A New, Highly Conductive, Lithium Salt/Nonionic Surfactant, Lyotropic Liquid‐Crystalline Mesophase and Its Application

Abstract

Highly conductive electrolyte materials are an essential part of many electrochemical systems, such as fuel cells, solar cells, batteries, electrochromic devices, and next-generation renewable-energy sources. The growing diversity in batteries and electrochemical cells increases the demand for novel electrolyte materials. For instance, in solar-cell applications, an electrolyte material with high viscosity and low volatility is desirable, together with high ionic conductivity. Electrolytes can be solids, gels, or liquids depending on the application. Gel electrolytes are advantageous when the conductivity in the solid form is not sufficient or the leakage or vaporization of the liquid electrolyte is a problem. Gel electrolytes can be aqueous or non-aqueous depending on the application type. While in some battery systems aqueous gel electrolytes have no use—for example, in Li ion batteries—they can be used in many rechargeable batteries, electrochemical capacitors, solar cells, and so on. Liquid-crystal gel electrolytes have also been investigated and are considered to be an important class of ordered materials for the above applications. A lyotropic liquid-crystalline (LLC) mesophase is formed by two main constituents: an amphiphile and a solvent. Common solvents are water, organic liquids, or ionic liquids. LLC-based electrolytes offer many advantages, like rigidity and high ionic mobility and can be an alternative to polymer electrolytes. Solvent-free LC systems (thermotropic LC) usually have low ionic conductivities at room temperature, typically around 10 6 Scm , whereas solvent-containing LLC systems have room-temperature ionic conductivities around 10 3 Scm . Usually high ionic conductivity in solvent-free LC electrolyte systems is achieved at high temperatures, that is, 150 8C and above. Recently we have shown that transition-metal aqua complex salts ([M ACHTUNGTRENNUNG(H2O)6]X2; in which M is a transition-metal cation and X is a suitable counterion), which have melting points close to room temperature, can also be used as solvents in the self-assembly process of some surfactants. The LLC mesophases of molten transition-metal-salt aqua complexes have important physical properties, such as high thermal stability (between 83 and 383 K), high ionic conductivity (room-temperature conductivities close to 2.0 10 4 Scm ), and nonvolatility. A highly concentrated aqueous electrolyte solution of an alkali metal salt can also act as a solvent in the assembly process of oligo(ethylene oxide) type surfactants, in which the highly concentrated electrolyte solution can be considered as an analogue of a molten salt. Their similarities arise due to strong ion–dipole (salt–water) interactions at high salt concentrations (highly concentrated refers to water/salt mole ratios of less than 8 in the case of lithium salts) and as a consequence, the heat of vaporization of water sharply increases. In this contribution, we have investigated the phase behavior and ionic conductivity of a new class of hydrated-salt/ surfactant mesophase, namely; LiNO3–H2O–C12EO10, LiCl– H2O–C12EO10, and LiClO4–H2O–C12EO10 systems, in which C12EO10 is C12H25 ACHTUNGTRENNUNG(OCH2CH2)10OH. The mesophase is a collaborative assembly of a hydrated salt species in the liquid phase and surfactant molecules. Earlier studies on salt– water–surfactant mesophases focus on the effect of salts on the phase behavior of surfactants in dilute aqueous solutions (18–1, water/salt mole ratio). Here, we demonstrate that as little as two water molecules per molecule of lithium salt is sufficient to form a LLC mesophase. At such a low water and high salt concentrations, the bulk properties of water are altered by the salt–water interactions and the salt– water couple collaboratively acts as the solvent in the LLC mesophase. An important outcome of the salt–water interaction is that the LLC mesophase is stable under ambient atmospheric conditions for years (see Supporting Information) and displays high ionic conductivity over a broad temperature range. The LLC samples were prepared by adding each ingredient: salt (LiNO3, LiCl, or LiClO4), surfactant (C12EO10), and water in the required amounts and the resulting mixture was then homogenized by constant shaking in a shaking water bath at 60–110 8C for 24 h. Under ambient conditions, the amount of water in the samples depends on the temperature, relative humidity, and the amount of salt in the mesophase, but always enough water remains in the samples to [a] C. Albayrak, Prof. . Dag Department of Chemistry, Bilkent University 06800, Ankara (Turkey) Fax: (+90)312-266-4068 E-mail : dag@fen.bilkent.edu.tr [b] Prof. A. Cihaner Department of Chemical Engineering and Applied Chemistry Atilim University 06836, Ankara (Turkey) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103705.