Synthesis of low-cost LiFePO₄ from Li₂CO₃ by a novel hydrothermal method and investigation on the reaction mechanism

Abstract

Phspho-olivine LiFePO₄ was synthesized from the relatively insoluble lithium source Li₂CO₃, proper iron and phosphorus sources (n(Li):n(Fe):n(P)=1:1:1) by a novel hydrothermal method. Afterwards, the optimal sample was mixed with glucose and two-step calcinated (500 °C and 750 °C) under high-purity N₂ to obtain the LiFePO₄/C composite. The resultant samples were characterized by X-ray diffraction (XRD), atomic absorption spectrometry (AAS), scanning electron microscops (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), elementary analysis (EA) and electrochemical tests. The results show that the optimal reaction condition is to set the reactant concentration at 0.5 mol·L⁻¹, the reaction temperature at 180 °C for 16 h duration. During the reaction course, an intermediate product NH₄FePO₄·H₂O was first synthesized, and then it reacted with Li⁺ to form LiFePO₄. The optimized LiFePO₄ sample with an average particle size (300 to 500 nm) and regular morphology exhibits a relatively high discharge capacity of 84.95 mAh· g⁻¹ at the first charge-discharge cycle (0.1C, 1C=170 mA·g⁻¹). Moreover, the prepared LiFePO₄/C composite shows a high discharge capacity of 154.3 mAh·g⁻¹ at 0.1C and 128.2 mAh·g⁻¹ even at 5C. Besides it has good reversibility and stability in CV test.