Low Potential Li Insertion in Transition Metal Oxides
- PDF / 447,294 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 68 Downloads / 267 Views
Synthesis and characterization. The molybdenum bronze,
[Na+(H 2 0)njy[MoO 3 ]Y-, was prepared as described previously, by reduction of MoO 3 in aqueous media.7 The final product analyzed (TGA and ICP) as ([Na(H 20) 2]0.2 6MoO 3, in agreement with previous reports. This material is referred to as "NaMoO 3". The lithium bronze, [Lio.07Nao.13(H 2O)0 .4 0 ]MoO 3, or "LiNaMoO 3" was prepared by the same method, substituting Li2MoO 4 as the buffer in place of Na 2MoO 4 . To obtain more highly lithiated bronzes, a cation exchange reaction was also carried out by ion-exchange of fresh NaMoO 3 (2g) with LiI in 50ml of butanol, as previously reported.8 Dehydration in vacuo for 24
601 Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
hours at 120'C yielded a material of stoichiometry [Lio.2oNao.o6(H2 0)0 .52]MoO 3, "LiMoO 3 ". The Sn-exchanged material ("SnNaMOO3", was prepared similarly, by exchange with SnCI 2oH2 0. X-ray powder diffraction (XRD) patterns were obtained on a Siemens D500 diffractometer equipped with a diffracted beam monochromator, using Cu-Ks radiation. A Varian Liberty 100-ICP-AES was used to determine the ratio of alkali (Li, Na) to molybdenum. XAFS studies were undertaken at LURE (Orsay, France) using X-ray synchrotron radiation emitted by the DCI storage ring (1.85 GeV positrons, average intensity of 250 mA). Data were collected in transmission mode at the Mo K edge (20,000 eV), using a a two crystal Si (311) monochromator. The study was not conducted using the L-edge due to lack of sufficient beam time. For each run, three EXAFS spectra were recorded from 19,900 to 21,000 eV with a 3eV step and two seconds of accumulation time per point. For each XANES run, two spectra were recorded from 19,950 to 20,150 eV with a 1 eV and one second of accumulation time per point. 9 The analysis was performed using standard XAS software. Electrochemical measurements.
The materials were evaluated (vs Li) as positive electrodes prepared from active material, and carbon black (Super S, Chemetals Inc.) using PVDF as a binder. The powders in the weight proportion 85, 12, 3, respectively, were mixed in cyclopentanone and spread onto a nickel disc. The electrodes were heated at 170'C in a vacuum for 2 hours prior to cell assembly in order to remove the free water. Dimethyl carbonate (DMC- Aldich) and ethylene carbonate (EC- Aldrich) were distilled under vacuum, mixed in the ratio 2:1 and LiPF6 was added to prepare the electrolyte as a 1.OM solution. Swagelock type cells were assembled in an argon filled glove box containing less than 2ppm of water and oxygen. Positive electrodes had a surface area of lcm2 and contained 1 - 2 mg of active material. Li metal, spread on a Ni disk, was used as the anode and reference electrode. A multichannel galvanostatic/potentiostatic system (Mac-PileTM) was used for the electrochemical study. The specific capacities obtained are normalized per gram of composite electrode (if not indicated), giving rise to an overestimation of the carbon contribution (vide infra). The
Data Loading...
