Ionically Conducting Glasses with Subambient Glass Transition Temperatures
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ABSTRACT Cryptands and crown ethers along with the lithium salt, LiCF 3SO 2N(CH 2)30CH3 (LiMPSA) were employed to produce a new type of amorphous electrolyte. The key to producing an amorphous phase was the mismatch between the cavity size of the macrocycle and the diameter of the cation. The addition of poly(bis-(2(2-methoxyethoxy)ethoxy)phosphazene) (MEEP) to the amorphous complex, LiMPSA/2.2.2 Cryptand, imparts improved electrochemical and viscoelastic properties. Conversely, when poly(sodium-4-styrenesulfonate) (PS4SS) is added to the amorphous complex, LiMPSA/2.2.2 Cryptand, the product crystallizes. The ionic conductivity of the MEEP rubbery electrolyte is a full order of magnitude higher when compared to the analogous PS4SS doped electrolyte (3.8x10` S cm-' (MEEP), 1.7x10 6 S cm-' (PS4SS) both at 305'K).
INTRODUCTION Inorganic superionic glasses and polymer electrolytes have attracted considerable attention as solid electrolytes for electrochemical devices. 1-3Presently both electrolytes fall short of the required combination of electrochemical and mechanical properties necessary for many applications. Inorganic superionic glasses have high ionic conductivity but they are usually brittle materials. By contrast, polymer electrolytes often have lower ionic conductivity but the desired mechanical compliance. Recently Angell and co-workers reported a new type of solid electrolyte that combined superionic glasses and polymer electrolytes.45 These 'rubbery electrolytes' or 'polymer-in-salt' electrolytes contain a supercooled mixture of lithium salts doped with poly(ethylene oxide) (PEO). Superionic glasses are observed to maintain their high ionic conductivity in the vitreous state. This phenomenon is best described as a decoupling of ionic motion from the structural relaxations of the glass matrix.6.7 Ionic motion is coupled closely to local motion in the polymer matrix of polymer electrolytes and consequently ionic conductivity decreases dramatically as the glass transition temperature (To)is approached. The supercooled mixture of lithium salts originally described by Angell are potentially explosive because strongly oxidizing perchlorate anions are combined with reducible organic materials.' The replacement of perchlorate and related oxidizing anions has recently been the subject of extensive research.9 -" Novel ionic liquids and molten salts have also been pursued as alternatives to the supercooled mixture of lithium salts. 9" In this investigation we explore the use of cation encapsulating macrocycles for the formation of amorphous electrolytes that do not contain strongly oxidizing anions. These macrocycles surround the cation and thereby reduce the coulombic interaction between the cation and anion. We found that if the cation matches the macrocycle cavity, the resulting complex is a solid with a low melting temperature (Tm), however if the macrocycle cavity is larger than the cation the complex is a glass with a subambient T8. The ionic glass formed can be described as a viscous liquid at room temperature
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