Dry Composite Polymer Electrolytes for Lithium Batteries
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511 Mat. Res. Soc. Symp. Proc. Vol. 496 ©1998 Materials Research Society
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Z' I[1 Figure 1: Time evolution of the impedance response of symmetrical Li/electrolyte/Li cells using a plain PEO-LiCIO 4 (A), a composite PEO-LiCIO 4 + 10 w/oy-LiA10 2 (B) or a composite PEO-LiC10 4 + 10 w/o zeolite (C) electrolyte (w/o = weight percent). The time of storage is indicated directly on the curves. Temperature: 106 'C. Frequency range: 0.1Hz - 65 kHz.
The figure clearly shows that, while in the plain electrolyte the width of the semicircle progressively increases with time (this suggesting a continuous growth of a passivation layer
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on the lithium electrode surface), in the two composite electrolytes the width does not vary consistently over a prolonged period of time (this suggesting the occurrence of a stable and compact passivation layer). These results confirm that the dispersed ceramic powder indeed promotes stabilization of the lithium interface in PEO-LiX electrolytes, probably by trapping water and other liquid impurities which otherwise would react with the lithium electrode with the continuous formation of not-uniform, porous passivation layers[5]I.
Lithium Salt
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Ceramic Additive
Mixing and weighing
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Hot pressing
Figure 2: Scheme of the preparation procedure of dry, hot-pressed, composite electrolyte membranes. 513
To further enhance interfacial stabilization, we have prepared ceramic powder-added composite electrolytes using a procedure where the presence of any liquid was avoided throughout the entire process. The procedure consisted in the direct reaction of the electrolyte components obtained by hot pressing an intimate mixture of PEO, LiX and the selected ceramic additive (Figure 2). This procedure excludes the presence of any residual solvent and gives uniform composite electrolyte membranes. These "dry" membranes have a conductivity similar to that offered by their parent electrolytes prepared by the common casting process (Figure 3). However, the dry character of the procedure induces a further consistent improvement in the stabilization of the lithium interfacial stability, as demonstrated by Figure 4 which shows in comparison the impedance response determined after a comparable amount of storage time of two similar cells using the two types of composite electrolytes. The semicircle-width response of the cell using the hot-pressed composi
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