Solid electrolyte

A solid electrolyte , and solid electrolyte , solid electrolyte or solid ionic conductors called, is a solid in which at least one type of ion is movable so that a worn by these ions electric current can flow. Solid electrolytes are electrically conductive, but unlike metals they have little or no electronic conductivity. Solid electrolytes are central components of high-temperature fuel cells , solid-state accumulators and batteries such as the lithium-iodine battery , and of some sensors , in particular the lambda probe , which is used in many motor vehicles.

The science of solid electrolytes and their conductivity is called solid state ionics ; for the history of the discovery of solid electrolytes see there .

Conductivity values ​​of solid electrolytes and superionic conductors

In most ion crystals, individual ions can move, so that there is low ion conductivity. One usually speaks of a solid electrolyte or solid ionic conductor only when the electrical conductivity σ is at least in the range 10 ⁻¹⁰ ≤ σ ≤ 10 ⁻⁴ S cm ⁻¹ . Solid electrolytes with particularly high conductivity are called fast ion conductors or super ion conductors . B. have a conductivity in the range 10 ⁻⁴ ≤ σ ≤ 10 ⁻¹ S cm ⁻¹ . Most of the known superionic conductors consist of a crystal lattice with large anions in which relatively small cations such as Ag ⁺ , Cu ⁺ or Li ^{+ have} a high mobility. Different authors delimit the range of superionic conductors differently, so "all substances that show ionic conductivities greater than 0.1" S cm ⁻¹ well below their melting point are referred to as superionic conductors or fast ionic conductors, or those that are at or near room temperature at room temperature have a conductivity σ of approx. 10 ⁻² S cm ⁻¹ .

Types of solid electrolytes

The solid electrolytes can be classified according to the mobile ion, so that anion conductors can be distinguished from cation conductors. Anion conductors can e.g. B. oxide or fluoride ion conductors, cation conductors are particularly known for protons, silver, lithium and sodium ions. There are also organic solid electrolytes, which are mostly based on polymers, and inorganic crystalline or amorphous solid electrolytes.

Probably the most important example of an oxide ion conductor is yttrium- stabilized zirconium dioxide , which is used in lambda sensors or at 650–1000 ° C in solid oxide fuel cells . The most important fluoride ion conductor consists of doped lanthanum fluoride . Sodium β-aluminate is used as the sodium ion conductor .

"Solid electrolytes" in electrolytic capacitors

The conventional electrolytic capacitors , known since 1896, contain a liquid electrolyte. This also applies to the so-called "dry" aluminum electrolytic capacitors , in which paper is soaked with liquid electrolyte so that it can no longer leak so easily. The liquid ion conductor can also be replaced by an electron-conducting solid, e.g. B. a conductive polymer . This is then called “solid electrolyte”, although - in contrast to the usual definition of a solid electrolyte given above - it is an electron conductor and not an ion conductor . Such “solid electrolytes” used in capacitors are primarily the organic polymers poly-3,4-ethylenedioxythiophene (PEDOT) and polypyrrole (PPy), or an inorganic oxide, namely the semiconducting manganese dioxide MnO ₂ .

Application examples

The central component of many ion-selective electrodes is the ion-selective membrane, which is usually formed by a solid electrolyte. For example, in the fluoride-selective electrode, the membrane sensitive to fluoride ions consists of a fluoride ion conductor, usually a single crystal of lanthanum fluoride .

Further applications for solid electrolytes are the membranes of many fuel cells or many sensors for gases, e.g. B. in CO ₂ sensors. Some galvanic cells or batteries also use solid electrolytes, in particular thin-film batteries, sodium-sulfur accumulators and the zebra battery . An important type of lithium battery with a lithium iodide solid electrolyte in use since 1972 is the lithium iodine battery used in most cardiac pacemakers . Solid electrolyte cells can be used for thermodynamic measurements. They are also used for heating elements or light sources in oxidizing gases including air, e.g. B. in the form of the Nernst lamp .

literature

• Ulrich Guth: Solid Electrolytes. In: Encyclopedia of Applied Electrochemistry. Springer, New York 2014, ISBN 978-1-4419-6996-5 , pp. 1989–1993, doi : 10.1007 / 978-1-4419-6996-5_317 .
• Ulrich Guth: Solid State Electrochemistry, Electrochemistry Using Solid Electrolytes. In: Encyclopedia of Applied Electrochemistry. Springer, New York 2014, ISBN 978-1-4419-6996-5 , pp. 2040-2041, doi : 10.1007 / 978-1-4419-6996-5_422 .

Individual evidence

1. ↑ Reinhard Nesper, Hansjörg Gruetzmacher, tungsten Uhlig: script Inorganic Chemistry Lecture I . Ionic bond and ionic compounds. Ed .: Reinhard Nesper, CCI The Creative Chemistry on the Internet, ETH Zurich. November 7, 2001, disorder and mobility, ionic conduction ( discussion of ionic conduction on the part of ETH Zurich [accessed on April 13, 2015]).
2. ↑ Roland Zeyher: Physics of superion conductors . In: Physics in Our Time . tape 13 , no. 6 . Verlag Chemie GmbH VCH / John Wiley & Sons, 1982, ISSN  1521-3943 , p. 183-193 , doi : 10.1002 / piuz.19820130603 .
3. ↑ CS Sunandana, P. Senthil Kumar: Theoretical approaches to superionic conductivity . In: Indian Academy of Sciences (Ed.): Bulletin of Materials Science . tape 27 , no. 1 . Springer India, February 1, 2004, ISSN  0250-4707 , p. 1-17 , doi : 10.1007 / BF02708477 .
4. ↑ Aluminum Polymer Electrolytic Capacitors (PZ-CAP) - Need of High Working Voltage for Solid Electrolytic Capacitor. Rubycon Corporation, accessed April 15, 2015 . 
5. ↑ Daihei Yamauchi: Latest Technological Trends for Conductive Polymer Aluminum Solid Electrolytic Capacitors. (PDF) Nichicon (Fukui) Corporation, accessed April 15, 2015 (see "Table 1: Types of cathode materials and conductivity").