| LiFePO4 cathode material is recognized as the safest power battery materials.It was themost potential cathode material for lithium batteries,which has long charging and discharging platform,stable nature,low price,environmental friendliness and many other advantages.There are two disadvantages of this materials:low electronic conductivity and low lithium ion transport rate,which hinder its development to higher specific capacity and poor high rate performance.Therefore,how to prepare LiFePO4 with regular morphology and excellent electrochemical performance has become the focus of research.At present,the synthesis methods of LiFePO4 mainly consist of one-step method and two-step method.They both have advantages and disadvantages.In order to compare the effects of different methods on LiFePO4,the morphology and properties of LiFePO4 synthesized by one or two steps were discussed.The carbon coatings were also discussed.The properties of LiFePO4 were characterized and measured by means of SEM,TEM,XRD,constant current charge-discharge and cyclic voltammetry,in order to explore the effects of morphological characteristics on electrochemical properties.Firstly,the precursor was synthesized by two-step method.The different time,temperature,pH and complexing agent on the morphology of FePO4 precursors during the hydrothermal synthesis of precursors were discussed.It is concluded that when the temperature was 120?,the best reaction time was 6 h at 1.5 pH and 1:1.2 ratio of iron to phosphorus.CTAB can also play the role of dispersing particles.Then,the structure of LiFePO4 synthesized by co-precipitation,micro-emulsion and hydrothermal methods was characterized,and the corresponding electrochemical properties were measured and compared.It was concluded that the sample particles obtained by co-precipitation method was the smallest.It reached the largest specific capacity of samples synthesized by microemulsion method,and the first charge and discharge specific capacity was 112 mAh·g-11 at 0.1 C ratio.The specific capacity of LiFePO4 batteries prepared by one-step method was low,but the Coulomb efficiency is high.Secondly,in order to maintain the integrity of the morphology.LiFePO4 was synthesized by one-step method.The pH,viscosity and solvent ratio on the morphology of LiFePO4 were discussed.The structure of LiFePO4 with three special morphologies synthesized by different solvents was compared.And we discussed the electrochemical performance of LiFePO4 with assembled batteries.It was concluded that when the pH of the solution was 9 and the volume ratio of ethylene glycol to water was 1:1,the morphology of LiFePO4 could be obtained.The regular shape of rhombic sample with diameter of about 1 micron can be obtained.The rod-like sample with diameter of about 400 nm can be obtained by using NMP as solvent,and the bulk sample with diameter of about 1micron can be obtained by using DMAC.Among them,the rod-like sample has the best total electrochemical energy,and the capacity reaches 125mAh·g-1at 0.1 C ratio for the first charge and discharge process.The specific capacity of samples synthesized by one-step method was generally higher than that by two-step method,but the efficiency was affected by the presence of a small amount of oxides.The structure and electrochemical properties of LiFePO4 coated with different carbon coatings were discussed.Ascorbic acid was used for in-situ carbon coating.When the carbon source content was 5%and 10%,the sample coatings formed by carbon source were 10 nm and 20 nm,respectively.The morphology of the samples synthesized by in-situ carbon coating method was related to the carbon source added.The particle size of the samples by using spirulina and superconducting carbon black as carbon sources was smaller than that of non-in-situ carbon coating.The first specific charge-discharge capacity of samples using Spirulina as templates was 131 mAh·g-1 at 0.1 C ratio. |
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