Abstract

cation and the spontaneous polarization of the ferroelectric ceramics. Also, the interfacial stability between ferroelectric ceramics added to composite polymer electrolytes and a lithium metal electrode is reported and discussed in terms of applications for lithium batteries. ExperimentalAll the polymer electrolytes described here were prepared by the solvent-casting technique using tetrahydrofuran (THF) as a carrier solvent. High-molecular-weight (Mw ϭ 6 ϫ 10 5 g/mol) PEO (Aldrich Chemical) and Li salts [LiClO 4 (Aldrich Chemical), LiBF 4 (Aldrich Chemical), LiPF 6 (Aldrich Chemical), LiCF 3 SO 3 (Aldrich Chemical), or Li[CF 3 SO 2 ) 2 N] (Fluka Chemika) were used as received.Barium titanate powders (Aldrich Chemical (0.6 to 1.2 m) and Osaka National Research Institute (1.8, 0.5, and 0.1 m) were dried under vacuum at 100ЊC for 24 h just prior to use. Lead titanate powders (Osaka National Research Institute) were calcined at 600ЊC for 6 h and then dried under vacuum at 200ЊC for 12 h. Lithium niobate powders (Aldrich Chemical) were dried under vacuum at 200ЊC for 12 h. Strontium titanate powders (Osaka National Research Institute) were calcined at 900ЊC for 12 h and then dried under vacuum at 200ЊC for 12 h. TiO 2 powders (Aldrich Chemical) were dried under vacuum at 150ЊC for 12 h just prior to use. X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to characterize ferroelectric ceramic powders. The characteristics of the ceramic particles used in this experiment are summarized in Table I. The BaTiO 3 with a particle size of more than 0.5 m showed a phase transition from the ferroelectric tetragonal phase to the paraelectric cubic phase at temperatures of 390-396 K. The amount of the endothermic heat absorbed by the phase transition depended on the particle size. The fine BaTiO 3 with 0.1 m indicated no phase transition. The amount of the endothermic heat is a measure of the content of the ferroelectric phase.Preparation of the composite electrolyte samples involved the dispersion of the selected ceramic powders and of the lithium salt in THF, followed by the addition of the polymer component (PEO). When the slurry was completely homogenized, it was heated and stirred simultaneously. The slurry gradually became a gel. The gel was cast onto a flat polytetrafluoroethylene sheet. The solvent was allowed to evaporate slowly at 40ЊC for 24 h; then the sheet was held at room temperature under vacuum for 48 h. These procedures yielded homogeneous and mechanically stable membranes with an average thickness of 300 m. All the steps in these preparation procedures and the experiments were carried out in an inert-gas atmosphere or under vacuum.

Insertion of lithium into, or extraction from, metallic host materials can produce volume changes, which lead to rapid mechanical disintegration and deterioration of Li-alloy electrode performance. The cycling properties of Sn-based composite electrodes can be significantly improved by optimizing the morphology and microstructure of lithium storage matrices. Decreasing the particle size of the metallic host powders to a submicron scale and using an intermetallic multiphase structure are effective ways for maintaining electrode mechanical and cycle stability. The fine host powders also enhance the kinetics of the electrochemical Li-alloying process. Besides the granular structure of host matrices, the cycling potential range and the identity of the organic electrolytes also have major influences on the electrode performance.

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NASICON-type Li 1+x Al x Ge 2-x (PO 4 ) 3 solid state lithium ionic conductors were synthesized by a sol-gel method using citric acid and ethylene glycol. The obtained precursors were sintered at various temperatures and the NASICON-type single phase was observed in a range of x = 0-0.6. The highest electrical conductivity was obtained for Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 sintered at 900 • C for 11 h in air. The total conductivity was 1.22×10 −3 S cm −1 at 25 • C, and the bulk and grain boundary conductivities were estimated by impedance profile analysis to be 1.70×10 −3 and 4.30×10 −3 S cm −1 , respectively. Sintered pellets of Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 were immersed in distilled water, saturated LiCl aqueous solution, and a saturated LiCl and LiOH aqueous solution at 50 • C for one week; X-ray diffraction patterns of these samples dried at 220 • C under vacuum showed no significant change from that of the pristine sample. The electrical conductivity of Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 was decreased to 1.4×10 −4 S cm −1 at 25 • C by immersion in distilled water, while immersion in the saturated LiCl aqueous solution increased the conductivity to 4.95×10 −3 S cm −1 at 25 • C. Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 was stable in the saturated LiCl and LiOH aqueous solution. Li 1.4 Al 0.4 Ge 1.6 (PO 4 ) 3 was unstable in contact with Li metal and Li-In alloy, but was stable in contact with Li 7 Ti 5 O 12 .