The Solid State Reaction In Hydrated Iron Phosphate Cathode Material Prepared Its Charge And Discharge Performance Studies

Ferric phosphate (FePO4) has attracted more and more attention as the delithiated form of LiFePO4 in connection with cathode material in lithium-ion batteries (LIBs) in the last few years due to its easy synthesis, inexpensiveness and a relatively high discharge voltage. Untill now, FePO4 is mainly prepared by precipitation and hydrothermal method. However, these proposed preparation methods are relatively complicated and are hard to get the pure phase of FePO4 Moreover, there are two intrinsic negative aspects of iron phosphate-based material for LIBs. One is the low electronic conductivity and the other is the low lithium ion diffusion coefficient. The two intrinsic drawbacks of iron phosphate-based material pose a bottleneck for the commercial applications. For the above problems, the content which we have studied in this thesis is as following:1. Solid-state reaction method at low heat has many advantages, such as few solvent, high selectivity, low energy consumption. Thus, it has become a noval synthetic technique for parparing nano size inorganic materials. In this work, FePO4·2H2O is firstly prepared by solid-state reaction method. The experimental results reveal that an approprate P source plays an important role on generation of FePO4·2H2O with high purity and crystallinity. Meanwhile, electrochemical performance of the as-prepared FePO4·2H2O and FePO4 are investigated. The corresponding electrochemical results indicate that the as-prepared FePO4·2H2O possess high discharge capacity and excellent cyclic performance.2. FePO4·2H2O has been synthesized by a solid-state reaction method at ambient-temperature. The influence of different synthesis conditions, such as P/Zn ratios, ageing temperature, aging time and surfactant, on structure, morphology and electrochemical performance of FePO4·2H2O sample are explored. X-ray diffraction (XRD) measurements indicate aging time does not affect structure of FePO4·2H2O sample, whenas a higher P/Fe atomic ratio and a lower the aging temperature result in easily the generation of FePO4·2H2O with amorphous structure. In addition, the spherical FePO4·2H2O can be obtained in the presence of PEG-6000. The results of electrochemical tests indicate that the spherical FePO4·2H2O sample has an excellent rate capability.3. Multiwalled carbon nanotubes (MWNTs) supported ferric phosphate hydrate nanocomposites (FePO4·2H2O/MWNTs) are prepared by a novel homogeneous precipitation. The structure and compounds of the composites are characterized by XRD, IR, TG-DTA. TEM images display that the ultrafine FePO4·2H2O nanoparticles with 20 nm particle size are highly uniformly dispersed on MWNTs surface. The discharge capacities show that FePO4·2H2O/MWNTs nanocomposites have a superior electrochemical performance. For example, FePO4·2H2O/MWNTs nanocomposites exhibit a high initial discharge capacity (129.9 mAh·g-1) and a stable capacity retention (114.3 mAh·g-1 after 20 cycles). The excellent electrochemical performance is attributed to the small particle size of FePO4·2H2O nanoparticles, the good electronic conductivity of MWNTs, and the particular three dimensional conductive networks of FePO4·2H2O/MWNTs nanocomposites.