5382
J. Phys. Chem. C 2007, 111, 5382-5388
Vibrational Spectroscopic Studies of Interactions in PEO-NaSCN-Montmorillonite Electrolytes Hucheng Zhang, Yang Zhao, Jianji Wang,* and Honghe Zheng School of Chemistry & EnVironmental Science, Henan Normal UniVersity, Xinxiang, Henan 453007, People’s Republic of China ReceiVed: June 19, 2006; In Final Form: December 23, 2006
Mixtures of NaSCN-montmorillonite (MMT) and the electrolytes of both poly(ethylene oxide) (PEO)MMT and PEO-NaSCN-MMT were prepared, and the MMT interfacial effect and the interactions of PEO with MMT and NaSCN were investigated by FT-IR and X-ray diffraction measurements at room temperature. It is shown from these experimental results that MMT surface possesses Lewis acid centers, and the interactions of MMT with PEO and PEO-NaSCN can occur several ways, including PEO intercalation, the solvation of PEO toward the interlayer cations, the grain boundary effect, Lewis acid-base interactions, and the epitaxial effect. When Na+-MMT content is high enough, the epitaxial effect cannot be observed in P(EO)20NaSCNNa+-MMT electrolytes, whereas such effect occurs significantly in PEO-Na+-MMT electrolytes. The Lewis acid-base interactions between PEO and Na+-MMT are the main interactions in the electrolytes with high Na+-MMT content. The grain boundary effect and the intercalation of PEO in MMT galleries become the dominant interactions in the electrolytes in which Na+-MMT content is less than 5%. Moreover, the effect of these interactions on the PEO conformation and the electrolytic performances is also analyzed in this paper.
1. Introduction When applied to high energy density rechargeable batteries, solid polymer electrolytes (SPEs) exhibit particularly attractive characteristics in that they are flexible, compact, free from leaks, and available in different geometries. Moreover, SPEs can serve not only as a medium to transport ions, but also as a separator to isolate the anode from the cathode in the batteries. Therefore, the research and development efforts on SPEs have become quite active since Wright1 discovered ionic conductivity in a PEOsalt complex in 1975. Poly(ethylene oxide) (PEO) is a favorable candidate for use as an electrolyte host due to its ability to dissolve salt well and having the proper structure to support ion transport. However, because the local relaxations of PEO segment are related to efficient cation transport in the electrolytes, various methods have been developed to increase the volume fraction of PEO amorphous phase and to improve the SPE conductivity at ambient temperatures. These include the development of copolymers or cross-linking polymers, the use of suitable plasticizers, and the incorporation of inorganic nanoparticles in the polymer host, such as SiO2, Al2O3, MgO, ZrO2, γ-LiAlO2, TiO2, BaTiO3, LiTaO3, and ZnO. It has been shown that the use of inorganic fillers not only decreases the PEO crystallinity and enhances the ionic conductivity, but also improves the mechanical stability and extends the thermal stability range of PEO-based electrolytes. In addition, an improved stability of the electrode-electrolyte interface and an enhancement of the cation transport number have also been reported, which are important factors controlling the performance of batteries. Montmorillonite (MMT) is classified as a 2:1 phyllosilicate;2 i.e., an individual layer consists of two silicate tetrahedral sheets * To whom correspondence should be addressed. Telephone: +86373-3325996. Fax.: +86-373-3326544. E-mail:
[email protected].
sandwiching an edge-shared metal hydroxide octahedral sheet. Because the silicate surface carries negative charges, some alkali metal and alkaline earth ions (such as Li+, Na+, K+, Ca2+, and Mg2+) can be combined to balance the charges between the layers. The distinct structure makes MMT possess three important characteristics, including the exchangeability of cations in interlayer regions, the intercalation of neutral molecules in the galleries, and small particle size (
