Graphene Modified Feof Nano-Structured Hybrid As High-Specific-Energy Cathode Materials for Li-Ion Batteries
Authors: Liu Yadong , Yang Fan , Yu Kang , Ren Yang , Sun Chengjun , Liu Yuzi , Ferreira Paulo Jorge , Xie Jian 
Published: February 2020

Lithium ion batteries as the major power source play a critical role in our life today 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 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. 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 limited in practice due to its low electronic conductivity and poor structure stability during charge/ discharge cycling.

Graphene, a single−layer of sp2 carbon, 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 for solving the low conductivity and structural stability is to incorporate the graphene sheets into the nanostructure FeOF so that the conductivity, stability and relocation can be comprehensively addressed. 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%. It has been demonstrated that the performance improvement can be attributed to the introduction of the graphene sheets which improved the electric conductivity and provide a substrate to stabilize the FeOF particles evidenced by the morphology observation and structure characterization. TEM (Fig. 2) 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 sheets provide substrates for FeOF nanoparticle to anchor on so that the formed metallic Fe particles don’t relocate, the intra- and inter-particle conductivity can be effectively improved by forming FeOF/graphene sandwiched structure, and finally these graphene sheets can hold the FeOF nanoparticles together to maintain the structural integrity. The incorporation of graphene sheets into the nanostructured metal oxides and metal oxyfluorides provide an effective and robust tool for tailoring the structure of nanomaterials.

(1) Kitajou, A.; Komatsu, H.; Nagano, R.; Okada, S. Journal of Power Sources
2013, 243, 494.

(2) Kucinskis, G.; Bajars, G.; Kleperis, J. Journal of Power Sources
2013, 240, 66.

Figure 1