Ionic Conductivity of Polymer-Ceramic Composites

Abstract

The paper analyzes ionic conductivity data in the PEO:LiBF 4 -MgO and PEO:LiBF 4 -BaTiO 3 composite electrolyte systems. The polymer and ceramic phases in the systems interact to provide a stable, amorphous phase. The ceramic phase is believed to impart a dual effect. It suppresses crystallization of and interacts with the polymer phase. The nature of the interaction is believed to be dipole-dipole and driven by a dielectric constant gradient. The nanosize ceramic powder provides higher conductivity values because of an enhanced interaction. At low temperatures ͑Ӎ20°C͒ the dipole-dipole interaction is favored, whereas at high temperatures ͑Ӎ100°C͒ a reverse trend is observed. A very high dielectric constant material such as BaTiO 3 provides little additional benefit.A variety of dielectric materials, such as polymers, glasses, ceramics, and their combinations, may be useful as solid electrolytes for high-energy density lithium rechargeable batteries, electrochromic devices, and electrochemical sensors. Among these materials, polymers have received considerable attention in the last two decades because of their low density, manufacturability, and capacity to accommodate volume changes as compared to true rigid, inorganic solid electrolytes. The subject of polymer electrolytes has been extensively covered by review papers and a monograph. 1,2 An ionically conducting solid material derived from polymer and ceramic phases is identified as a polymer-ceramic composite in this paper. This composite type of material with significant ionic conductivity thus becomes a subset of solid electrolytes and has recently received considerable attention. Two review papers 3,4 have been published on the topic in the last three years. An analysis 4 of a broader range of composite electrolytes reveals that the incorporation of ceramic components in a polymer matrix leads to enhanced conductivity, cationic transport number, and electrode-electrolyte interfacial stability.The ionic conductivity, , of composite electrolytes is dependent upon many variables such as the characteristics ͑chemistry, size, and volume fraction͒ of the ceramic component, thermalization parameters ͑heating rates, hold time, cycling, etc.͒, properties of the polymer component, degree of reactivity between the polymer and ceramic phases, and temperature. The purpose of this paper is to investigate the relative importance of these variables in the PEO:LiBF 4 -MgO/BaTiO 3 system and extend the observations to the general class of polymer-ceramic composite materials. ExperimentalThe PEO:LiBF 4 -MgO/BaTiO 3 composite electrolyte films were made by a melt-casting technique using reagent-grade Union Carbide ͑mol wt 2,000,000͒ poly͑ethylene͒ oxide ͑PEO͒, lithium tetrafluoroborate (LiBF 4 ), magnesium oxide ͑MgO͒, and barium titanate (BaTiO 3 ). The ͓O͔:͓Li͔ ratio of the polymer complex was 8:1. The mean particle sizes of MgO, 20 nm ͑nano͒ and 5 m ͑micro͒ were used. Similarly, two barium titanate materials with mean particle sizes of 70 nm and 1 m were employed. The nanos...

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Studies on PEO-based sodium ion conducting composite polymer films

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