Multicomponent Materials as Li + Conductors
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Multicomponent Materials as Li+ Conductors Susan Babinec, Huiqing Zhang, Andrew Talik, Valeriy Ginzburg, and Nicole Wagner Corporate Materials Research & Development, The Dow Chemical Company Midland, MI 48672 ABSTRACT The conductivity and mechanical stiffness of ethylene-oxide based Li+ conducting solid polymer electrolytes (SPE) were measured and compared for a variety of single- and two-phase systems. Our objective was to determine whether there are simple correlations between these two properties despite the fact that these systems are truly complex. Results show that molecular architecture dominates both transport and mechanical behavior of single phase systems, thereby eliminating broad correlations. Conductivity was additionally found to require not only facile local chain dynamics but also a sufficient concentration of vacancies, as per the site hopping model. Two phase systems also show complex behaviors, but offer a broader range of both conductivity and stiffness values, and good conductivity/stiffness correlations. INTRODUCTION Nearly thirty years ago P. Wright discovered Li+ conduction in “SPE”, and soon after M. Armand initiated an era of their exploration in Li-based batteries [1, 2]. The ideal material remains somewhat elusive, however, as the performance requirements are quite contradictory: the ionic conductivity of a liquid and the mechanical strength of a solid. Many single phase materials have been extensively developed in the attempt to balance the demands, with limited success. Two phase materials are recently of interest for their potential to more naturally balance the requirements, and to offer interesting and potentially beneficial interfacial effects, provided that morphologies are carefully designed [3,4]. All SPE, and in fact all conductive materials, follow the universal conductivity equation (1) where σ is conductivity in S/cm, ni is the concentration of the ith species, µi is the mobility of the ith species, and qi is it’s charge. Many excellent research groups have analyzed SPE transport and have thereby provided several mechanistic models and performance generalities. One important understanding is that crystalline materials are not conductive, and that transport occurs only in the amorphous regimes, with the Li+ hopping from one adjacent site to the other in a semi-concerted fashion. Another key finding is that the plot of ln σ vs. 1/T is not linear, as with classic Arrhenius behavior (2A), but instead is often curved. This observation led to the now well accepted correlation between local polymer chain dynamics and Li+ transport. Specifically, SPE typically provide the best fit to the Vogel-Tamman-Fulcher (VTF) empirical equation which relates the temperature dependence of a variety of properties (conductivity here) to polymer mechanics as per equation (2B); where B is a constant and T0 is a reference temperature, typically taken as 50K over Tg. σ = Σ ni ∗ µi ∗ qi (1) σ = σ0 K exp (-Ea/RT) (2A) (2B) σ = σ0 exp [-B/(T-T0) ]
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While these detailed studies do rel
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