Ionic conductors

8/13/2019 Ionic conductors

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Ionic conductors

Ionic solids contain defects that allow the migration ofions in an electric field

Some solid materials have very high ionicconductivities at reasonable temperatures

– useful in solid state devices

mobile interstitialmobile vacancy

Applications of solid ionic conductors

Membranes in separation processes

Electrolytes in sensors

Electrolytes in fuel cells and batteries

– should be a poor electronic conductor

Electrode materials in solid state batteries

– should be a good electronic and ionicconductor



Factors effecting the conductivity

σ = n Z e µ Conductivity is influenced by 1)the carrier concentration n,

2) the carrier mobility µ Usually, defects act as the charge carriers

– not many defects in most ionic solids

– mobility is usually low at room temperature

< 10-10Insulators

10-3-104Semiconductors

103-107MetalsElectronic conductors

10-1-103Liquid electrolytes

10-1-103Solid Electrolytes

< 10-16 – 10-2Ionic crystalsIonic conductors

Conductivity (S m-1)Material

Ionic conductivity in NaCl

NaCl is a poor ionicconductor

Conduction involvesmigration of cationvacancies

Cation vacancies are present due to

– doping - extrinsic defects

– Schottky defects - intrinsicdefects



Conduction is an activated process

µ = µ0 exp (-Ea/kT) - Arrhenius equation

Temperature dependence of conductivity

σ = (σ0/T) exp(-Ea/kT) – Contribution from mobility and defect formation



Idealized conductivity for NaCl

At low T conductivity is

dominated by mobility of

extrinsic defects

At High T, conductivity is

due to thermally formed

(intrinsic) defects

Intrinsic versus extrinsic conductivity

Extrinsic conductivity

– σ = (σ0/T) exp(-Ea/kT)

– carrier concentration is fixed by doping

Intrinsic conductivity

– carrier concentration varies with temperature – σ = (σ’0/T) exp(-Ea/kT) exp(-∆HS/2kT)

– slope of plot gives Ea + ∆HS/2



Cation vacancy migration mechanism

Cations can not hop from site to site via a

direct route

– not enough space

Cations migrate via an interstitial site

– this is a tight squeeze and requires energy

Experimental conductivity of NaCl

Broadly as expected – Get deviation at low T due

to vacancy pairing

– Get deviation at high T due

to screening of mobile

defects by defects of

opposite charge» Debye-Huckle type model



Energetics of ionic conduction in NaCl

0.27-0.50Dissociation of vacancy –

Mn2+ pair

~1.3Dissociation of vacancy pair

2.18-2.38Formation of Schottky pair

0.90-1.10Migration of Cl-

0.65-0.85Migrationof Na+, Em

Activation energy (eV)Process

AgCl

The predominant defect in AgCl is cationFrenkel

Cation interstitials are more mobile than cationvacancies

Cation interstitials can migrate by one of twomechanisms

– direct movement

– indirect movement



Migration mechanism in AgCl

Two possible pathways for interstitial migration:

1) move directly from interstitial to interstitial

2) interstitial displaces regular cation onto

interstitial position

Migration actually occurs by second pathway

Evidence for the indirect mechanism

Both charge and mass transport through a crystal

can be measures

– conductivity gives charge mobility

– diffusion measurements using radiolabelled Ag+ gives

mobility of Ag+

Charge is transported twice as fast as Ag+ ions

suggesting the indirect mechanism is correct



Doping in AgCl

Doping AgCl with a divalent impurity like Cd2+

reduces the ionic conductivity of the specimen

There is an equilibrium between cation vacancies

and Ag+ interstitials

– doping increases vacancy concentration

– doping decreases interstitial concentration

Cd2+ doped AgCl

Schematic showing effect of Cd2+ impurity

on conductivity – Presence of Cd2+ reduces

number of Ag+ interstitials and hence

lowers conductivity

Get minimum in conductivity

curve when doped – at high

impurity concentrationsconductivity is dominated by

cation vacancy migration, at

low concentrations interstitial

migration dominates



Solid electrolytes

There is a technological need for solids that havevery high ionic conductivities

Such materials are referred to as FAST IONCONDUCTORS

They include:

– α AgI

– Na β alumina

– NASICON, Na1+xZr 2[(PO4)3-x(SiO4)x]

– Stabilized zirconias

Ionic conductivity of some good solid electrolytes



β - alumina

Na1+xAl11O17+x/2 (β) and Na1+xMgxAl11-xO17 (β”) aregood sodium ion conductors at moderate temperatures

Na ions have high mobility and can be ion exchangedwith a wide variety of other cations

M2O.x Al2O3 x = 5 - 11

– M = Alkali+, Cu+, Ag+, Ga+, In+, Tl+, NH4+

– x = 5-7 usually produces β” material

– x = 8 - 11 gives β material

– β” material usually stabilized by addition of Li+

or Mg2+

The structures of β and β” alumina



The structure of β - alumina

Conduction plane of β alumina



The sodium sulfur cell

Sodium sulfur cells have ahigh energy density

– useful for electric vehicles

There are safety concerns

– molten sodium

2Na(l) --> 2Na+ + 2e-

2Na+ + 5S(l) + 2e- ---->

Na2S5(l)

Sodium sulfur phase diagram Need to operate at high temperatures

Can not fully discharge cell (solidifies)



Silver iodide

At low temperatures AgI adopts either a Wurtzite

or zinc blende structure

– Ag+ fills half of the tetrahedral holes in a close packed

I- array

Above 146o C it transforms to a BCC structure

with the Ag+ filling a small fraction of the

available tetrahedral sites

– the cation sublattice “melts”

σ ~ 130 Sm-1

The structure of α - AgI



Cation sites in α - AgI

Ionic conduction in α - AgI

There are many possible sites for Ag+

– 12 tetrahedral

– 24 trigonal

– 6 octahedral

There are only 2 Ag+ ions per unit cell!

– these ions are found disordered on the tetrahedral sites

Motion between sites is facile – ~0.05 eV activation barrier



RbAg4I5

AgI is polymorphic. The high

temperature α phase has a

high ionic conductivity

associated with a melted Ag+

sublattice

At low T ionic conductivity

drops

RbAg4I5 discovered while

trying to find materials that stillhad α – AgI structure at low T

RbAg4I5

Highest room temperature ionic conductivity ofany crystalline solid, 0.25 S cm-1

– Not stable < ~25 °C



Cu2HgI4

Material shows an order disorder phasetransition similar to AgI

– color change at phase transition

– marked increase in ionic conductivity at phasetransition

Structure has FCC array of I- with cationsfilling tetrahedral holes

– at low T cations are ordered – at high T they are disordered over all sites

The structure of Cu2HgI4 at low T



Stabilized zirconias

Y2O3 and CaO can be

dissolved in ZrO2

– creates a lot of oxygen

vacancies

At high temperatures

the defects are mobile

– oxide ion conductor

Applications of stabilized zirconia

Oxide conductors are of use for

– oxygen sensors

» based on concentration cell, can be used to measure O2 inexhaust gases, molten metals …

– fuel cell membranes

ZrO2 is only usable at high temperatures



An oxygen sensor

An O2 concentration cell can be built

E = [2.303RT/4F] log(p’/pref )

Fuel cells

Fuel cells are

devices for the

direct conversion

of fuels such as

CH3OH, H2, CO to

electrical energy



Solid oxide fuel cells

Fuel cells offer an

efficient and clean

way of using fossil

fuels, but

– high cost

– thermal cycling

problems

Solid oxide fuel cell performance

from a paper by S.C. Singhal in Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells, 1995



Electrochromic devices

Color changes such

as those needed in

smart windows can

be achieved by

moving ions into a

suitable solid

Lithium batteries

Batteries based on

lithium are attractive

as they can be light a

have a very high

voltage output

– Considerable current

research on cathodes

and electrolytes for

these devices

Ionic conductors

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