This document discusses the electrical properties of solid inorganic materials. It begins by defining solid electrolytes as crystalline solids that conduct electricity via the movement of ions. Some key solid electrolyte materials discussed include silver iodide (AgI), sodium beta-alumina, and lithium cobalt oxide (LiCoO2). Applications of solid electrolytes mentioned include use in solid oxide fuel cells, lithium-ion batteries, oxygen gas sensors, and as separators in electrochemical cells.

1. ELECTRICAL PROPERTIES OF
SOLIDS
Paper I – Inorganic Materials-Properties-II
- Jaiswal Priyanka Balister
- M.Sc. II (Inorganic)
- Semester IV
- Mithibai College (2015-16)
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2.  Solid Electrolytes
• Electrolyte is a substance that conducts electricity through the movement of ions.
• Most electrolytes are solutions or molten salts, but some electrolytes are solids and
some of those are crystalline solids.
• Different names are given to such materials:
1. Solid Electrolyte
2. Fast Ion Conductor
3. Superionic Conductor
• Solid electrolytes are an unusual group of materials which have high ionic
conductivity with negligible electronic conductivity.
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3.  Ionic v/s Electronic Conductivity
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• Let’s begin by comparing the properties of ionic conductors with the conventional
electronic conductivity of metals.

4.  General Characteristics: Solid Electrolytes
1. A large number of the ions of one species should be mobile. This requires a
large number of empty sites, either vacancies or accessible interstitial sites.
Empty sites are needed for ions to move through the lattice.
2. The empty and occupied sites should have similar potential energies with a
low activation energy barrier for jumping between neighboring sites.
3. The structure should have solid framework, permeated by open channels. The
migrating ion lattice should be “molten”, so that a solid framework of the
other ions is needed in order to prevent the entire material from melting.
4. The framework ions (usually anions) should be highly polarizable. Such ions
can deform to stabilize transition state geometries of the migrating ion
through covalent interactions.
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5.  Molten Sublattice (1/2 Melting)
• In the best ionic conductors one ion becomes so mobile that for all intensive purposes
those ions are in a “molten” state.
• This behavior can be seen in part from the entropies of the observed phase transitions,
where the Ag (and F respectively) sublattice melts prematurely.
(poor ionic conductor) b-AgI  a-AgI (excellent ionic conductor)
T = 146 ºC, DS = 14.5 J/mol-K
a-AgI  molten AgI
DS = 11.3 J/mol-K
Compare with the entropy of melting of 24 J/mol-K for NaCl.
solid PbF2  molten PbF2
DS = 16.4 J/mol-K
Compare with the entropy of melting of 35 J/mol-K for MgF2
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6.  Solid Electrolyte Materials
• Ag+ Ion Conductors
• AgI & RbAg4I5
• Na+ Ion Conductors
• Sodium b-Alumina (i.e. NaAl11O17, Na2Al16O25)
• NASICON (Na3Zr2PSi2O12)
• Li+ Ion Conductors
• LiCoO2, LiNiO2
• LiMnO2
• O2- Ion Conductors
• Cubic stabilized ZrO2 (YxZr1-xO2-x/2, CaxZr1-xO2-x)
• d-Bi2O3
• Defect Perovskites (Ba2In2O5, La1-xCaxMnO3-y, …)
• F- Ion Conductors
• PbF2 & AF2 (A = Ba, Sr, Ca)
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7.  Ag+ Ion Conductors
b-AgI
• Stable below 146 ºC
• Wurtzite Structure (tetrahedral coordination)
• s = 0.001 S/cm – 0.0001 S/cm
a-AgI
• Stable above 146 ºC
• BCC Arrangement of I-, molten/ disordered Ag+
• s ~ 1 S/cm, EA=0.05 eV
• Conductivity decreases on melting
RbAg4I5
• Highest known conductivity at room temperature
• BCC Arrangement of I-, molten/disordered Ag+
• s ~ 0.25 S/cm (25 ºC), EA=0.07 eV
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Structure of a-AgI, showing interstitial
cation sites: large hatched circles, iodide
ions; squares, octahedral 6b sites; solid
circles, tetrahedral 12d sites; open circles,
24h sites

8.  Na+ Ion Conductors
• Na3Zr2PSi2O12 (NASICON)
• Framework of corner sharing ZrO6
octhahedra and PO4/SiO4 tetrahedra
• Na+ ions occupy trigonal prismatic
and octahedral sites, ¼ of the Na+
sites are empty
• EA ~ 0.3 eV
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NaAl7O11 (Na2O.nAl2O3)
• FCC like packing of oxygen.
• Every fifth layer ¾ of the O2- ions are
missing, Na+ ions present.
• These layers are sandwiched between
spinel blocks.
• 2D ionic conductor.

9.  Applications of Solid Electrolytes
• There are numerous practical applications, all based on electrochemical cells, where ionic
conductivity is needed and it is advantageous/necessary to use solids for all components.
• In such cells ionic conductors are needed for either the electrodes, the electrolyte or both.
• Solid electrolytes have found applications in several areas, including
(a) gas sensors
(b) Separators
(c) solid oxide fuel cells
(d) solid-state batteries.
• In addition, solid electrolytes have been used in the construction of solid electrolyte cell
reactors (SECRs), in which heterogeneous catalytic reactions have been studied. Also,
SECRs have been used as chemical co-generative fuel cells, i.e., for the simultaneous
production of electricity and useful compounds.
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Electrolyte
Anode Cathode
Useful
Power
e- 

10.  Schematic of a Solid Oxide Fuel Cell
• Solid oxide fuel cells (SOFCs) are a class
of fuel cells characterized by the use of a
solid oxide material as the electrolyte.
• SOFC is an electrochemical conversion
device that produces electricity directly
from oxidizing a fuel.
• SOFCs use a solid oxide electrolyte to
conduct negative oxygen ions from the
cathode to the anode.
• The electrochemical oxidation of the
oxygen ions with hydrogen or carbon
monoxide thus occurs on the anode side.
• They operate at very high temperatures,
typically between 500 and 1,000 °C.
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11.  Advantages of SOFCs over Fuel Cells
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12.  Schematic of Rechargeable Li+-Battery
• A Li-ion battery is a type of rechargeable battery in
which Li-ions move from the negative electrode to
the positive electrode during discharge and back
when charging.
• Li-ion batteries use an
intercalated lithium compound as one electrode
material.
• Handheld electronics mostly use LIBs based on
lithium cobalt oxide (LiCoO2), which offers high
energy density, but presents safety risks.
• Lithium iron phosphate (LiFePO4), Lithium
manganese oxide (LMnO) and Lithium nickel
manganese cobalt oxide (LiNiMnCoO2) offer lower
energy density, but longer lives and inherent safety.
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13.  O2 Gas Sensor
• The partial pressure of oxygen in the
sample gas, PO2(sample), can be
determined from the measured potential,
V, via the Nernst equation.
V = (RT/4F) ln[(PO2(ref))/(PO2(sample))]
• Because of the low ionic conductivity at
low temperatures, the sensor is only useful
above 650 ºC.
• High concentration of anion vacancies is
necessary for O2- hopping to occur.
• Its two electrodes provide an output
voltage corresponding to the quantity of
oxygen in the exhaust relative to that in the
atmosphere.
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14.  Separator Schematic
• A separator is a permeable membrane placed
between a battery’s anode and cathode.
• The main function of a separator is to keep the
two electrodes apart to prevent electrical short
circuits, while also allowing the transport of
ionic charge carriers that are needed to close the
circuit during the passage of current in
an electrochemical cell.
• Solid ion conductors, can serve as both separator
and the electrolyte.
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Diagram of a battery with a
polymer separator

15.  References
1. A.R. West, “Solid State Chemistry and it’s Applications”, Chapter 13, Wiley
(1984)
2. C.N.R Rao and J. Gopalakrishnan, “New Directions in Solid State Chemistry”,
pp. 409-416, Cambridge (1997)
3. A. Manthiram & J. Kim, “Low Temperature Synthesis of Insertion Oxides for
Lithium Batteries”, Chem. Mater. 10, 2895-2909 (1998).
4. J.C. Boivin & G. Mairesse, “Recent Material Developments in Fast Oxide Ion
Conductors”, Chem. Mater. 10, 2870-2888 (1998).
5. Craig Fisher (Japan Fine Ceramic Institute)
http://www.spice.or.jp/~fisher/sofc.html
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