Polymer Electrolytes

Fundamentals and Applications

César Sequeira, Diogo Santos

• Publisher: Woodhead Publishing
• Year: 2010
• Print ISBN: 9781845697723
• eBook ISBN: 9781845699772
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Polymer electrolytes are electrolytic materials that are widely used in batteries, fuel cells and other applications such as supercapacitors, photoelectrochemical and electrochromic devices. Polymer electrolytes: Fundamentals and applications provides an important review of this class of ionic conductors, their properties and applications.Part one reviews the various types of polymer electrolyte compounds, with chapters on ceramic polymer electrolytes, natural polymer-based polymer electrolytes, composite polymer electrolytes, lithium-doped hybrid polymer electrolytes, hybrid inorganic-organic polymer electrolytes. There are also chapters on ways of characterising and modelling polymer electrolytes. Part two discusses applications such as solar cells, supercapacitors, electrochromic and electrochemical devices, fuel cells and batteries.With its distinguished editors and international team of contributors, Polymer electrolytes: Fundamentals and applications is a standard reference for all those researching and using polymer electrolytes in such areas as battery and fuel cell technology for automotive and other applications.- Provides an important review of this class of ionic conductors, their properties and applications in practical devices- Explores categories of polymer electrolytes and conductivity measurements- Features a comprehensive analysis of current developments in polymer electrolytes and highlights a new type of polymer electrolyte

Part I

Types and development of polymer electrolytes

1

Introduction to polymer electrolyte materials

C.A.C. Sequeira and D.M.F. Santos, Technical University of Lisbon, Portugal

Abstract:

Polymer electrolytes are electrolytic materials that offer many advantages in the area of large, high energy density batteries for electric propulsion and in fuel cells for electric vehicle or stationary applications. At the other end of the spectrum, polymer electrolytes are of interest for small, portable electronic devices where the battery represents a significant proportion of the device’s size and weight. This chapter focuses on relevant structural, physical and electrical properties of these materials and on the related sectors where there has been considerable industrial input.

Key words

polymer electrolyte classes

polymer electrolyte properties

batteries

fuel cells and capacitors

photoelectro chemical devices

electrochromic applications

1.1 Introduction

Polymers are at the forefront of the materials revolution; their uses are expected to grow at a faster rate (~ 15% per year) than that of any other structural compound. This trend may even be accentuated as polymers with new functionalities appear. We shall discuss here polymers having electrolyte properties for which important applications are foreseen.

Ionic conductivity is usually restricted to salts in the molten state or their solutions in polar solvents. Despite their ubiquity, the understanding of such complex media is far from complete. On the other hand, a few remarkable solids with high ion mobility have been discovered, ranging from PbF₂ by Faraday (1839) to recent newcomers including β-alumina and sulphide glasses. In such materials, one of the sublattices is in a quasi-molten state while still held by a rigid framework. Clays, zeolites, and natural and synthetic ion exchange resins may be considered as intermediate materials between solutions and the above-mentioned solid electrolytes. This is particularly true in the sense that a fixed network bears immobilised electrical charges while the discrete counter-ions are mobile, but only when the coulombic interactions are screened by a polar solvent. Water or alcohols are usually required as both anion (hydrogen bonding) and cation (electron pairs) solvation is needed (Gray, 1997). The motion of ions in polymeric matrices in the absence of a solvent is a relatively new phenomenon whose existence and importance have been recognised only in the past decades (Armand, 1986; Bruce, 1995a; Gray, 1997; Linford, 1987; Lipkowski and Ross, 1994; MacCallum and Vincent, 1987; Skotheim, 1988; Takahashi, 1989; Vincent, 1987a; Wright, 1975) and corresponds to a different concept; the macromolecule itself acts as a solvent for a salt which becomes partially dissociated in the matrix, leading to electrolyte behaviour. No small molecules are required for the conducting process, though the polymer–salt complex is routinely made from a solution of both constituents.

An interesting parallel can be drawn between electronically and ionically conducting polymers: the latter (colourless) donate electron pairs from heteroatoms included in the macromolecular array from ‘s’ orbitals to Lewis acids (cations), while the former (highly absorbing) release single electrons from their conjugated ‘π’ system upon oxidative doping, with the formation of polarons (radical cations) possibly as dimers (bipolarons). Polymer electrolytes are to solutions what redox polymers (polypyrrole, polythiophene) are to metals. As a rule, electronic conductivity is best in spatially regular systems (crystals) while ionic conductivity prefers disordered matter.

The ionic conductivity of polymer electrolytes is typically 100 to 1000 times less than exhibited by a liquid- or ceramic-based electrolyte. Although higher conductivities are preferable, and indeed a great deal of effort has gone into improving the bulk conductivity of polymer electrolytes over the years, 100-fold or 1000-fold increases are not essential, as a thin film electrochemical cell configuration can largely compensate for the lower values.

Many polymer electrolyte materials will exhibit to a greater or lesser extent the following properties:

adequate ionic conductivity for practical purposes;

low electronic conductivity;

good mechanical properties;

chemical, electrochemical and photochemical stability;

ease of processing.

These critical attributes are necessary if the materials are to be considered as practical replacements for their liquid counterparts. In addition, their properties, particularly conductivity and transport properties, should be sufficiently practical to stimulate their development when compared with other highly conducting solid electrolyte materials. Since 1978, when Michel Armand first introduced polyether–alkali–metal salt complexes to the solid state community as potential materials for electrochemical devices, there has been an enormous amount of research carried out on these (particularly high molecular weight poly(ethylene oxide)–lithium salt) systems, to obtain a better understanding of their fundamental properties and to use this information for the development of new generations of polymer electrolyte materials that are commercially more attractive (Armand, 1983; Plancha et al., 1997a).

1.2 Categories of polymer electrolytes

Rather confusingly, the term polymer electrolyte is widely used in fuel cell technology to refer to ion-selective polymers which, when hydrated, have two intermingled phases – the solid polymer region and a liquid-like region. An example is DuPont’s Nafion which is a fluorinated polymer with sulphonate end groups. The liquid phase is sometimes described as composed of micelles which are channel-like regions filled with liquid. Electrostatic regions in the ionic structure of the polymer backbone, with the mobile charges in water, help to form these regions which contain a liquid phase. There have been attempts to develop similar single ion conductive polymers for lithium batteries but, as yet, a...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Part I: Types and development of polymer electrolytes
• Part II: Applications
• Index