| LiFePO4 is considered as the most promising cathode material for lithium-ion batteries(LIBs)because of its high capacity,low cost,high safety and environmental benignity.However,due to its low lithium ion diffusivity rate,low electronic conductivity and energy density,unsatisfied performance at low temperature,LiFePO4 is limited in a wide range of applications in the field of power LIBs.At present,researchers mostly use solvothermal method to synthesize high-performance LiFePO4 nanocrystals,but this method is not suitable for large-scale production because of its low yield and high cost.Although the conventional hydrothermal synthesis method has the advantages of low cost,uniform particle size and controllable morphology,the synthesized samples have large grain size and unsatisfied electrochemical performance,which can't be used in industrial production.Therefore,it is urgent to develop a low-cost hydrothermal method to synthesize high-performance LiFePO4 nanocrystals.In this thesis,LiFePO4 nanocrystals with high-performance were synthesized by microwave-assisted hydrothermal method based on the understanding of the nucleation and growth mechanism of LiFePO4,the low temperature performance and energy density of LiFePO4 were systematically studied,which laid the foundation for promoting the industrialization of LiFePO4 synthesized by hydrothermal method.The main contents and conclusions are as follows:(1)Based on the understanding of LaMer mechanism,high-performance LiFePO4 nanocrystals were synthesized by microwave-assisted hydrothermal method and the excess lithium source in the filtrate was recycled,which reduced the cost of raw materials.Furthermore,the volumetric specific yield is 1.3 mol L-1,which is 30%higher than that of the conventional hydrothermal method.The nanocrystals synthesized by hydrothermal method have[010]and[100]orientations.LiFePO4 nanocrystals,synthesized by two types of LiOH,have similar discharge specific capacity at each rate and excellent cycle stability.The discharge specific capacity of LiFePO4 is 122 mAh g-1 after 1000 cycles at the high rate of 3C(charge/discharge in 20 min)and the retention rate is as high as?88%.The migration impedance(Rm)of Li+ions in the active material lattice is proposed for the first time,which is of great significance for understanding the intercalation behavior of LIBs.(2)In order to improve the energy density of LiFePO4,LiMn0.5Fe0.5PO4 solid solution nanocrystals were synthesized by microwave-assisted hydrothermal method.The discharge capacity of LiMn0.5Fe0.5PO4 solid solution is 168 mAh g-1 at 0.1C,and the energy density is 625 Wh kg-1,which is 16%higher than that of LiFePO4(540 Wh kg-1).The results of in situ electrochemical impedance spectroscopy(EIS)analysis show that the synthesis of LiMn0.5Fe0.5PO4 solid solution can effectively alleviate the Jahn-Teller effect of Mn3+ and ameliorate the electrochemical performance of LiMn0.5Fe0.5PO4 solid solution.After long cycle test,the adhension of electrode active material and current collector is not tight,which leads to the increase of charge transfer resistance(Rct)and the decrease of Li+diffusion coefficient,then it hinders the migration of Li+ions and the capacity of LiMn0.5Fe0.5PO4 solid solution decays after long cycle test.Using Li4Ti5O12 as the negative electrode,the discharge capacity of full cell is 118 mh g-1at a high rate of 10C.Furthermore,the full cell discharge capacity is 124 mAh g-1 after 500 cycles at 1C and retaining 92.5%of the initial capacity.The excellent electrochemical performance of the full cell lays a foundation for the practical application of LiMn0.5Fe0.5PO4.(3)In order to understand the excellent electrochemical performance of LiMn0.5Fe0.5PO4 solid solution synthesized by hydrothermal method,LiMn0.25Fe0.75PO4,LiMn0.5Fe0.5PO4 and LiMn0.75Fe0.25PO4 solid solutions were synthesized by microwave-assisted hydrothermal method and the(de)lithiation mechanism of the three samples was studied.The results of electrochemical characterization results show that LiMn0.5Fe0.5PO4 solid solution has the highest specific capacity and energy density,and good cycle stability.Mossbauer spectroscopy was used to characterize the three samples after carbon coating.The results show that LiMn0.5Fe0.5PO4/C has less types of iron ion defects and low content of FeLi anti-site defect.Low defect types and defect concentration are favorable for the migration of Li+ions.The phase transformation mechanism of the three solid solution samples was characterized by operando X-ray diffraction.The three solid solution materials with the same morphology show different delithiation and lithiation mechanisms.LiMn0.5Fe0.5PO4 solid solution has a single-phase transition in the whole process of charge and discharge,whereas the other two solid solution samples have a two-phase transition.The sample with single-phase transition has smaller lithium miscibility gap and higher number of activated particles than that of samples with two-phase transition.In situ electrochemical impedence spectroscopy and galvanostatic intermittent titration were used to analyze the diffusion kinetics of Li+ions in the three solid solutions.The results show that the Rct value of LiMn0.5Fe0.5PO4 is smaller than that of the other two samples,and LiMn0.5Fe0.5PO4 has the highest Li+ion diffusivity value.Therefore,LiMn0.5Fe0.5PO4 solid solution has superior electrochemical performance.(4)The unsatisfied electrochemical performance of LiFePO4 at low temperature limits its wide application.In order to improve the electrochemical performance of LiFePO4 at low temperature,LiFePO4 nanocrystals synthesized by microwave-assisted hydrothermal method were composited with carbon nanotubes to prepare three kinds of LiFePO4/CNT composite electrode.The low temperature electrochemical properties of LiFePO4/CNT were systematically studied.The surface and cross-sectional morphology of the composite electrode were observed by scanning electrom microscopy,it was demonstrated that in the three composite electrode materials,LiFePO4/CNT1.6 has the best amelioration effect and carbon nanotubes play the role of conductive network.The surface resistance of the LiFePO4/CNT1.6 composite electrode is the smallest.The results of low temperature electrochemical test show that LiFePO4/CNT1.6 electrode has the best low temperature performance and the discharge capacity is 98 mAh g-1 at-10 ?,which is 29%higher than that of the un-composite electrode.Besides,the long cycle stability of LiFePO4/CNT1.6 composite electrode is the best With the decrease of temperature,the values of interface resistance Reei,Li+ions migration resistance Rm and charge transfer resistance Rct increase,whereas the diffusion coefficient of Li+ions decreases,which hinders the migration of Li+ions in the lattice of active material in the electrode,resulting in the capacity degradation of LIBs at low temperature. |
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