Synthesis And Improved Electrochemical Performance Of Iron Oxides Composite As Anode Materials For Li-ion Batteries

With the increasing demand of Li-ion batteries (LIBs) for promising power sources in electric vehicles (EV) and hybrid electric vehicles (HEV), much attention has been paid to alternative anode materials with high capacity, long cycling lifetime and high safety. So far, transition metal oxides have been intensively investigated as anode materials for LIBs due to their high specific capacities. Among them, iron oxides are considered to be promising anode materials for LIBs because of their high specific capacity, low cost, natural abundance and environmental friendliness, etc. However, there still exist critical problems about iron oxides, such as intrinsically low electrical conductivity, large volume expansion/contraction during Li-ion insertion/extraction and large voltage hysteresis, which result in their poor cycling stability and low initial coulombic efficiency, and then limit their practical applications. This work is aiming at improving the cycling performance and initial coulombic efficiency of iron oxides by synthesizing composite anodes. The main research contents and results are as follows:(1) The Fe2O3@Ag hetestructure nanorod composite was prepared by hydrothermal route and subsequent traditional silver mirror reaction, by coating the surface of Fe2O3nanorods with Ag nanoparticles. The Fe2O3@Ag hetestructure nanorod composite electrode shows better electrochemical performance than pure Fe2O3nanorod electrode. After50cycles at0.1C, the Fe2O3@Ag nanorod composite still delivers a discharge capacity of631.3mAh g-1, what’s more, the initial coulombic efficiency has a great enhancement, reaches83.5%. These improvements are attributed to the Ag nanoparticles, which greatly enhanced the electrical conduction of the electrode.(2) A graphene-encapsulated hollow Fe3O4microsphere (GE-H-Fe3O4-MS) ex-situ composite was successfully synthesized by electrostatic interactions and the subsequent reduction of hydrazine, using the hollow Fe3O4microspheres and graphene oxide as raw materials. The high electrical conductivity and stress buffering effect of the graphene nanosheet improved the electrochemical performance of GE-H-Fe3O4-MS electrode to some extent. After80cycles at200mA g-1, the reversible capacity of it is471.9mAh g-1, higher than the pure H-Fe3O4-MS electrode.(3) A hierarchical Fe3O4microsphere/graphene nanosheet (H-Fe3O4-MS/GNS) in-situ composite was synthesized by a facile one-pot solvothermal route and the subsequent high temperature annealing. In this composite, GNS and H-Fe3O4-MS are connected together by strong covalent coupling, the synergistic effect of them ensure composite material has an excellent electrochemical performance. The specific reversible capacities of the H-Fe3O4-MS/GNS in-situ composite electrode after70cycles are1171.6mAh g-1at200mA g-1and940.4mAh g-1at500mA g-1. The reversible discharge capacities of it at200,500,1000,2000and3000mAg-1are1015,992,935,852and745mAh g-1, respectively, what’s more, when the current density is lowered down to200mAg-1, the discharge capacity of the composite swiftly recovers to1016mAh g-1after60cycles.