University of Louisville,
Keywords: graphene, foam, incommensurate, CVD, Ni particles
Summary:Development of high capacity Lithium-ion battery anode materials will have a profound and direct impact on current commercial and emerging markets such as portable electronics, electric vehicles, and electric grid storage. Recently, incommensurate graphene (IMLG) has been found as a high capacity new anode material enables reversible perform four times higher capacity (1540 mAh/g) than commercial graphite [1-3]. Incommensurate graphene is a multilayer graphene which has a weak interaction within layers with the absence of commensurately-stacking order within adjacent layers. This multilayer graphene may act as a single layer with modified electronic configuration. Due to the unique capacity of storing a large amount of lithium, scale-up synthesis of high-quality IMLG structures promises the feasibility of the rapid development of lightweight, cost-efficient, high-capacity rechargeable batteries. Here we report the study of the feasible synthesis of incommensurate graphene foam by cost-efficient Chemical Vapor Deposition technique using commercially available Nickel particles as a catalyst and 3D template. Methane gas is used as a hydrocarbon source to decompose thin graphene film on the catalyst surface at 950-1100˚C temperatures under low pressure. Defect-free incommensurately-stacked graphene is acquired in the form of foam after removing the Nickel from the 3D graphene/Ni network. Morphology and chemical composition of graphene/Ni network are studied by SEM/EDS analysis. The final Ni-free product contains 97-99% carbon which is predominately sp2 as confirmed by X-ray photoelectron spectroscopy. The stacking/none-stacking structure of graphene network is studied by X-ray diffraction, selected area electron diffraction (SAED) and Raman spectroscopy analysis. We develop systematically incommensurately-stacking multilayer graphene structures by controlling low concentration of defects. Defect analysis is performed by Raman mapping analysis that confirms defects are only ~ 0.02% of carbon assigned to the boundary defects, mainly. Large in-plane crystallites are produced over well-interconnected micron-sized curved graphene sheets with separation of few-micron-size pores. Crystallite size varies from 460 up to 580 nm as it is estimated by Raman D and G bands intensity ratio. The algorithm is developed to estimate incommensurate-commensurate ratio in the graphene foam by using Raman mapping analysis. Various regions of samples were analyzed to find the set of Raman G and 2D peaks to classify incommensurate and commensurate stacking. Highly incommensurateness (~ 90%) of graphene layers is achieved reproducibly which is chemically and thermally stable (up to 500˚C) and exhibits 2-4 ohm high electrical conductivity. This technique provides high reproducibility of high-quality incommensurate graphene foam, and it is feasible for large scale production. We found that crystallinity in-plane and incommensurateness within the layers are important factors for the exceptionally increased reversible capacity of storing Lithium, and consequently for the battery performance. In fact, the weakened interplanar interaction of graphene layers enables easy and full penetration of lithium atoms, followed by flexible adjustment of the layers for stable long-term stable cycling of battery cells.