Sol-Gel Coating Improves Structural Stability of LiCoO 2 Cathodes during Electrochemical Cycling
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then compared with transmission spectra results generated using a finite-element method. The agreement between the plane-wave method and finite-element calculations was very good. Results from this experiment showed that 12-fold symmetry structures had bandgaps that are nearly independent of propagation direction. As these tilings increase in complexity, so do their number of symmetry elements, rendering them more isotropic. JUNE LAU
Pulse-Generation Limited to Five Optical Cycles Using Negatively Chirped Optical Parametric Oscillator Researchers at the University of St. Andrews in the United Kingdom have demonstrated the feasibility of using optical parametric oscillators (OPOs) with chirped grating crystals to produce mid-infrared pulses with a bandwidth as narrow as five optical cycles. In the July 15 issue of Optics Letters, T. Beddard, M. Ebrahimzadeh, T.D. Reid, and W. Sibbett report that crystals of aperiodically poled lithium niobate (APPLN) effectively recompressed dispersionbroadened laser pulses of known frequency, resulting in the bandwidth restriction. The recompression is due to the chirped grating period of the crystals. A 1-mm-long APPLN crystal with eight different linearly chirped gratings was used in the study. The least-chirped grating had a uniform period of 22.96 µm, while the most-chirped had a period ranging from 21.89 µm to 23.55 µm. The average grating period was constant throughout the series, and the crystal was studied in both positively and negatively chirped orientations. When the crystal was pumped collinearly by a Ti:sapphire laser, the OPO generated nearly transform-limited 53-fsduration pulses at a center wavelength of 3 µm in the best-case scenario. This optimum output (approximately five optical cycles at FWHM intensity) was found to correspond to a negatively chirped grating with periods from 22.00 µm to 23.54 µm. Pulses emanating from the positively chirped OPO orientation were consistently broader. The development of numerical models to explain the observations is planned. TIM PALUCKA
Sol-Gel Coating Improves Structural Stability of LiCoO2 Cathodes during Electrochemical Cycling A team of researchers from Samsung SDI Company and Seoul National Univer-
MRS BULLETIN/AUGUST 2000
sity has greatly improved the structural stability of LiCoO2 cathodes by sol-gel coating LiCoO2 powders with SnO2. The coated materials were heat-treated to 400°C or 500°C and retained 87% and 84% of their initial capacities after 47 cycles between voltages of 4.4 V and 2.75 V. This was attributed to a nonuniform Sn distribution throughout the particles and the resulting suppression of the monoclinicto-hexagonal phase transition commonly observed in uncoated materials upon cycling. Lithium transition-metal oxides are used as cathode materials for lithium-ion cells. The most stable among the α-NaFeO 2 structure types is Li1-xCoO2. According to the scientists, “Li1-xCoO2 cycling stability is excellent over the limited composition range 0 < x < 0.5, but the stability rapidly deteriorates for x > 0.5.” T
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