| Lithium titanate(Li4Ti5O12),as a anode materia with spinel structure has the characteristics of "zero strain"(little changes in crystal structure during reciprocating lithium ion intercalation and deintercalation process),has attracted a substantial attention,largely because of high safety,stable structure,good cycling stability and rate capability.Despite these advantages,the application of lithium titanate(Li4Ti5O12)has been limited due to its intrinsic drawbacks,such as low ionic and electronic conductivity,large polarization in charge and discharge.Unlike conventional cathode materials,the energy storage mechanism of iron fluoride is simultaneously the deintercalation of Li+ and the phase conversion reaction with Li metal,with a high theoretical specific capacity of 712 mAh/g(3e').Even if take place of le-also has 237 mAh/g theoretic specific capacity,and high operating potential(2.7 V).However,its wide band gaps,strong ionic bonds,and poor electrical conductivity lead to low specific capacity and fast capacity fading,which in turn limits its practical applications.In this work,the electrochemical properties of Li4Ti5O12 and FeF3·0.33H2O were improved from the viewpoint of the conductivity by doping,the details are as follows:Firstly,Li4Ti5O12 materials were prepared by two-step solid-phase method using TiO2 and Li2CO3 as raw materials,and Ag with high electron conductivity was introduced into the surface of Li4Ti5O12 by electroless plating.The influence of the electroless plating time on the electrochemical performance of Li4Ti5O12 was studied.The results show that:prolonging the electroless plating reaction time does not have much effect.On the contrary,the Li4Ti5O12/Ag material prepared by the electroless plating reaction for 10 min has the best performance and the sample particle size distribution is uniform.The initial discharge specific capacities were 182.3 mAh/g and 126.1 mAh/g at 0.2 C and 10 C respectively,after 50 cycles at 1 C the capacity fade to 149.7 mAh/g,it's capacity retention rate was 92%,showing excellent cycle performance and rate performance.Appropriate amount of electroless plating Ag is not only conducive to the Li-deintercalation/intercalation reaction,accelerates the reaction kinetics and thus enhances the diffusion efficiency of Li+,but also reduces the polarization to obtain a higher conductivity,effectively improving the material's rate capability and reversible capacity.In the work,Fe(NO3)3·9H2O and 40 wt%.HF solutions were used as raw materials to prepare pure phase FeF3·0.33H2O materials by hydrothermal method.The system of hydrothermal temperature(0C),hydrothermal time(h),and calcination temperature(?)and calcining time(h)were the investigation factors,four factors and four levels of L16(45)orthogonal experiment table were used to explore the optimal preparation conditions of FeF3·0.33H2O cathode materials.The precursor calcining temperature was judged by TG-DSC,and the physical properties of the products were characterized by XRD at different temperatures.The electrochemical performance of the products was tested by a charge and discharge test.The results show that the optimal preparation conditions of FeF3·0.33H2O cathode materials is hydrothermal reaction at 120 ? for 6 h,and then heating at 230 ?for 3 h in a tube furnace.The initial discharge capacity of the sample at a voltage range of 1.5 to 4.5 V at 0.1 C was 238.5 mAh/g.In order to improve the electrochemical performance of FeF3·0.33H2O cathode materials,Fe1-2x/3MnxF3·0.33H2O(x=0.01,0.03,0.05,0.07)cathode materials were synthesized by hydrothermal method.The effects of Mn2+ doping with different content on the physical and electrochemical properties of the cathode material were investigated.The initial discharge of Fe1-2x/3MnxF3·0.33H2O materials at 0.1 C was 246.1 mAh/g(x=0),261.8 mAh/g(x=0.01),284.0 mAh/g(x=0.03),275.6 mAh/g(x=0.05)and 234.8 mAh/g(x=0.07),respectively.The data shows that when the Mn2+ doping amount is 0.03,the first discharge capacity is the highest.Using the high conductivity of carbon materials to increase the electronic conductivity of ferric fluoride,Fe0.98Mn0.03F3·0.33H2O/C material was prepared by doped with varying amounts of acetylene black(10 wt.%,15 wt.%,20 wt.%).At 0.1 C rate,the discharge capacity of Fe0.98Mn0.03F3·0.33H2O/C doped with 10wt.%,15 wt.%and 20 wt.%acetylene carbon is 305.8 mAh/g,353.7 mAh/g and 325.7 mAh/g,respectively.It was found that the Fe0.98Mn0.03F3·0.33H2oO/C material doped with 15 wt%of acetylene blacks can give the highest specific capacity.Suggesting that appropriate amount of Mn2+ and carbon doping can reduce the polarization of the material,and improve the conductivity of the material,and also its cycle stability and rateability are improved. |
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