accords with the objective Li_2−xFeSi_1−xP_xO₄/C. Also, Fe with zero-valence could be found as an impurity with the addition of 20 at% NH₄H₂PO₄ due to overreduction in the system. The synthesized pure Li_2−xFeSi_1−xP_xO₄/C (x = 0.03) displayed the highest discharge capacity of 179 mA h g⁻¹ in the first cycle, the best discharge capacity retention and the most reliable redox reversibility of the coulombic efficiency (approximately 100%), compared with the synthesized materials with Fe₂O₃ or Li₄SiO₄ impurities.