- Novel donor-typed ternary graphite intercalation compounds (GICs) are synthesized and characterized containing (1) pyrrolidinium cations with DMSO co-intercalates; (2) alkali metal cations with crown ether co-intercalates; and (3) alkali metal cations with polyethylene glycol co-intercalates. Structural and compositional data are obtained for the stable products. A new ether-amine co-intercalate exchange method is developed for the preparation of ether containing GICs which were previously difficult to prepare, and these studies lead to the discovery of a secondary ether-amine co-intercalate reaction that results in a unique pillaring phenomenon in GIC galleries.
New GICs containing N, N-n-alkyl substituted pyrrolidinium cation intercalates (Pyn.m, n, m= alkyl chain lengths) are obtained via cationic exchange from stage-1 donor-type GIC [Na(ethylenediamine)1.0]C15. Powder X-ray diffraction and thermogravimetric analyses give the GIC structures and compositions. [Py4.8]C47·0.71DMSO and [Py8.8]C48 with intercalate monolayers are obtained as stage-1 GICs with gallery expansions of 0.48 nm, whereas [Py1.18]C47 and [Py12.12]C80·0.25DMSO are stage-1 GICs with intercalate bilayers and gallery expansions of 0.81 nm. The gallery dimensions require that alkyl-chain substituents orient parallel to the encasing graphene sheets. Graphene sheet charge densities are correlated to the intercalate footprint areas. Smaller intercalate cations such as Py1.4, Py4.4, and Py1.8 either form high-stage GICs, or do not form stable intercalation compounds. These results, along with those reported for graphite intercalation of other quaternary ammonium cations, illustrate the trend for larger intercalates to form more stable and lower-stage GICs.
Layered host-polymer nanocomposites comprising polymeric guests between inorganic sheets have been prepared with many inorganic hosts, but there is limited evidence for the incorporation of polymeric guests into graphite. In Chapter 3, the preparation, and structural and compositional characterization of GICs containing polyether bilayers are reported for the first time. The new GICs are obtained by either (1) reductive intercalation of graphite with an alkali metal in the presence of an oligo or polyether and an electrocatalyst, or (2) co-intercalate exchange of an amine for an oligo or polyether in a donor-type GIC. Structural characterization of products using powder X-ray diffraction, Raman spectroscopy, and thermal analyses support the formation of well-ordered, first-stage GICs containing alkali metal cations and oligo or polyether bilayers between reduced graphene sheets.
Crown ethers are well established as co-intercalates in many layered hosts, but there are no reports of crown ethers incorporated into graphite. In Chapter 4, the preparation of the first GICs containing crown ethers is described. These GICs are obtained either by reductive intercalation of an alkali metal-amine complex followed by co-intercalate exchange, or by the direct reaction of graphite with a crown ether, alkali metal, and an electrocatalyst. Structural and compositional characterization of these new GICs using powder X-ray diffraction, thermal analysis, and GC/MS indicates the formation of well-ordered, stage-1 bilayer galleries.
There has been a major effort recently to develop new rechargeable sodium-ion electrodes. In lithium ion batteries, LiC6 forms from graphite and de-solvated Li cations during the first charge. With sodium ions, graphite only shows a significant capacity when Na+ intercalates as a solvated complex, resulting in ternary GICs. Although this chemistry has been shown to be highly reversible and to support high rates in small test cells, these GICs can require >250% volume expansion and contraction during cycling. In Chapter 5, the first examples of GICs that reversibly sodiate/desodiate without any significant volume change are demonstrated. These pillared GICs are obtained by electrochemical reduction of graphite in an ether/amine co-solvent electrolyte. The initial gallery expansion, 0.36 nm, is less than half of that in diglyme-based systems, and shows a similar capacity. Thermal analyses suggest the pillaring phenomenon arises from the in situ polymerization of co-intercalates during the initial reduction.