Graphene-Incorporated Nano-Structured Iron Oxyfluoride with Excellent Performance As Cathode Materials for Li-Ion Batteries
Authors: Liu Yadong , Yang Fan , Yu Kang , Li Chenzhao , Ren Yang , Sun Chengjun , Liu Yuzi , Xie Jian 
Published: February 2020

Lithium-ion batteries (LIBs) are the major power sources for electronic and transportation applications due to their high specific energy, good cyclability and environmental friendliness1, 2 Intercalation compounds were commercially employed as the positive (cathode) electrode material. However, they have a limited capacity due to the mono-valence change of host materials and the accompanied 1-e- transfer process, which accommodates 1-Li ion intercalation. These materials can’t meet the growing demand on higher specific energy and energy/power density. To achieve high specific energy cathode materials, efforts are being made to find alternative cathode materials for LIBs: (1) materials with transition metal ions capable of multi valence changes, and (2) materials with high potentials (vs. Li/Li+). Compared with intercalation cathode materials, conversion cathode materials can store multiple electrons instead of 1 electron of the intercalation materials. However, the conversion materials have not been used as practical LIB cathode materials due to the low electric conductivity, and structural stability.

Iron oxyfluoride (FeOF) was proposed3, 4 as a promising candidate because its high theoretical specific capacity of 885 mAh/g, leading to an exceptionally high theoretical specific energy of 2938 Wh/kg for 3- electron reactions. Graphene was employed to solve the low conductivity and structural stability. As the results from the improved FeOF structure, the nanostructured FeOF with the incorporated graphene sheets shows the superior performance to its blank. The FeOF/G initial specific capacity reached 621 mAh/g (Fig. 1) with an initial Coulombic efficiency of 94.7%, while the blank FeOF initial specific capacity can reach 586 mAh/g but has a drastically low Coulombic efficiency of 38.7%. SEM and TEM (Fig. 2) were employed to observe the morphology. 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 discharged species through bonding them to their original sites without migration, in which the graphene sheets serve as a “cage” to trap the discharged species. Thus, the FeOF/Graphene composite can keep the (de)lithiation reaction reversible during discharge and charge process and could be a promising cathode material for Li-ion batteries.

Fig. 1 Charge/discharge curves of FeOF (BLK) and FeOF/graphene (GRP).

Fig. 2 SEM image (a) and TEM image (b) of FeOF/Graphene hybrid material.

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Figure 1