Synthesis And Modifications Of Li₂FeSiO₄Cathode Materials For Lithium Ion Batteries

As one of the cathode materials for lithium ion batteries, polyanion-type Li2FeSiO4has drawn intensive attention and made researchers devote a lot of energies to the study, due to its excellent cycling stability, good safety, low cost and environmental friendliness. However, the intrinsic low conductivity of Li2FeSiO4restricts its electrochemical performance, especially the performance at high rates. In this dissertation, several synthesis methods were designed and developed to fabricate Li2FeSiO4materials with different sizes, compositions and morphologies, which could insert/extract more than one Li+per Li2FeSiO4molecule and thus had extra-large specific capacities. Furthermore, by embedding Li+conductor materials inside and doping highly charged ions into the compounds, the rate performances of the materials were greatly improved.Li2FeSiO4/C materials were synthesized successfully with rheological phase method for the first time, with multi-wall carbon nanotubes coated by SiO2layer (MWCNTs@SiO2) and Li2SiO3nanorods-microspheres (NR-MS) hierarchical structure as the precursors respectively. XRD (X-ray diffraction), SEM (scanning electron microscopy), TEM (transmission electron microscopy) and EA (elemental analysis) were used to characterize the Li2FeSiO4/C. CV (cyclic voltammetry), EIS (electrochemical impedance spectroscopy) and galvanostatic charge-discharge were carried out to measure the electrochemical performance of Li2FeSiO4as cathode materials for lithium ion batteries. The EIS result indicated the interfacial mass transfer resistance of cell, with the Li2FeSiO4/C was synthesized using MWCNTs@SiO2with35.72%SiO2as precursor, was much lower.1.32Li-ion per formula could be extracted from Li2FeSiO4when it was discharged at a current density of0.1C on the first cycle. Discharge specific capacities of170,160,146.6,126,110.5mAh g-1were obtained when the current densities were0.5C,1C,2C,5C, IOC, respectively. The hierarchical structures of Li2SiO3NR-MS precursors were retained in the Li2FeSiO4/C products. A discharge specific capacity of136.5mAh g-1was obtained for these Li2FeSiO4/C microspheres after40cycles at the rate of0.1C.Li2FeSiO4/C was synthesized with improved hydrothermal assisted sol-gel route. In the synthesis process, oxidation-resisted, inexpensive and soluble Fe3+salt was used as the iron source and the carbon source was added into precursor solutions simultaneously. Using the above method, Li2FeSiO4/C materials were fabricated with citrate acid, PEG (polyethylene glycol), triblock copolymer P123(poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide)) as organic carbon sources respectively. XRD, SEM, TEM, EA, Raman spectroscopy and galvanostatic charge-discharge measurements were used to characterize the structure?morphology and electrochemical performance of the materials. The results showed that the Li2FeSiO4/C with the P123as carbon source had the most pores in the structure and the coating carbon on particle surface were the most ordered. The morphologies of Li2FeSiO4/C were influenced by the amounts of P123, which revealed the coating carbon from organic carbon sources played an important role in the crystalline growing process. Discharge specific capacities of119.2and104.4mAh g-1were achieved at the rates of1C and5C for the Li2FeSiO4/C that was synthesized by optimizing the amount of P123.Li2FeSiO4/C was synthesized by solvent evaporation-induced synthesis (SEIS) method. Li2FeSiO4/C nanoparticles were synthesized when pure ethanol was used as the solvent, while Li2FeSiO4/C nanoworms were obtained when the solvent was water-ethanol mixture with a special volume ratio. The structures and morphologies of the materials were characterized with XRD, SEM, TEM, EA, Raman spectroscopy and N2adsorption-desorption isotherm. For Li2FeSiO4/C nanoparticles, the charge-discharge results showed that1.39Li+per molecule could be inserted/extracted into/from the Li2FeSiO4reversibly at0.1C rate and room temperature. Besides Fe2+/Fe3+oxidation, some Fe3+ions were also oxidized into Fe4+ions, as suggested by the ex-situ Mossbauer spectra of the cathode materials in the first charge process. We believe that it is this Fe3+/Fe4+redox reaction that led to the more than one lithium ion insertion/extraction and the super-large specific capacities of the material. The materials also showed excellent rate capability. Discharge specific capacities of180,150,120mAh g-1were obtained at the high rates of1C,5C,10C, respectively. The Li2FeSiO4/C nanoworms synthesized by SEIS method were Li2FeSiO4nanoparticles coated and connected by amorphous carbon. The nanoworms had a lot of mesopores inside. The above special morphology of the Li2FeSiO4/C nanoworms could be used to explain the superior high rate performances of the materials. At the rates of5C,10C and20C, the discharge specific capacities can stabilize at137,109,92mAh g-1after200cycles, respectively.A series of Li2FeSiO4/C composites containing Li+-conductor Li3PO4were fabricated by the SEIS method using pre-synthesized Li3V2(PO4)3as additive. The XRD results showed that when the synthesis precursors of Li2FeSiO4existed, the Li3V2(PO4)3additive transformed into Li3PO4by the post-calcination. TEM images showed that Li3PO4and Li2FeSiO4crystals were coexisted in the particles of composites. The charge-discharge tests indicated that, by adjusting the amount of Li3V2(PO4)3additives in the synthesis, an appropriate amount of Li3PO4in the composite can be obtained, which can improve the rate performance of the cathode material effectively. When the mole ratio of Li3V2(PO4)3was adjusted to6%, the discharge specific capacities of173.2,157.4and130.7mAh g-1were obtained at the current densities of166,332and830mA g-1for the composite.Li2-yFeSii-yPyO4/C compounds (y=0,0625,0.125,0.25) were synthesized by doping P into the Si4+site in the molecule by SEIS method using NH4H2PO4as the P source. The XRD results suggested that the cell volume became smaller when appropriate amount of P (y<0.125) was induced into the lattice of Li2FeSiO4. The galvanostatic charge-discharge measurements showed that the rate performance of Li2-yFeSi1-yPyO4/C reached the best when y was0.0625. At the rates of166,332and830mAg-1, the discharge specific capacities can stabilize at163.9,156.3and143.3mAh g-1. EIS results showed that the interfacial mass transfer resistance between electrode and electrolyte was lower after doping P in Si4+sites.