In Situ Synthesis Of Carbon Based Nano Iron Compounds And Their Electrochemical Performance

Lithium ion batteries (LIBs), a green, pollution free energy storage device, have been broadly used to improve the people’s production and life style, to promote social progress and development. However, with the development of our society, the demand for higher performance of LIBs has been more intensive. The main method to meet this demand is to develop a new electrode material. With a special mechanism of storing Li ions by conversion reactions, iron based transition metal compounds are the promising candidate of the electrode material. However, when used as the electrode materials, their low conductivity and large volume change between charge and discharge limit their application. By forming a coating structure with nano-carbon materials can improve their electrochemical performance and make the commercial application come true.The purpose of this subject is to find a facile way to synthesis the carbon coating structure to improve the electrochemical performance of FeF2 and FeS when used as the electrode materials in LIBs.First, we obtained two kinds of carbon based nano composites by the in situ co-pyrolysis of ferrocene with NH4F and sulfur respectively at 500℃ for 3h, carbon nanotubes capsulated with FeF2 nanorods (FeF2@CNTs) and graphene loaded with FeS nanosheets coated by graphene with the structure named of "sheet on sheet". And we also investigated the formation mechanism of the two structures by comparing the different samples obtained from different reaction conditions. (1) The formation mechanism of FeF2@CNTs can be summarized as the synergistic growth mechanism. F-1 from NH4F reacted with Fe forming FeF2 nano-rods and catalyze the formation of CNTs. At the same time, the CNTs would also make sure the one-dimensional growth of the FeF2 nano-rods and formed the FeF2@CNTs at last. (2) The growth of FeS@G/G contains two steps:first, ferrocene and sulfur reacted forming amorphous carbon layers loaded with FeS nanoparticles, then amorphous carbon layers grew into graphene sheets and FeS nano-particles grew into FeS nano-sheets with the formation of graphene coatings, eventually grew into FeS@G/G.Electrodes made from these two structures showed higher lithium storage capacity, better cyclic stability and high rate performance. (1) FeF2@CNTs nano-rods were used as the cathode material. The first reversible capacity of FeF2@CNTs electrode when current density was 50mAg-1 was 263mAhg-1 and there was almost no capacity fade after 50 cycles. When the current density increased to lAg-1, the first reversible capacity of FeF2@CNTs nano-rods was 133 mAhg-1, and it still kept at 93 mAhg-1 after 50 cycles. (2) FeS@G/G nano-sheets were used as anode materials. When the current density was 50mAhg-1, the first rechargeable capacity was 934.7mAhg-1, and it maintained at 847.5mAhg-1 after 80 cycles. When the current density increased to lAg-1, the first reversible capacity of FeS@G/G nano-sheets was 643.9 mAhg-1, the reversible capacity can still stay at 568.9mAhg-1 after 80 cycles, which indicated good rate-performance.