![Superionic ice conducting](https://upload.wikimedia.org/wikipedia/commons/6/64/Superionic_ice_conducting.svg)
In
materials science, fast ion conductors are
solid
Solid is one of the State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...
conductors with highly mobile
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
s. 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, g ...
, 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 cell
A solid oxide fuel cell (or SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic electrolyte.
A ...
s. 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 the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...
.
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 medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
s which have no regular structure and fully mobile ions. Solid electrolytes find use in all solid-state
supercapacitor
A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than other capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable ba ...
s,
batteries
Battery most often refers to:
* Electric battery, a device that provides electrical power
* Battery (crime), a crime involving unlawful physical contact
Battery may also refer to:
Energy source
*Automotive battery, a device to provide power t ...
, and
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
s, and in various kinds of
chemical sensor
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s.
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 particle ...
s act as charge carriers. One notable example is
superionic water
Superionic water, also called superionic ice or ice XVIII is a phase of water that exists at extremely high temperatures and pressures. In superionic water, water molecules break apart and the oxygen ions crystallize into an evenly spaced latti ...
.
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.
![Electronic ionic conductivity diagram](https://upload.wikimedia.org/wikipedia/commons/7/70/Electronic_ionic_conductivity_diagram.JPG)
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
Yttria-stabilized zirconia (YSZ) is a ceramic in which the cubic crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" ( Zr O2) and "yttria" ( Y2 O3) ...
, 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 (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, Caustic (substance), caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime (material), lime''" co ...
to give an oxide conductor that is used in
oxygen sensor
An oxygen sensor (or lambda sensor, where lambda refers to air–fuel equivalence ratio, usually denoted by λ) or probe or sond, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analysed.
It was ...
s 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, porcelain ...
s function as ion conductors. One example is
NASICON
NASICON is an acronym for sodium (Na) superionic conductor, Super Ionic CONductor, which usually refers to a family of solids with the chemical formula Na1+xZr2SixP3−xO12, 0 < x < 3. In a broader sense, it is also used for similar compounds wh ...
, (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 Beta-alumina solid electrolyte (BASE) is a fast ion conductor material used as a membrane in several types of molten salt electrochemical cell. Currently there is no known substitute available. β-Alumina exhibits an unusual layered crystal structur ...
. 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
A sodium–sulfur battery is a type of molten-salt battery constructed from liquid sodium (Na) and sulfur (S). This type of battery has a high energy density (its energy density is 5 times that of a lead-acid battery), high efficiency of charge/ ...
.
Fluoride ion conductors
Lanthanum trifluoride
Lanthanum trifluoride is a refractory ionic compound of lanthanum and fluorine.
The LaF3 structure
Bonding is ionic with lanthanum highly coordinated. The cation sits at the center of a trigonal prism. Nine fluorine atoms are close: three at ...
(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 transducer (or sensor) that converts the activity (chemistry), activity of a specific ion dissolved in a Solution (chemistry), solution into an electrical potentia ...
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 scientist who contributed to the study of electromagnetism and electrochemistry. 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 gray coloration. The silver contamination arises because AgI is hig ...
(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
Copper(I) iodide is the inorganic compound with the formula CuI. It is also known as cuprous iodide. It is useful in a variety of applications ranging from organic synthesis to cloud seeding.
Copper(I) iodide is white, but samples often appear ...
(CuI),
rubidium silver iodide Rubidium silver iodide is a ternary inorganic compound with the formula RbAg4I5. Its conductivity involves the movement of silver ions within the crystal lattice. It was discovered while searching for chemicals which had the ionic conductivity prop ...
(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 obje ...
, 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 transducer (or sensor) that converts the activity (chemistry), activity of a specific ion dissolved in a Solution (chemistry), solution into an electrical potentia ...
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 mi ...
, conductive 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 (XIIA2+VIB4+X2−3), known as ...
ceramics –
strontium titanate
Strontium titanate is an oxide of strontium and titanium with the chemical formula Sr Ti O3. At room temperature, it is a centrosymmetric paraelectric material with a perovskite structure. At low temperatures it approaches a ferroelectric phase ...
,
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
gel
A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still di ...
s, such
polyacrylamide
Polyacrylamide (abbreviated as PAM) 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, mainly fo ...
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 (''Gracilaria'') and "tengusa" (''Gelidiaceae''). As found in nature, agar is ...
, etc. are fast ion conductors
*A salt dissolved in a polymer – e.g.
lithium perchlorate
Lithium perchlorate is the inorganic compound with the formula LiClO4. This white or colourless crystalline salt is noteworthy for its high solubility in many solvents. It exists both in anhydrous form and as a water of crystallization, trihydra ...
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. Ion#Anions and cations, Polycations and polyanions are polyelectrolytes. These groups dissociation (chemistry), dissociate in aqueous solutions (water), making the pol ...
s and
Ionomer
An ionomer () ('' iono-'' + ''-mer'') is a polymer composed of repeat units of both electrically neutral repeating units and ionized units covalently bonded to the polymer backbone as pendant group moieties. Usually no more than 15 mole percent ...
s – e.g.
Nafion
Nafion is a brand name for a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Dr. Walther Grot of DuPont. Nafion is a brand of the Chemours company. It is the first of a class of synthetic polymers with ...
, 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 (June 12, 1918 – February 17, 2012) was one of the pioneers of the integrated circuit. While also pioneering the photo-voltaic effect, light-emitting diodes and History of the battery#Invention, lithium batteries, he innovated ...
.
As a space-charge layer has nanometer thickness, the effect is directly related to
nanoionics
Nanoionics is the study and application of phenomena, properties, effects, methods and mechanisms of processes connected with fast ion transport (FIT) in all-solid-state nanoscale systems. The topics of interest include fundamental properties of ...
(nanoionics-I). Lehovec's effect is used as a basis for developing
nanomaterials
*
Nanomaterials describe, in principle, 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-based approach to nan ...
for portable lithium batteries and fuel cells.
See also
*
Mixed conductor
Mixed conductors, also known as mixed ion-electron conductors (MIEC), are a single-phase material that has significant conduction ionically and electronically. Due to the mixed conduction, a formally neutral species can transport in a solid and th ...
References
{{Authority control
Electric and magnetic fields in matter
Electrochemical concepts