Computational, Electrochemical and 7 Li NMR Studies of Lithiated Disordered Carbons Electrodes in Lithium Ion Cells
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represent radial concentration profiles of the chemical species in the electrochemical cell that are associated with the NMR resonances in a conventional NMR spectrum. For example, the 5% dispersion in Li-7 nutation frequencies across a 100 pim layer at the surface of the carbon coated working electrode reported here is used to record 7Li NMR spectra at three different radial positions within the 100 pm layer. Since the gradient of the B, field (VB,=-A/r2 ) is not constant, the radial spacing between NMR spectra is also not constant. A one-dimensional radial concentration image or profile for any chemical species observed in the NMR spectrum is obtained by plotting the intensity or integral of the resonance versus the radial position at which the spectrum was recorded. The capacity and power output of an electrochemical cell relies, in part, on the presence of electrolyte at all of the accessible surface area of the carbon particles. In this work, we report the in situ 7Li NMR image of solvated lithium ions in an electrolyte wetting a carbon electrode that was derived from pillared clays as templates and pyrene, styrene, trioxane, propylene and ethylene as the organic precursor [1,2]. The carbon was coated onto the central conductor of a
toroid cavity
detector and served as the cathode electrode in the electrochemical cell. EXPERIMENTAL The synthesis of the calcined pillared clays (PILCs) as well as the loading with pyrene, styrene, trioxane/pyrene, ethylene and propylene carbon precursors, has been described in detail elsewhere [ 1,2]. Electrochemical characterization of the carbon anodes was performed using an Arbin 2400 station cell cycler. Details regarding the preparation of the electrode were described by Sandf et al. [6]. Gaussian 94 was used for all ab initio calculations [7]. Geometry optimizations were performed using the Hartree-Fock method with a 3-21 G basis set. However, for the Li3-C6o complex, the restricted open-shell-Hartree-Fock (ROHF) method was used in order to avoid spin contamination. For the NMR experiments, approximately 1.0 g of electrolyte (IM LiPF6 in ethylene carbonate:diethyl carbonate) was placed in a cylindrical electrochemical cell that formed part of the toroid cavity NMR detector. The cell was assembled from a glass tube with an inside diameter of 10.4 mm and length 22.0 mm. Circular rubber septa were used to seal the tube at both ends. A gold wire counter electrode 1.0 mm in diameter was formed into a 7-turn helix and place against the inner wall of the glass tube. A 0.62 mm diameter copper wire served as a working electrode and passed through the center axis of the cell, piercing both septa. Each end of the working electrode was soldered to a copper rod for mounting the cell in the NMR detector. The working electrode also functioned as the central conductor of the NMR detector. The carbon-based material was made into a slurry as described previously [1,2,6]. The sealed end of a capillary tube was used to coat the working electrode, as it was rapidly rotated in a horizontal or
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