Study On Synthesis And Electrochemistry Performance Of LiFePO₄ As Cathode Materials For Lithium-ion Batteries

Olivine-structured LiFePO4, as the new anodic material of lithium battery, which is considered to be one of the best suitable anode materials, possesses the following merits: high safety, favorable thermostability and cycle performance, inexpensive, environmental friendly etc. Therefore LiFePO4 has become hot spots of the battery development and research around the world.FePO4 and LiOH·H2O were used to synthesize LiFePO4, reducing the precursor and materials type, lowering material prices, as well as simplifying synthesizing process, which is benefit for the large scale production. During LiFePO4 synthesizing progress, the structure, topography and compozation have great effect on the performance of LiFePO4. Therefore, FeCl3·6H2O, Fe(NO3)3·9H2O and FeSO4 were used as ferrum source, NH4H2PO4 and Na2HPO4·12H2O were used as phosphorus source. FePO4·2H2O and NH4Fe2(PO4)2(OH)·2H2O were obtained. Through controlling different material and adding order obtained oblate spheroid, oblate spheroid with sunk center, schistose as well as flocculent FePO4.The aforementioned FePO4 were used as ferrum and phosphorus source, LiOH was used as lithium source, dextrose was used as reducing agent and carbon source, synthesizing LiFePO4/C through high temperature solid method. TG-DSC analysis indicated that the LiFePO4/C preparing temperature should be higher than 500℃; XRD pattern showed that LiFePO4/C prepared using different FePO4, were all well-crystallized at the temperature of 650℃. SEM micrograph showed that, the prepared LiFePO4/C were random and cylindrical shape, and obviously different with the precursor. LiFePO4/C prepared using FePO4 with different topography, were undertaken charge/discharge cyclic test, the result indicated that, the first discharge specific capacity at 0.5C of LiFePO4/C prepared using oblate sphriod and schistose FePO4 were 126.42 mAh·g-1 and 127.35mAh·g-1 respectively, the discharge specific capacity after 20 charge and discharge cycle were 122.83 mAh·g-1 and 123.43 mAh·g-1 respectively. The first discharge specific capacity at 0.5C of LiFePO4/C prepared using flocculent FePO4 was 106.31mAh·g-1, the discharge specific capacity after 20 charge and discharge cycle was 104.47mAh·g-1. LiFePO4/C prepared using FePO4, which synthesized with different materials, were undertaken charge/discharge cyclic test, the result showed that the first discharge specific capacity at 0.5C of LiFePO4/C prepared using FeSO4 and NH4H2PO4 was 82.22 mAh·g-1 and the discharge specific capacity after 20 charge and discharge cycle was 78.84mAh·g-1; The first discharge specific capacity at 0.5C of LiFePO4/C prepared using FePO4, which synthesized with other materials, had litter discrepancy and the highest value was 128.88 mAh·g-1, the discharge specific capacity after 20 charge and discharge cycle was 125.27 mAh·g-1 Thus it can be seen the optimal electrochemical performance of LiFePO4/C were prepared using oblate sphriod and schistose FePO4 precursor.