In-Situ X-Ray Characterization of LiMn 2 O 4 : A Comparison of Structural and Electrochemical Behavior
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the electrochemical reactions. In fact, as described, only the gel system could be used in this cell since liquid electrolyte would lead to wetting of the beryllium window and allow its subsequent participation in the redox reactions. Over 20 diffraction peaks are seen from the cell components themselves over the 20 range studied. This number of artifact diffraction peaks makes observation of the diffraction pattern from the sample of interest problematic at best. Additionally, very long data acquisition times are required (on the order of 12 hours) for a single pattern. This severely limits real-time data collection on samples to only the slowest charge or discharge rates. For in-situ XRD electrochemical studies, the material used for housing the cell must meet several criteria. First, it must be relatively transparent to the X-ray radiation so as to minimize the absorption of both the incident and diffracted rays. Second, it should have few if any diffraction
lines at, or near angles where the sample diffracts. Third, it should be chemically stable to the solvent system utilized, as well as when in contact with the electrode materials. Fourth, it must be electrochemically stable and preferably inert, so as not to interfere with the electrochemical characteristics of the active electrode materials. Fifth, it should be impervious to the solvent system utilized so it will not dry out during the course of the experiment. Sixth, the mechanical characteristics of the material should be such that compression can be applied to the cell to ensure good electrical contact and minimum interfacial resistance. Finally, a method of sealing the cell material to itself and other materials, such as the electrical feedthroughs, must be available. The two materials that were selected for the in-situ cell were polypropylene and polyethylene (Imil thickness). In order to determine unit cell parameters, high quality X-ray data must be collected. This seemingly simple requirement is not so easily met in an electrochemical cell primarily due to the fact that the electrode can and usually does swell after cell activation and upon charging and discharging. As a result of this swelling the electrode surface can move out of the focal plane of the X-ray source. This movement will likely result in displacement of the diffracted energy and incorrect determination of cell parameters. In the system described here, an internal standard is present that ensures precise determination of the diffraction angles and hence the unit cell parameters. EXPERIMENTAL Cell Design Porous electrodes were prepared and consisted of 83% active material, 8% Teflon used as a binder, and the balance as carbon. The electrodes were prepared using a standard mixing, knead, and roll technique. The final electrode thickness was 11 mil and was approximately 50% porous. The active material used was LiMn 20 4 obtained from FMC and Chemetals and LiCoO 2 obtained from FMC. The porous electrodes were pressed into an aluminum current collector grid in such a way so that the alumi
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