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Slow recrystallization in the polymer electrolyte system poly(ethylene oxide)n –LiN(CF3SO2)2 Ludvig Edmana) Lawrence Berkeley National Laboratory, Materials Science Division, University of California, Berkeley, California 94720
Anders Ferry Department of Materials Engineering, Monash University, Clayton, 3168 VIC, Australia
Marca M. Doeff Lawrence Berkeley National Laboratory, Materials Science Division, University of California, Berkeley, California 94720 (Received 21 December 1999; accepted 20 June 2000)
Thermal and ion-transport properties of the salt-in-polymer system poly(ethylene oxide)n–LiN(CF3SO2)2 [P(EO)nLiTFSI] were investigated for compositions ranging from n ⳱ 5 to n ⳱ 50. Particular attention was paid to the region n ⳱ 8 to 10 where a crystallinity gap previously had been reported. We concluded that the absence of distinct melting transitions for salt-rich compositions (n ⳱ 5 to 12) was attributable to the extremely slow kinetics of recrystallization of this system following a heat treatment. The results further indicated that it was primarily the nucleation process that was inhibited by the [(bis)trifluoromethanesulfonate imide] (TFSI) anion. As a corollary, the ionic conductivity was strongly dependent on the thermal history of samples, and an enhancement of up to 300% was observed in the ambient temperature ionic conductivity for pre-heated salt-rich samples.
I. INTRODUCTION
Salt-in-polymer electrolytes (SPEs) represent a class of materials with intriguing structure-transport property relationships.1,2 They are formed by the dissolution of an inorganic salt in a polar host polymer, thus creating an ion-conducting plastic material.3,4 These materials belong to an unusual subset of coordination compounds in the solid state. In particular, the modes of ionic transport and their relationship to structural rearrangements of the polymer host have been extensively studied in order to elucidate fundamental mechanisms.5,6 The main driving force in these studies is the great potential for electrochemical applications (e.g., thin-film batteries, fuel cells, electrochromic devices, etc.), although the fundamental mechanisms involved in ion transport also inspire research in this area.7 The propensity of common SPEs to form nonconducting crystalline complexes at ambient temperature, however, limits their usefulness for various devices. Avenues to inhibit the formation of such domains include the inclusion of nano- or micrometer-sized fillers,8,9,10,11 and recently also fullerenes.12 a)
Address all correspondence to this author. e-mail: [email protected] Present address: Department of Experimental Physics, Umeå University, S-901 87 Umeå, Sweden.
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J. Mater. Res., Vol. 15, No. 9, Sep 2000
One of the most promising candidates for use in battery applications include the system poly(ethylene oxide) (PEO) complexed with the salt LiN(CF3SO2)2 [lithium (bis)trifluoromethanesulfonate imide; LiTFSI]. The large and bulky TFSI anion is thought to suppress the detrimental formation of crystalline noncon
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