Synthesis And Electrochemical Performance Of Multi- Structural, Multi-Morphological And Multi-Dimensional Iron Oxide/Carbon Composites As Anode Materials For Lithium-Ion Batteries

Iron oxides are promising anode materials for lithium ion batteries due to their high theoretical capacity, nontoxicity, lower cost and abundant in earth crust. However, large scale application of iron oxides is suspended by their poor cycling and rate stability induced by the huge volume changes during lithiation/delithiation process and the low electronic conductivity. This thesis focuses mainly on the controllable design and synthesis of iron oxides with multi-structure, multi-morphology and multi-dimension, and the introduction of high conductive carbon in different forms on the purpose of solving these problems, and to improve the overall electrochemical performance of the iron oxides. Effects of the structure and morphology on the electrochemical performance of the iron oxides are investigated.A facile spray drying method is promoted in the synthesis of Fe2O3 nanoparticle by using iron chloride and ammonia as starting materials combining a calcination of the spray drying of the precursor. Fe2O3/C composites with nano Fe2O3 particles dispersed evenly in the carbon matrix were prepared by ball milling of the spray drying precursor and a high conductive acetylene black followed by air calcination. The composites show improved cycling stability. A capacity of 725 mAh/g at 100 mA/g is achieved after 50 cycles with capacity retention of about 70% for the composite with a carbon content of 60 wt%. The method is of high efficiency and cost-effective and are great potential in large scale application.Bi-morphological carbon coated Fe3O4 composites with mainly of micron/ sub-micron octahedral and a few amount of nano-spheres are synthesized by a chemical vapor deposition (CVD) method by using a spray dried Fe2O3 nanoparticles as the starting materials and a high concentration C2H2 as deposition gas. During the CVD process, Fe2O3 nanoparticles were reduced into Fe3O4 and recrystallized into octahedra.. An initial reversible capacity of 570 mAh/g is reached for the composite deposited for 20 min with a carbon content of 19 wt% at 100 mA/g, and the capacity retention after 60 cycles reaches 96%. The method is of very short preparation period and the bi-morphological micro/submicro and nano-sized product is favorable in obtaining high tap density and reducing the agglomeration of the iron oxide particles during cycling. Novel structured amorphous iron oxide/carbon (FeOx/C) composites were prepared by a spray drying method combining calcination by using critic acid, iron chloride and ammonia as raw materials The crystallinity of the iron oxide can be greatly adjusted by adjusting the compositions of the raw materials, especially the ammonia. The amorphous composites show excellent thermodynamic cycling performance for lithium-ion battery anodes. A capacity of 1004 mAh/g after 700 cycles at 100 mA/g and a capacity of 570 mAh/g at 2 A/g can be achieved. The lithium storage mechanism of the amorphous FeOx/C composites is also studied.Unique structures of carbon coated Fe2O3 composites with hybride structure of crystalline Fe3O4 nanowires and nanoparticles, and ultrathin carbon coated mesoporous Fe2O3 nano-flakes are successfully synthesized by using ferrocene as the iron and carbon sources, ammonium bicarbonate and ammonium sulphate as assisted reagents, respectively. Fe3O4 nanowires show high radial electronic conductivity, and the carbon coating accommodates partially the the volume change during lithiation/delithiation, favoring the structure stability. The hybrid structure of carbon coated Fe3O4 nanowires and nano-particles exhibits an initial reversible capacity of 911 mAh/g at 100 mA/g and a discharge capacity of 984 mAh/g after 100 cycles. The one dimensional structure of the nanowire is well preserved after 100 cycles. Mesoporous Fe2O3 nanoflakes with carbon content of only 5.5 wt% shows an initial capacity of 910 mAh/g at 100 mA/g and the capacity shows an increased trend with cycling which reaches 1080 mAh/g after 120 cycles. A reversible capacity of 420 mAh/g could be achieved at 2 A/g. In situ TEM study shows that the carbon coated mesoporous Fe2O3 nanoflakes could effectively accommodate the volume changes during lithiation and maintain the structure stability. The carbon coating not only promotes the electronic conductivity of the nanoflake, but also serves as a skeleton in supporting the Fe2O3 flakes. The large facet size and ultra thin thickness together with the porous nature favor quite the connection between flakes and electrolyte. The facile and high yield method in synthesizing carbon coated Fe3O4 nanowires and mesoporous Fe2O3 nanoflakes could provide new ideas and inspiration in the application of iron oxides in other fields.