Research On Preparation And Electrochemical Performance Of Spherical Fe FeF₃·0.33H₂O As Cathode Material

Due to the poor electronic conductivity,iron?III?fluoride has poor cycle performance and rate performance,which is not suitable for using as lithium ion battery cathode material directly.But iron?III?fluoride owns potential value for research that it could participate in the multi-electrons transfer reaction and its higher theoretical specific capacity compared to the conventional cathode material.Iron?III?fluorides with different crystal water content have different crystal structure.FeF3?0.33H2 O has the best electrochemical performance in all of these crystalline materials.However,the method of direct synthesis of pure FeF3?0.33H2 O is rarely reported currently,so it has great significance to find a simple and environmentally friendly synthesis method to promote the electrochemical performance study of FeF3?0.33H2 O.Beside,reducing the particle size or doping moderate amounts of other elements could improve the conductive ability of FeF3?0.33H2 O.In this paper FeF₃·0.33H₂O was synthesized by one-step using solvothermal method at 180 ?.The synthesis method is simple and environmental with good reproducibility.The rate of production is 69.7%.This paper also researches the reaction temperature of sample preparation.The primary particles size of FeF3?0.33H2 O is controlled in 200-500 nm with taper-shape;part of primary particles can loosely gang together and form some broken spheres.Using prepared FeF3?0.33H2 O as lithium ion battery cathode material have charge and discharge cycle test.It can deliver a high initial discharge specific capacity of 386 mAh/g,100 th cycle discharge specific capacity is 99 mAh/g,200 th cycle discharge specific capacity is 81 mAh/g at 0.2C?1C = 237 mA/g?between 1.5 and 4.5 V.Except for the first time,the coulomb efficiency is nearly 100% in cycle test.The discharge specific capacity is 55 mAh/g,29 mAh/g,16 mAh/g at 2C,5 C,10 C,respectively.The radius of Mg²⁺ is close to Fe³⁺.F space can form if Mg²⁺ enter the crystalline structure of FeF3?0.33H2 O,which will improve the conductive ability of the material.For the first time,this paper tries to use different proportions of Mg element to doped into FeF3?0.33H2 O,synthesized Fe1-xMgxF3-x·0.33H2O?x = 0.09,0.13,0.17?as cathode material.It was clear that the right amount of Mg doped with no damage to the crystalline structure of FeF3?0.33H2 O after some tests,and Mg element was evenly distributed in the sample.Grain size of the materials are uniform;particle is porous spherical.The particle size of spherical Fe0.87Mg0.13F2.87·0.33H2 O is minimum in these materials,which is about 600 nm.The porous spherical particles are made up of smaller primary nanoparticles which are 50-100 nm with taper-shape.Fe0.87Mg0.13F2.87·0.33H2 O has the most excellent electrochemical performance in different Mg doped materials.Charge and discharge cycle tests show that 100 th cycle discharge specific capacity is 158 mAh/g and 200 th cycle discharge specific capacity is 139 mAh/g at 0.2C between 1.5 and 4.5 V,compared with FeF3?0.33H2 O increased by 59.60% and 71.60%,respectively,which shows the right amount of Mg doping improved the specific capacity and cyclic stability of FeF3?0.33H2 O.At the same time,Mg doping also improved the rate performance of FeF3?0.33H2 O,especially the specific capacity at large rate was increased significantly.The discharge specific capacity of Fe0.87Mg0.13F2.87·0.33H2 O is 106 mAh/g,77 mAh/g,47 mAh/g at 2C,5C and 10 C,compared with FeF3?0.33H2 O,increased by 92.73%,165.52%,193.75%,respectively.