
In
materials science
Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries.
The intellectual origins of materials sci ...
, fast ion conductors are
solid
Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...
conductors with highly mobile
ions. These materials are important in the area of
solid state ionics
Solid-state ionics is the study of ionic-electronic mixed conductor and fully ionic conductors (solid electrolytes) and their uses. Some materials that fall into this category include inorganic crystalline and polycrystalline solids, ceramics, gla ...
, and are also known as solid electrolytes and superionic conductors. These materials are useful in batteries and various sensors. Fast ion conductors are used primarily in
solid oxide fuel cells. As solid electrolytes they allow the movement of ions without the need for a liquid or soft membrane separating the electrodes. The phenomenon relies on the hopping of ions through an otherwise rigid
crystal structure
In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...
.
Mechanism
Fast ion conductors are intermediate in nature between
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line solids which possess a regular structure with immobile ions, and liquid
electrolyte
An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble Salt (chemistry), salts, acids, and Base (chemistry), bases, dissolved in a polar solven ...
s which have no regular structure and fully mobile ions. Solid electrolytes find use in all solid-state
supercapacitor
alt=Supercapacitor, upright=1.5, Schematic illustration of a supercapacitor
upright=1.5, A diagram that shows a hierarchical classification of supercapacitors and capacitors of related types
A supercapacitor (SC), also called an ultracapacitor, ...
s,
batteries, and
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
s, and in various kinds of
chemical sensors.
Classification
In solid electrolytes (glasses or crystals), the ionic conductivity σ
i can be any value, but it should be much larger than the electronic one. Usually, solids where σ
i is on the order of 0.0001 to 0.1 Ω
−1 cm
−1 (300 K) are called superionic conductors.
Proton conductors
Proton conductor
A proton conductor is an electrolyte, typically a solid electrolyte, in which H+ are the primary charge carriers.
Composition
Acid solutions exhibit proton-conductivity, while pure proton conductors are usually dry solids. Typical materials a ...
s are a special class of solid electrolytes, where
hydrogen ion
A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particl ...
s act as charge carriers. One notable example is
superionic water
Variations in pressure and temperature give rise to different phases of ice, which have varying properties and molecular geometries. Currently, twenty-one phases, including both crystalline and amorphous ices have been observed. In modern histor ...
.
Superionic conductors
Superionic conductors where σ
i is more than 0.1 Ω
−1 cm
−1 (300 K) and the activation energy for ion transport ''E''
i is small (about 0.1 eV), are called
advanced superionic conductors. The most famous example of advanced superionic conductor-solid electrolyte is
RbAg4I5 where σ
i > 0.25 Ω
−1 cm
−1 and σ
e ~10
−9 Ω
−1 cm
−1 at 300 K. The
Hall (drift) ionic mobility in RbAg
4I
5 is about 2 cm
2/(V•s) at room temperatures. The σ
e – σ
i systematic diagram distinguishing the different types of solid-state ionic conductors is given in the figure.

No clear examples have been described as yet, of fast ion conductors in the hypothetical advanced superionic conductors class (areas 7 and 8 in the classification plot). However, in crystal structure of several superionic conductors, e.g. in the minerals of the pearceite-polybasite group, the large structural fragments with activation energy of ion transport ''E''
i < ''k''
BT (300 К) had been discovered in 2006.
Examples
Zirconia-based materials
A common solid electrolyte is
yttria-stabilized zirconia, YSZ. This material is prepared by
doping Y
2O
3 into
ZrO2. Oxide ions typically migrate only slowly in solid Y
2O
3 and in ZrO
2, but in YSZ, the conductivity of oxide increases dramatically. These materials are used to allow oxygen to move through the solid in certain kinds of fuel cells. Zirconium dioxide can also be doped with
calcium oxide
Calcium oxide (formula: Ca O), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term '' lime'' connotes calcium-containing ...
to give an oxide conductor that is used in
oxygen sensors in automobile controls. Upon doping only a few percent, the diffusion constant of oxide increases by a factor of ~1000.
Other conductive
ceramic
A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...
s function as ion conductors. One example is
NASICON, (Na
3Zr
2Si
2PO
12), a sodium super-ionic conductor
beta-Alumina
Another example of a popular fast ion conductor is
beta-alumina solid electrolyte. Unlike the usual
forms of alumina, this modification has a layered structure with open galleries separated by pillars. Sodium ions (Na
+) migrate through this material readily since the oxide framework provides an ionophilic, non-reducible medium. This material is considered as the sodium ion conductor for the
sodium–sulfur battery.
Fluoride ion conductors
Lanthanum trifluoride
Lanthanum trifluoride is a refractory ionic compound of lanthanum and fluorine. The chemical formula is .
The LaF3 structure
Bonding is ionic with lanthanum highly coordinated. The cation sits at the center of a trigonal prism. Nine fluorine ...
(LaF
3) is conductive for F
− ions, used in some
ion selective electrode An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a simple membrane-based potentiometric device which measures the activity of ions in solution. It is a transducer (or sensor) that converts the change in the concent ...
s.
Beta-lead fluoride exhibits a continuous growth of conductivity on heating. This property was first discovered by
Michael Faraday
Michael Faraday (; 22 September 1791 – 25 August 1867) was an English chemist and physicist who contributed to the study of electrochemistry and electromagnetism. His main discoveries include the principles underlying electromagnetic inducti ...
.
Iodides
A textbook example of a fast ion conductor is
silver iodide
Silver iodide is an inorganic compound with the formula Ag I. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a grey colouration. The silver contamination arises because some samp ...
(AgI). Upon heating the solid to 146 °C, this material adopts the alpha-polymorph. In this form, the iodide ions form a rigid cubic framework, and the Ag+ centers are molten. The electrical conductivity of the solid increases by 4000x. Similar behavior is observed for
copper(I) iodide (CuI),
rubidium silver iodide (RbAg
4I
5), and Ag
2HgI
4.
Other Inorganic materials
*
Silver sulfide
Silver sulfide is an inorganic compound with the formula . A dense black solid, it is the only sulfide of silver. It is useful as a photosensitizer in photography. It constitutes the tarnish that forms over time on silverware and other silver ob ...
, conductive for Ag
+ ions, used in some
ion selective electrode An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a simple membrane-based potentiometric device which measures the activity of ions in solution. It is a transducer (or sensor) that converts the change in the concent ...
s
*
Lead(II) chloride
Lead(II) chloride (PbCl2) is an inorganic compound which is a white solid under ambient conditions. It is poorly soluble in water. Lead(II) chloride is one of the most important lead-based reagents. It also occurs naturally in the form of the min ...
, conductive for Cl
− ions at higher temperatures
*Some
perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as , known as the perovskite (stru ...
ceramics –
strontium titanate,
strontium stannate – conductive for O
2− ions
*
Zr(HPO4)2.\mathitH2O – conductive for H
+ ions
*
UO2HPO4.4H2O (hydrogen uranyl phosphate tetrahydrate) – conductive for H
+ ions
*
Cerium(IV) oxide
Cerium(IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2. It is an important commercial produc ...
– conductive for O
2− ions
Organic materials
*Many
gels, such
polyacrylamide
Polyacrylamide (abbreviated as PAM or pAAM) is a polymer with the formula (-CH2CHCONH2-). It has a linear-chain structure. PAM is highly water-absorbent, forming a soft gel when hydrated. In 2008, an estimated 750,000,000 kg were produced, ...
s,
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from " ogonori" and " tengusa". As found in nature, agar is a mixture of two components, t ...
, etc. are fast ion conductors
*A salt dissolved in a polymer – e.g.
lithium perchlorate in
polyethylene oxide
Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular we ...
*
Polyelectrolyte
Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are t ...
s and
Ionomers – e.g.
Nafion, a
H+ conductor
History
The important case of fast ionic conduction is one in a surface space-charge layer of ionic crystals. Such conduction was first predicted by
Kurt Lehovec
Kurt Lehovec (12 June 1918 – 17 February 2012) was a Czech-American physicist. He one of the pioneers of the integrated circuit. While also pioneering the Solar cell, photo-voltaic effect, light-emitting diodes and History of the battery#Invent ...
.
As a space-charge layer has nanometer thickness, the effect is directly related to
nanoionics (nanoionics-I). Lehovec's effect is used as a basis for developing
nanomaterials
Nanomaterials describe, in principle, chemical substances or materials of which a single unit is sized (in at least one dimension) between 1 and 100 nm (the usual definition of nanoscale).
Nanomaterials research takes a materials science ...
for portable lithium batteries and fuel cells.
See also
*
Mixed conductor
References
{{Authority control
Electric and magnetic fields in matter
Electrochemical concepts