Controllable Synthesis And Electrochemical Properties Of LiFePO₄Nano-and Microcrystals

Olivine-structured LiFePO4is potential as a lithium-ion cathode material. One impediment to the wider using of LiFePO4, however, is its low intrinsic conductivity. In rencent years, various approaches such as carbon coating, foreign metal doping, morphology control, and reducing the particle size have been successfully adopted. Among the effective approaches available, reducing the particle size of LiFePO4is a much more efficient way. Its main purpose is shortening the transport path lengths of Li+ions and increasing the high rate charge-discharge property.In this paper, we aimed at the controllable synthesis of micro-nano LiFePO4. By optimizing the hydrothermal synthetic process and exploring new synthetic method, the particle size of LiFePO4was reduced and the electrochemical property was increased. The samples were characterized by XRD, FE-SEM, HRTEM, FTIR, BET, Charge-discharge system and CV. The growth mechamism of LiFePO4has been discussed.1. The effects of the adding sequence of the precussors and the content of EG on the structure of the obtained LiFePO4samples were studied. The results showed that the Li-PO4-Fe sequence has the advantage of preparing smaller LiFePO4particles that the Fe-PO4-Li sequence. The morphology and particle size of the obtained samples can be adjusted by the changing of EG/W ratio. The addition of EG can reduce the particle size of the obtained samples and increase the electrochemical property.A facile precursor-separated hydrothermal synthesis (PSHS) method was developed to synthesize LiFePO4nano-crystallines where the iron sulfate solution precursor was packed in polymer film to divide it from Phosphoric acid and Lithium hydroxide before reaction. This method avoided the probably generated Fe3+at the precursors mixing process. The precursors were not mixed until the temperature reached a certain value. The edges of the polymer film started to melt and the FeSO4solution was released at the impact of pressure. The particle size was reduced to nanoscale (about50-200nm) without any additives. The as-prepared product showed excellent electrochemical properties especially rate capability. The reaction mechanism of this method was also studied. Then the controllable LiFePO4particles was synthesized by different EG/W ratios. The particle size was reduced by the increasing of EG content. The BET result showed that the specific surface area was also increased regularly. The high rate performance of the obtained samples wasincreased.The LiMnPO4material was synthesized by the PSHS method. The effects of pH and Mn/Fe ratio were investigated. The result shows that the pH has great influence on the phase purtity of LiMnPO4. The Fe2+was added to obtain LiMn1-xFexPO4/C (x=0.2,0.5,0.8) materials. The electrochemical properties of LiMn1-xFexPO4/C (x=0.2,0.5,0.8) were better than LiMnPO4and there are two platforms of the charge and discharge profiles. The sample of LiMn0.5Fe0.5PO4/C has the highest discharge capacity of134mAh g-1at0.1C and76mAh g-1at IOC.