Study On Morphology Control And Performance Of LiFe_1-xMn_xPO₄（x=0-1.0）

LiFe_1-xMn_xPO₄ was obtained by replacing Fe with partial Mn in LiFePO₄,it combines the advantages of relative high electrical conductivity of LiFePO₄ and relative high voltage of LiMnPO₄,which can improve the working voltage of LiFePO₄ material and then improve the specific energy of the material.As the olivine-type materials has only one-dimensional lithium ion diffusion channel,its crystal morphology,particle size,crystallinity and so on play a key role in the electrochemical performance of the material.In this paper,LiFe_1-xMn_xPO₄ with different morphologies was synthesized by hydrothermal method at the temperature of 160°C using LiOH,FeSO4·7H2O,MnSO4·H2O and H3PO4 as raw materials.Based on the analysis of the phase and morphology of the reaction intermediate product and the final product,the mechanism of crystal morphology change during hydrothermal synthesis was revealed for the first time.And the effects of different Mn contents（x=0,0.4,0.5,0.6,0.7,1.0）on the morphology and properties of the products were studied.First,LiFePO₄ crystals were prepared at a heating rate of 0.5,1.5,3,5°C/min,respectively.The results show that the heating rate does not affect the composition of the product but greatly affect morphology and performance of the material.The dissolution rates of the two precipitated precursors Li₃PO₄ and Fe₃（PO₄）₂?8H₂O at different temperatures are different,resulting in the change of the concentration and ratio of the constitutive crystal Fe²⁺and PO₄^3-in the reaction system,which is the root cause of the product showing different morphologies.For LiFePO₄,the appropriate heating rate is3°C/min.It shows that the particle size,sheet thickness and discharge specific capacity are the best of the samples.Secondly,the effect of the introduction of Mn on the growth mechanism of the LiFe_1-xMn_xPO₄ crystal was studied.The results show that the results show that after introducing Mn,the precursor of the reaction is(Fe_0.5Mn_0.5)₃（PO₄）₂·8H₂O,and its stable existence is 80°C,Which is lower than the stable existence temperature（135°C）of precursor in LiFePO₄ system the reaction precursors change from Fe₃（PO4）₂·8H₂O to(Fe_0.5Mn_0.5)₃（PO₄）₂·8H₂O after doping Mn.The stable temperature range of the former is less than 135°C,and the stable temperature of the latter is about 80°C.It was known that the growth of crystal at low temperature is easy to produce ion mixing.Therefore,the heating rate should be increased as much as possible to shorten the time of the reaction system at low temperature.Thirdly,the effects of different heating rate on the morphology and properties of the LiFe_1-xMn_xPO₄ were studied.The samples of x=0.4 and x=0.5 were prepared by using the heating rate of 3°C/min and 5°C/min,respectively.The results show that the particle size,film thickness and electrochemical properties of the materials prepared at 5°C/min are batter,regardless of the value of x.Since the addition of Mn reduces the dissolution temperature of the precursor,a high heating rate is required to rise to the high temperature range as soon as possible.It is proved that the rapid heating is beneficial to improve the electrochemical performance of the products.Finally,the LiFe_1-xMn_xPO₄ samples with different x values（x=0.4,0.5,0.6,0.7,1.0）were synthesized by hydrothermal method and at a heating rate of 5°C/min.The results show that the x value has little effect on the morphology of the prepared samples,but it has a great influence on the particle size and the sheet thickness.With the increase of x value,it first decreased and then increased.When the x value was 0.6,the electrochemical propertie of the prepared materials were the best.