Preparation And Electrochemical Performence Of Iron Fluride/Carbon Cathode For Lithium Ion Battery

Cathode is an important part of lithium-ion batteries,its capacity and energy density have a significant impact on the overall performance of lithium-ion batteries.Due to the high theoretical specific capacity,high energy density,high discharge platform and low cost of raw materials,Fe F₂has become a promising cathode material for lithium-ion batteries.However,the large volume change of Fe F₂during the charging and discharging process of the battery will cause the electrode structure to easily collapse,and the conductivity of the discharge product（Li F）is poor.These factors have seriously hindered the electrochemical performance of Fe F₂.In this paper,the synthesis of Fe F₂nanomaterial,carbon doping modification and electrochemical properties of Fe F₂were studied.The main research contents are as follows:（1）Fe F₂@C cathode material was synthesized by solvothermal method,and the synthesis route was continuously optimized by changing a series of experimental conditions,so as to provide suitable synthesis conditions for subsequent doping.The experimental results show that lower solvothermal temperature and higher carbonization temperature can generate more impurities,shorter reaction time and less surfactant addition can’t make the sample form a good uniform morphology.The electrochemical Li-storage capacity of the products increase firstly and then decrease with the increase of reaction time and surfactant addition.The results show that the Fe F₂@C cathode material obtained at 220°C for 16 h,0.8 g surfactant and 500°C carbonization temperature has the best purity,crystallinity,morphology,and electrochemical performance.（2）In order to further improve the electrochemical performance,Fe F₂@KB cathode material was prepared by doping KB under the optimal synthesis conditions,Fe F₂@KB is a dice-like micron particle formed by self-assembly of spherical particles with the size of 15 nm.The dice-like particles have a particle size of 8μm and are coated with a carbon shell of 5 nm.The special structure of Fe F₂@KB makes its electrochemical performance greatly improved.The reversible capacity of Fe F₂@KB is 208.4 m A h g^-1after 50 cycles at the current density of 0.06A g^-1.At a high current density of 0.6 A g^-1,Fe F₂@KB still has a reversible capacity of 94.6 m A h g^-1after 200 charge-discharge cycles,and the capacity retention rate is 95%.In addition,the high pseudocapacitance effect（81%）at high current density and high ionic diffusion coefficient(6.8×10^-14cm S^-1)also contributed to the improvement of the electrochemical performance of Fe F₂@KB.（3）Using molten salt method synthesized the Fe F₂@C cathode material,and the samples with the best composition,morphology and electrochemical performance were obtained by changing the type of molten salt and reaction temperature.The results show that the sample prepared with Zn Cl₂as molten salt at 350℃has high purity and good morphology.The whole sample shows a rod-like structure covered by carbon shells.The diameter of the rod-like particles is 200-600 nm and the length is 5-10μm.It has a reversible capacity of 182.8 m Ah g^-1after50 cycles at a current density of 0.06 A g^-1.In the rate performance test,the stable specific capacity of the sample is 204.6 m A h g^-1,the capacity retention rate is 99.8%.In addition,by comparing the electrochemical performance of samples with different carbon content,we found that the pseudocapacitance effect and ionic diffusion coefficient in samples increased with the increase of carbon content.