Research On Preparation And Electrochemical Performance Modification Of Cobalt Fluoride As New Lithium Cathode Material

In recent years metal fluorides have received extensive attention for its high theoretical specific capacity and voltage when used as a new type of cathode materials for lithium ion battery. Among them, cobalt fluoride have great potential for its high theoretical specific capacity of 553 m Ah g-1, and a high theoretical potential of 2.854 V vs. Li+/Li. But only few studies of cobalt fluoride have been reported, a comprehensive study is needed. The biggest challenge for practical application of cobalt fluoride as cathode materials is the poor cycle performance caused by its intrinsic low ion and electron conductivity. In this article, cobalt fluoride is synthesised through a one-step hydrothermal method. Regarding to the problem of poor conductivity of the material,we use surface coating and in suit synthesis to improve the electrochemical performance of cobalt fluoride. X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Nitrogen adsorption-desorption techniques, Thermal gravimetric analysis, Cyclic voltammetry and Discharge/charge measurements are applied to characterize the physical properties and electrochemical performance of as-obtained cobalt fluoride composites.In our study, with ionic liquid as green fluorine source, cobalt fluoride consists of1-3 μm microspheres in diameter is synthesized through a one-step hydrothermal method. Carbon nanotubes was used as conductive carbon to assemble batteries. When cycled under 50 m A g-1, the obtained Co F2 delivers a capacity of 500 m Ah g-1 in first cycle, but only 12 m Ah g-1is retained after 50 cycles. The decrease of the capacity is caused by the intrinsic low conductivity, the poor reversibility and the dissolution of active material in electrolyte.In order to improve the conductivity of material and reduce the dissolution of active material in the electrolyte, a layer of glucose molecules is adsorbed on the surface of the cobalt fluoride particle and forms a carbon layer through pyrolysis. When cycled the as-obtained Co F2@C with a current density of 50 m A g-1, it delivers a capacity of602 m Ah g-1 in first cycle, and 44 m Ah g-1 after 50 cycles is retained. It can deliver a capacity higher than 40 m Ah g-1 after 50 cycles even cycled with a high current density of 100 m A g-1 and 200 m A g-1. This result indicated that the carbon coating could improve the cycling stability and the capacity retention.An in suit growth method was used to fabricate a Co F2/graphene composite. After the graphene was uniformly dispersed in the solution, nucleation and growth of Co F2 occurs on the surface of graphene, which ensures a firm combination of cobalt fluoride and graphene. After covered by graphene, the formation of SEI film was suppressed,leading to a decrease of discharge capacity in first cycle. However, the cycle performance is improved significantly. When cycled with a current density of 50 m A g-1,it delivers a capacity of 62 m Ah g-1 after 50 cycles. Combination with graphene was proved to be an effective way to improve the electrochemical performance of Co F2.In this study, two approaches of modification have been applied to solve the problems of intrinsic conductivity and dissolution of Co F2. Further more, the comprehensive characterization of Co F2 composites fill the gap of Co F2 as cathode material for Li-ion batteries.