Study Of Synthesis And Application On Lithium Iron Phosphate Cathode Materials

This dissertation focuses on the synthesis of LiFePO4, the fabrication of Li-ionbatteries, and the integration of battery systems. In order to improve the shortcomings ofLiFePO4, such as large surface area, low electrical conductivity, and low electrochemicalactivity, we prepared LiFePO4and LiFePO4/C using hydrothermal method and high-speedcentrifugal spray-drying technique. We studied the preparation techniques of LiFePO4andhave realized its industrialization. We also designed the power battery system for pureelectric vehicles based on18650-type batteries.We synthesized LiFePO4by the hydrothermal method using LiOH H2O, H3PO4andFeSO47H2O as the raw materials, and optimized the synthesis conditions. The carbon-freeLiFePO4shows an initial discharge capacity of113mAh g-1. We also synthesized the solidspherical LiFePO4/C by the high-speed centrifugal spray-drying method with LiH2PO4,Fe2O3and glucose as the raw materials. The LiFePO4/C composite has a high tap density of1.56g cm-3.We studied and optimized the preparation technique of LiFePO4through the selectionof raw materials, experimental equipments and synthesis methods. Finally, we formulatedthe raw material standard and industrial production equipments LiFePO4. We now can massproduce LiFePO4materials with many advantages, including high specific capacity, lowcost, good safety, and other good comprehensive properties. We have successfully appliedthem to the commercial batteries.We studied the influence of conductive additives, such as CNT, Super-P, and KS-6,on the electrochemical performance of LiFePO4/C composite. CNT could be uniformlydispersed in the LiFePO4/C composite, forming abundant conductive networks. Theaddition of CNT could effectively reduce the internal resistance of the electrode, increasethe utilization efficiency of active materials, and improve the initial coulombic efficiencyof the battery. LiFePO4/C composite without CNT only delivers a low discharge capacityof139.1mAh g-1, where a high internal resistance of3.25mΩ is observed. In contrast, ahigh discharge capacity of146.3mAh g-1can be obtained by adding CNT while theinternal resistance is decreased to1mΩ. For the CNT-free electrode, its discharge capacity at7C rate is only81.34%of that at1C rate. However, for the CNT-added electrode, itsdischarge capacity at9C rate was almost the same as that at1C rate. Moreover, thecapacity retention of the CNT-added electrode after2165cycles is91.78%, indicating thatthe electrode shows a good cycle performance.We also studied the electrochemical performance of LiFePO4electrodes pasted oncarbon-coated aluminum foils. Using carbon-coated aluminum foils could not only improvethe adhesion of LiFePO4, but also provide a good conductive layer between LiFePO4particles and aluminum foil to reduce the contact resistance in between. It is beneficial tothe improvement of the electrode’s rate performance. The as-fabricated electrode shows anincrease in discharge plateau by0.3–0.4V at10C rate, and also delivers a15%higherdischarge capacity than the conventional electrode directly pasted on aluminum foil at15Crate.Finally, we developed a316.8V/48Ah power battery system for pure electric vehicles.This system consists of battery pack, battery container, battery management system (BMS),vehicle communications wiring harness, battery high/low voltage wiring harness, heatersand high/low voltage connectors, etc.32batteries were connected first in parallel, and then99batteries were connected in series. The nominal voltage of the whole battery pack is316.8V, the working voltage range is247.5346.5V, the total energy is15.2KWh, theweight of whole package is about182Kg (the weight of batteries is about112Kg), and theenergy density of the whole package was about83.6Wh Kg-1.