Graphene Modified Feof Nano-Structured Hybrid As Cathode Materials for Li-Ion Battery

Abstract

Lithium-ion batteries (LIBs) are the major power sources for electronic and transportation applications due to their high specific energy, good cyclability and environmental friendliness. Intercalation compounds were commercially employed as the positive (cathode) electrode material. However, they have a limited capacity, which are not likely to meet the growing demand on specific energy and energy/power density. Transition-metal oxides, fluorides and oxyfluorides have attracted a lot of interest due to their ability to deliver high electrochemical specific energy arising from 2-3 electrons transferred. FeOF was proposed as a promising candidate1 because it has a high theoretical specific capacity, 885 mAh/g (3-electron process) and 590 mAh/g (2-electron process), leading to an exceptionally high specific energy of 1328 Wh/kg. However, the electrochemical performance of FeOF is drastically different in practice due to its low electronic conductivity and poor structure stability during charge/discharge cycling process. To overcome these obstacles, conducting carbon matrices have been introduced to absorb the volume changes and to improve the structural stability of the electrodes. Graphene, a single−layer of sp2 carbon lattices, has been considered as one of the most attractive carbon materials for its excellent charge carrier mobility, mechanical robustness and thermal and chemical stability2. Our approach is to prepare a mixture of precursor of FeOF and graphene oxide in solution and to ensure a better bonding between FeOF and GO through a hydrothermal procedure. The resulted hybrid material showed much improved initial columbic efficiency and cyclability compared with the blank FeOF material. The initial specific capacity reached 621 mAh/g (Fig. 1) and the capacity retention was 78.9% (specific capacity 493 mAh/g) after 100 cycles at 0.1 C rate. It has been demonstrated that the performance improvement could be attributed to introduction of graphene which improved the electric conductivity and provide a substrate to stablize the FeOF particles by morphology observation and structure characterization. SEM (Fig. 2) and TEM were employed to observe the morphology change after introducing graphene. The structure evolution during charge/discharge process were also characterized by in situ XAS and high resolution XRD. The results showed that the graphene nano-sheets serve as substrates to stabilize the structure of FeOF and form a framework to stabilize the Fe clusters through bonding them to their original sites without migration. Thus, the FeOF/Graphene composite can keep the (de)lithiation reaction reversible during discharge and charge process. Figure 1