Electrochemical Properties for Solid Polymer Electrolyte/Salt Systems in Lithium Secondary Batteries

Abstract

We establish a new thermodynamic framework to describe the electrochemical properties of solid polymer electrolyte/salt systems. The Pitzer-Debye-Hückel expression is introduced to the modified double lattice-nonrandom model to take into account longrange electrostatic forces between molecules. After developing full descriptions for the electrochemical properties of a poly͑eth-ylene oxide͒ ͑PEO͒/NaCF 3 SO 3 system, the open-circuit voltage ͑OCV͒, diffusion coefficient, and ionic conductivity are calculated and compared with experimental data. We predict OCVs and diffusion coefficients of PEO/ZnBr 2 and PEO/ZnI 2 systems with the parameters obtained from phase diagrams and ionic conductivity data. The quantitative description according to the proposed model is in good agreement with the experimental data.Ionically conducting polymers were first discovered nearly three decades ago and were subsequently used as electrolytes in solidstate batteries. These polymers are solutions of salts in polymers in which a macroscopically solid state is achieved by entanglement. Microscopically they behave as liquids. The polymer electrolytes are generally composites of poly͑ethylene oxide͒ ͑PEO͒ and a salt such as LiClO 4 , LiAsF 6 , LiCF 3 SO 3 , and LiPF 6 . These materials show a common characteristic, namely the interdependence between measured properties and the nature and proportion of the phases present in the polymer electrolytes. It is believed that the ionic conduction is a property of the amorphous phase and that ion association, ionpolymer interactions, and local relaxations of the polymer strongly influence the ionic mobility. 1 Phase diagram studies of PEO-based electrolyte systems were initiated mainly as a consequence of previous ionic conductivity studies which showed significant variations of the ionic conductivity as a function of different experimental parameters, thermal history being one of the most important. Robitaille et al. 2 and Bésner and Prud'homme 3 reported complete phase diagrams for PEO/NaSCN and PEO/KSCN. Stainer et al. 4 constructed the phase diagram for PEO/NH 4 SCN. Lee and Crist 5 reported the phase behavior for PEO/NaSCN mixtures, which shows a eutectic point. Robitaille and Fauteux, 6 Fauteux, 7 and Fauteux and McCabe 8 studied extensively the phase behaviors of binary solid polymer electrolyte/salt systems such as PEO/LiX͑X = CF 3 SO 3 − , ClO 4 − , AsF 6 − , N͑CF 3 SO 2 ͒ 2 − , N͑CF 3 SO 3 ͒ 3 − ͒, and PEO/NaI. Gorecki et al. 9 described a phase diagram of PEO/Li͓͑CF 3 SO 2 ͒ 2 N͔ from the physics point of view. Labrèche et al. 10 also reported a partial phase diagram for PEO/Li͓͑CF 3 SO 2 ͒ 2 N͔ and employed tetraethylenesulfamide as a plasticizer for the same system. Recently Bae's group 11-14 has investigated this subject, introducing various thermodynamic models that describe phase behaviors of solid polymer electrolyte/salt systems. These theories consider various factors such as specific interaction, pressure, and local composition and agree with experimental data remarkably.Using t...

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