Self-assembled Nano-micro Structure LiFePO₄and Its Electrochemical Performace

Olivine LiFePO4is acknowledged to be the most promising cathode material for power batteries in electric vehicles (EVs) because of its brilliant cycling performance, high theoretical capacity and non-toxicity. However the process of its commercialization is limited to the low electrical conductivity and low tap-density, which can’t meet the requirement of power cell. Focusing on the above problems, we started work as follows:(1) The olivine lithium ion phosphate (LiFePO4) was prepared via solvothermal reactions using ethylene glycol (EG) as the solvent and urea as the additive respectively. The crystalline structure, particle morphology, and surface microstructure were characterized by high-energy synchrotron XRD, SEM, and FT-IR spectroscopy respectively. The influence of different reaction temperature and the content of urea on the morphologies and structures are investigated in details. And the electrochemical properties were investigated by charge/discharge test. The results show that the structure of obtained LiFePO4is flower-like, which is composed of LiFePO4nanosheets. The electrolyte is easier to permeate the spherical LiFePO4particles through the open porous. The tap density of the flower-like LiFePO4is1.2g·cm-3. It delivers discharge capacities of152mAh g-1at0.1C,134mAh g-1at0.2C,118mAh g-1at1C and104mAh g-1at2C. The discharge voltage platform has remained at about3.3V at2C.(2) The hierarchical microstructured LiFePO4sphere which self-assembled by nanoplates has been successfully synthesized via the low-temperature solvothermal reaction. These spheres show a uniform size distribution of14μm and are hierarchically constructed with two-dimensional nanoplates of20nm thickness. These tiny plates are densely aggregated in an ordered fashion. This hierarchical structure gives a relatively high tap density of1.4g·cm-3. The physical properties of the product were characterized by XRD, SEM, TEM, BET, and Raman spectroscopy. A reasonable formation mechanism is proposed on the basis of the results of time-dependent experiments and the effects of organic compounds during the formation of spherical LiFePO4. The charge-discharge test showed that the discharge capacity is158mAh g-1at0.1C, and103mAh g-1at2C. The LiFePO4sphere has good cycle performance in various charge-discharge rates.