Fe-based Cathode Material Of Lithium-ion Batteries

The advantages of iron-based lithium cathode material include abundant resources,environmental benignity, low price and high-coulombic efficiency. In this dissertation, theiron-based lithium cathode material have been focused on Fe₂（SO₄）₃、FePO₄andLiFePO₄. We hope to develop Fe-based cathode materials of lithium ion batteries withlower cost, simpler technology and high performance by further studying of theirelectrochemical properties. The conclusions have been summarized as following:（1）The electrochemical properties of Fe₂（SO₄）₃were studied. Raw materials ofFe₂（SO₄）₃was sintered in air with different temperature and time.The charge-dischargeresults showed that Fe₂（SO₄）₃samples sintered at400℃with9h had the bestelectrochemical performance. Its initial discharge capacity could reach85%of thetheoretical capacity and maintained60mAh/g in the end. Further analysis of the reactionmechanism was done. On basis of CV measurement, the redox peaks were found to be at3.92and3.37V. The electrochemical impedance measurements have shown that theexchange current density is10-2¹0-4mAcm-2. The mechanism of Fe₂（SO₄）₃charge-discharge reaction is two phases at high state of charge and single phase at low stateof charge. We speculated that only one lithium ion is reversible inintercalation/de-intercalation reaction.（2）The electrochemical properties of FePO₄were studied. Raw materials of FePO₄were sintered in air at different temperature with9h. The XRD results showed FePO₄thatexisted both amorphous and crystalline structures. When temperature rises up to that400℃,the amorphous structure gradually change to crystal structure and FePO₄materials willcompletely crystallization at700℃.Meanwhile, FePO₄samples sintered at400℃with9h had the best electrochemical performance. The further studies of synthesizing FePO₄synthsized by co-precipitation and modified commercial FePO₄by coating showed thatsynthetic FePO₄sintered below400℃had a high capacity due to their loose structure,however, their cycle stability turn poor after30cycles; for coated commercial FePO₄, itsinitial capacity can improved with a certain range. However, due to the Li+intercalationand de-intercalation, the integrity of surface materials may be destroyed and itselectrochemical performance becomes poor.（3）The electrochemical properties of LiFePO₄synthesized by using FePO₄werestudied. The LiFePO₄was synthesized by solid consistency method at differenttemperatures. The XRD、SEM etc results showed that LiFePO₄samples sintered at the550℃had the best electrochemical performance, its charge capacity can reach120mAh/g andstill retained stable after100cycles. In order to further study the influence of raw materialFePO₄, we synthesized LiFePO₄and FePO₄compounds by controlling the content of LiOH,the results showed that FePO₄did not take part in the charge-discharge reaction ofLiFePO₄, and instead directly contribute a small percentage of capacity as cathodematerials. On basis of the above-mentioned work, we tried to modify the LiFePO₄bycoating or doping La/Ce oxides. Facts proved that after the doping by1wt.%La/Ce oxides,the specific capacity of LiFePO₄improved20mAh/g and reached140mAh/g at0.1C.Moreover, its rate performance also improved, especially LiFePO₄doped with1wt.%La2O3, the capacity still maintained90mAh/g at5C.