The Synthesis Of Lithium Iron Phosphate LiFePO₄ And Its In-situ High Pressure Studies

As a new replaceable energy material, lithium ion batteries have attracted a lot of research interests. Lithium ion battery has high capacity, and is environmentally recycled. Now, the studies of lithium ion batteries are mainly concentrated in battery capacity and enhanced electrochemical performance. By improving the anode materials of lithium ion batteries to higher battery capacity, it has become the current research direction. Among these anode materials, lithium iron phosphate LiFePO₄ has been investigated quite thoroughly. LiFePO₄ has low toxicity and good reversibility, and the big anions in crystal structure can stable the whole structure. By carbon-encapsulating or metal ions doping, it can improve the capacity of LiFePO₄.In this thesis, at the protective atmosphere with nitrogen,Li₂CO₃, Fe₂O₃ and glucose NH4H2PO₄ used as raw materials, carbon-encapsulated LiFePO₄ materials have been synthesized by carbonthermal reduction method. The XRD analysis shows the agreements with the standard PDF card. Meanwhile, the crystal structure of LiFePO₄ was studied, and besides its Raman active vibration modes at ambient pressure.Raman spectroscopy and synchrotron radiation X-ray diffraction technique are used for in-situ studies of LiFePO₄ under high pressure. The Raman spectra and the changes of lattice constants at high pressure have been presented. High-pressure Raman spectra show that all Raman modes are shifted to higher wave numbers with the increase of the pressure. Some Raman modes related with the vibrations of PO₄ cluster are obviously enhanced at high pressure. While releasing the pressures, no internal stress is found. The lattice constants and the unit cell volume become smaller continuously with the pressure increasing pressure.The doping effects of different kinds of metal ions have been studied on the base of carbon-encapsulated LiFePO₄. In this thesis, doping of Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, Co³⁺, and Mn⁴⁺ metal ions have been investigated, and the samples are characterized by XRD and Raman spectroscopy. The results show that the metal ions can be easily doped into LiFePO₄, without any new structure change. Namely, they can from solid solution with LiFePO₄ as LiFe_1-xM_xPO₄ (M = Zn²⁺ Cu²⁺, Ni²⁺, Co²⁺, Co³⁺, Mn⁴⁺).