Preparation Process And Electrochemical Performance Optimization Of Difluoride-Carbon Composite Cathode

Iron difluoride（FeF₂）is a phase-transition-type cathode material for lithium-ion battery.During the electrochemical reaction,iron is converted back and forth between Fe²⁺and Fe,and every 1 mole of FeF₂ participating in the reaction goes hand in hand with 2 moles of electron transfer.Due to this characteristic,FeF₂ has a high theoretical specific capacity(571 m Ah g^-1)and is regarded as an ideal cathode material for high energy density Li-ion batteries.However,this material has poor electronic conductivity,large volume expansion and serious voltage hysteresis during the electrochemical reaction,which makes the electrochemical performance of FeF₂ unsatisfactory.Among them,poor electronic conductivity is an urgent problem to be resolved,and it is usually necessary to composite FeF₂ with carbon materials with good conductivity to enhance the conduction of electrons,thereby improving the electrochemical performance.Most of the researchers have made carbon with rich pore size through structural design,which is combined with FeF₂ to enhance the conduction of electrons.At present,no one has systematically reported the preparation of difluoride/carbon composite cathode.Therefore,starting from the selection of conductive materials,this paper takes the pure-phase FeF₂ synthesized in the laboratory as the research object,and uses the ball-milling method to prepare the composite cathode of FeF₂ and different carbons,so as to find a conductive material suitable for FeF₂,and explore its electrochemical reaction mechanism.The main contents include the following two aspects:（1）FeF₂ was synthesized by solution method,and it was ball-milled mechanically with six different conductive carbon materials.The electrochemical properties of six composite cathodes were studied,and the most suitable conductive carbon material for FeF₂ was obtained.Electrochemical tests and microstructure characterization were carried out to explore the electrochemical evolution behavior of FeF₂ during the charging and discharging process.The results showed that an inexpensive conductive graphite-FeF₂ composite cathode had excellent electrochemical performance.During the electrochemical reaction,a stable cathode electrolyte interface（CEI）layer was formed on the surface of FeF₂,which effectively prevented the dissolution of active materials.In addition,the cathode composited with FeF₂ and low-cost conductive carbon fibers also showed good electrochemical performance,and at a current density of 114 m A g^-1,the battery could be cycled stably for 800 times,and the discharge specific capacity remained at 300 m Ah g^-1.It was benefited from the one-dimensional conductive framework,which enhanced the transfer of electrons and the growth of stable CEI during the reaction process.（2）A three-dimensional conductive frame is built by applying the above-mentioned composite material of FeF₂ and conductive carbon fiber to a three-dimensional current collector nickel foam.At the same time,the catalytic cracking effect of metal nickel on acetylene is used,and the acetylene is cracked into amorphous carbon by chemical vapor deposition method,which is filed on the surface of the active material to form a carbon coating layer.By studying the relationship between its electrochemical performance and microstructure,it was found that the carbon coating layer greatly improved the electrochemical performance of the electrode,which can be stably cycled for 400 times at a current density of 285 m A g^-1.The discharge specific capacity remains at 450 m Ah g^-1.The content of carbon coating layer was studied,and it was found that the electrode with a coating time of 3 minutes had the best performance.