A broad review of recent research work on the preparation and the remarkable properties of intercalation compounds of graphite, covering a wide range of topics from the basic chemistry, physics and materials science to engineering applications.

Intercalation compounds of graphite

Oxidative wet-chemical delamination of graphene from graphite is expected to become a scalable production method. However, the formation process of the intermediate stage-1 graphite sulfate by sulfuric acid intercalation and its subsequent oxidation are poorly understood and lattice defect formation must be avoided. Here, we demonstrate film formation of micrometer-sized graphene flakes with lattice defects down to 0.02% and visualize the carbon lattice by transmission electron microscopy at atomic resolution. Interestingly, we find that only well-ordered, highly crystalline graphite delaminates into oxo-functionalized graphene, whereas other graphite grades do not form a proper stage-1 intercalate and revert back to graphite upon hydrolysis. Ab initio molecular dynamics simulations show that ideal stacking and electronic oxidation of the graphite layers significantly reduce the friction of the moving sulfuric acid molecules, thereby facilitating intercalation. Furthermore, the evaluation of the stability of oxo-species in graphite sulfate supports an oxidation mechanism that obviates intercalation of the oxidant. Scalable graphene production from graphite via an intercalation-oxidation-reduction process is still hampered by low reproducibility and many lattice defects. Here, the authors show that reducing molecular friction by using highly crystalline graphite and mild oxidizing conditions is the key to high quality graphene.

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Effect of friction on oxidative graphite intercalation and high-quality graphene formation

Electrochemical doping of bisulfate ions into single wall carbon nanotube (SWNT) bundles has been studied using coulometry, cyclic voltammetry, mass-uptake measurements, and Raman scattering experiments. A spontaneous charge-transfer reaction is observed prior to the application of an electrochemical driving force, in sharp contrast to previous observations in the graphite−H2SO4 system. A mass increase of the SWNT sample and a concomitant upshift of the Raman-active tangential mode frequency indicate oxidation (i.e., removal of electrons) of the SWNT bundles. In fact, using Raman scattering, we were able to separate the spontaneous and electrochemical contributions to the overall charge transfer, resulting in the value of an upshift of 320 cm-1 per hole, per C-atom introduced into the carbon π-band by the bisulfate (HSO4-) dopant. This value may prove to be a universal measure of charge transfer in acceptor-type SWNT compounds. At a critical electrochemical doping, the SWNT bundles are driven into an “ove...

http://eri.louisville.edu/papers_pres/Sumanasekera%20JPC%201999.pdf

Electrochemical Oxidation of Single Wall Carbon Nanotube Bundles in Sulfuric Acid

Graphite intercalation compounds (GIC) possess a broad range of unique properties that are not specific to the parent materials. While the stage transition, changing the number of graphene layers sandwiched between the two layers of intercalant, is fundamentally important and has been theoretically addressed, experimental studies revealed only macroscopic parameters. On the microscale, the phenomenon remains elusive up to the present day. Here we monitor directly in real time the stage transitions using a combination of optical microscopy and Raman spectroscopy. These direct observations yield several mechanistic conclusions. While we obtained strong experimental evidence in support of the Daumas–Herold theory, we find that the conventional interpretation of stage transitions as sliding of the existing intercalant domains does not sufficiently capture the actual phenomena. The entire GIC structure transforms considerably during the stage transition. Among other observations, massive wavefront-like perturb...

Direct real-time monitoring of stage transitions in graphite intercalation compounds.

Abstract Electrochemical energy storage systems using graphite as both the negative and the positive electrode have been proposed as “dual-graphite cells”. In this kind of electrochemical system, the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode, both based on graphite, during the charging process. On discharge, cations and anions are released back into the electrolyte. So far, the systems proposed in literature are primarily based on Li+ and PF6- intercalation/de-intercalation into/from graphite from non-aqueous organic solvent based electrolytes. As the positive electrode potential during charging always exceeds 4.2 V vs. Li/Li+, the organic electrolyte starts to decompose at these highly oxidizing conditions resulting in insufficient discharge/charge efficiencies. The replacement of organic solvent by ionic liquids (ILs) leads an increased stability of the electrolyte towards oxidation and thus to remarkably higher efficiencies as well as an increased cycling stability. In fact, ionic liquids provide extended anodic electrochemical stability and in addition, no solvent co-intercalation occurs in parallel to anion intercalation at high potentials. Here, we present highly promising results for “dual-ion cells” based on a graphite cathode and an ionic liquid based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). As the compatibility of this IL with graphite anodes is poor, alternative anodes such as metallic lithium or lithium titanate (Li4Ti5O12, LTO) are used. Consequently, the “dual-graphite” cell is renamed to “dual-ion” cell. In addition, the calculation of the specific energy of these systems will be in the focus of the discussion.

Hervé Fuzellier

Papers

Phase transitions and aging effects of the graphite intercalation compound alpha -C5n-HNO3.

Sulfate graphite intercalation compounds: New electrochemical data and spontaneous intercalation

In-plane electrical resistivity of nitric acid intercalated graphite

Uranyl-sulfate graphite intercalation compounds II. Structural and spectroscopic studies

Uranyl-sulfate graphite intercalation compounds—I. Their synthesis and stability