Transcript Ionic Conductivity and Solid Electrolytes

Schematic of Rechargable Li Battery
Li-ion batteries are among
the best battery systems in
terms of energy density
(W-h/kg & W-h/L). This
makes them very attractive
for hybrid automobiles &
portable electronics.
Taken from A. Manthiram &
J. Kim, Chem. Mater. 10,
2895-2909 (1998).
Chem 754 - Solid State Chemistry
Cathode Materials Considerations
1. The transition metal ion should have a large work function
(highly oxidizing) to maximize cell voltage.
2. The cathode material should allow an insertion/extraction of a
large amount of lithium to maximize the capacity.
High cell capacity + high cell voltage = high energy density
3. The lithium insertion/extraction process should be reversible
and should induce little or no structural changes. This prolongs
the lifetime of the electrode.
4. The cathode material should have good electronic and Li+ ionic
conductivities. This enhances the speed with which the battery
can be discharged.
5. The cathode should be chemically stable over the entire voltage
range and not react with the electrolyte.
6. The cathode material should be inexpensive, environmentally
friendly and lightweight.
Chem 754 - Solid State Chemistry
LixTiS2
•Structure type is CdI2, hcp packing of
anions, octahedral Ti
•Li intercalates between the I- layers
•Pure TiS2 is a semi-metal, conductivity
increases upon insertion of Li (high
electronic conductivity)
Li Inserts in
this layer
•Lithium insertion varies from 1  x  0
•10% expansion, TiS2  LiTiS2
•Capacity ~ 250 A-h/kg
•Voltage ~ 1.9 Volts (This is the major
limitation of the TiS2 cathode)
•Energy density ~ 480 W-h/kg
Li Inserts in
this layer
Chem 754 - Solid State Chemistry
Li1-xCoO2
•LiMO2 structures are ordered
derivatives of rock salt (ordering occurs
along alternate 111 layers)
•Li intercalates into octahedral sites
between the edge sharing CoO2 layers
•Good electrical conductor
•Lithium de-intercalation varies from 0 
x  0.5 and is reversible
•Capacity ~ 45 A-h/kg
•Voltage ~ 3.7 Volts
•Energy density ~ 165 W-h/kg
•Cobalt is expensive (relative to Ti, Ni
and Mn).
Chem 754 - Solid State Chemistry
Li1-xMn2O4
•Structure type is defect spinel
•Mn ions occupy the octahedral
sites, while Li+ resides on the
tetrahedral sites.
•Rather poor electrical conductivity
•Lithium de-intercalation varies
from 0  x  1, comparable to
Li1-xCoO2
•Presence of Mn3+ gives a JahnTeller distortion that limits cycling.
High Li content stabilizes layer like
structure.
•Capacity ~ 36 A-h/kg
•Voltage ~ 3.8 Volts
•Energy density ~ 137 W-h/kg
•Mn is cheap and non-toxic.
Chem 754 - Solid State Chemistry
Solid Oxide Fuel Cells
A fuel cell generates electricity and heat by electrochemically
combining a gaseous fuel and an oxidizing gas, via an ion conducting
electrolyte, typically at elevated temperatures (eg 800-1000 ºC)
Typical Fuels 2H2 + O2 (from the air)  H2O
2CO + O2 (from the air)  2CO2
Advantages vs. Conventional Power Generation Methods
(e.g. Steam Turbines)
Higher conversion efficiency
Lower CO2 emissions
See http://www.spice.or.jp/~fisher/sofc.html for more details
Chem 754 - Solid State Chemistry
Schematic of a Solid Oxide Fuel Cell
Chem 754 - Solid State Chemistry
Taken from http://www.spice.or.jp/~fisher/sofc.html
Materials Issues (SOFC)
Cathode (Air Electrode) & Anode (H2/CO Electrode)
–High electronic conductivity
–Chemical and mechanical stability (at 600-900 ºC in oxidizing
conditions for the cathode and in highly reducing conditions for the
anode)
–Thermal expansion coefficient that matches electrolyte
–Sufficient porosity to facilitate transport of O2 from the gas phase
to the electrolyte
Electrolyte (Air Electrode)
–Free of porosity
–High oxygen ion conductivity
–Very low electronic conductivity
Interconnect (between Cathode and Anode)
–Free of porosity
–High electronic conductivity and negligible ionic conductivity
–Stable in both oxidizing and reducing atmospheres
–Chemical and thermal expansion compatibility with other components
Chem 754 - Solid State Chemistry
Favored Materials (SOFC)
Cathode (Air Electrode)
–(La1-xCax)MnO3 (Perovskite)
• (La1-xSrx)(Co1-xFex)O3 (Perovskite)
• (Sm1-xSrx)CoO3 (Perovskite)
• (Pr1-xSrx)(Co1-xMnx)O3 (Perovskite)
Anode (H2/CO Electrode)
–Ni/Zr1-xYxO2 Composites
Electrolyte (Air Electrode)
–Zr1-xYxO2 (Fluorite)
• Ce1-xRxO2 , R = Rare Earth Ion (Fluorite)
• Bi2-xRxO3 , R = Rare Earth Ion (Defect Fluorite)
• Gd1.9Ca0.1Ti2O6.95 (Pyrochlore)
• (La,Nd)0.8Sr0.2Ga0.8Mg0.2O2.8 (Perovskite)
Interconnect (between Cathode and Anode)
–La1-xSrxCrO3 (Perovskite)
Chem 754 - Solid State Chemistry
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.
Because of the low ionic
conductivity at low
temperatures, the sensor is
only useful above 650 ºC.
V = (RT/4F) ln[{(PO2(ref.)}/{(PO2(sample)}]
See http://www.cambridge-sensotec.co.uk/sensors_explained.htm for details
Chem 754 - Solid State Chemistry
Design Principles: O2- Conductors
•High concentration of anion vacancies
–necessary for O2- hopping to occur
•High Symmetry
–provides equivalent potentials between occupied and vacant sites
•High Specific Free Volume (Free Volume/Total Volume)
–void space/vacancies provide diffusion pathways for O2- ions
•Polarizable cations (including cations with stereoactive
lone pairs)
–polarizable cations can deform during hopping, which lowers the
activation energy
•Favorable chemical stability, cost and thermal
expansion characteristics
–for commercial applications
Chem 754 - Solid State Chemistry
Phase Transitions in ZrO2
Room Temperature
Monoclinic (P21/c)
7 coordinate Zr
4 coord. + 3 coord. O2-
High Temperature
Cubic (Fm3m)
cubic coordination for Zr
tetrahedral coord. for O2-
Chem 754 - Solid State Chemistry
Effect of Dopants: ZrO2, CeO2
•Doping ZrO2 (Zr1-xYxO2-x/2, Zr1-xCaxO2-x) fulfills two purposes
–Introduces anion vacancies (lower valent cation needed)
–Stabilizes the high symmetry cubic structure (larger cations
are most effective)
•We can also consider replacing Zr with a larger cation (i.e. Ce4+) in
order to stabilize the cubic fluorite structure, or with a lower
valent cation (i.e. Bi3+) to increase the vacancy concentration.
Compound
Zr0.8Y0.2O1.9
Ce0.8Gd0.2O1.9
d-Bi2O3
r4+
Specific Free
(Angstroms)
Volume
0.86
0.31
1.01
0.38
1.17
0.50
Conductivity
@ 800 ºC
0.03 S/cm
0.15 S/cm
1.0 S/cm (730 C)
Bi2O3 is only cubic from 730 ºC to it’s melting point of 830 ºC. Doping is necessary to
stabilize the cubic structure to lower temps.
Chem 754 - Solid State Chemistry
Gd2Ti2O7 Pyrochlore
The pyrochlore structure can be derived
from fluorite, by removing 1/8 of the
oxygens, ordering the two cations and
ordering the oxygen vacancies.
By replacing some of the Gd3+ with Ca2+
oxygen vacancies in the A2O network are
created, significantly increasing the
ionic conductivity (at 1000 ºC):
Gd2Ti2O7
s = 1  10-4 S/cm, EA = 0.94 eV
Gd1.8Ca0.2Ti2O6.95
s = 5  10-2 S/cm, EA = 0.63 eV
There is an opportunity to obtain
mixed electronic-ionic conductivity in
the pyrochlore structure.
M2O6 Network
A2O Network
Chem 754 - Solid State Chemistry
Ba2In2O5 Brownmillerite
The brownmillerite structure can be
derived from perovskite, by removing
1/6 of the oxygens and ordering the
vacancies so that 50% of the smaller
cations are in distorted tetrahedral
coordination.
Tetrahedral
Layer
In Ba2In2O5 at 800 ºC the oxygen
vacancies disorder throughout the
tetrahedral layer, and the ionic
conductivity jumps from 10-3 S/cm to
10-1 S/cm.
BaZrO3-Ba2In2O5 solid solutions
absorb water to fill oxygen vacancies
and become good proton conductors
over the temperature range 300-700
ºC.
Octahedral
Layer
Chem 754 - Solid State Chemistry
Aurivillius and BIMEVOX phases
Bi2WO6 is a member of the Aurivilius structure
family. The structure contains 2D perovskitelike sheets made up of corner sharing octahedra,
stacked with Bi2O22+ layers.
Bi4V2O11 is a defect Aurivillius phase, better
written as (Bi2O2)VO3.5, where 1/8 of the oxygen
sites in the perovskite layer are vacant.
Conductivity at 600 ºC is the highest ever
reported for an O2- conductor ~ 0.2 S/cm.
Only the perovskite oxygens are mobile.
Normally Bi4V2O11 undergoes phase transitions
upon cooling that lower it’s ionic conductivity,
but doping onto the V site stabilizes the HT
phase. These phases are generally called
BIMEVOX phases. (Bi2O2)V0.9Cu0.1O3.35 has a
conductivity of 0.01 S/cm at 350 ºC !!
Chem 754 - Solid State Chemistry
Summary O2- Conductors
•It is generally true that dopants have to be added either to
introduce vacancies, or to stabilize the high temperature/high
symmetry phase
•Among fluorite based O2- conductors both doped CeO2 and Bi2O3
have higher conductivities than stabilized ZrO2, but both are
less chemically stable. In particular they are prone to reduction.
This limits their use.
•Brownmillerite conductors show high conductivity, but are prone
to become electrically conducting under mildly reducing
conditions. They show promise as proton conductors.
•Ionic conductors based on Bi4V2O11 (BIMEVOX) show very high
conductivity for low temperature applications.
Chem 754 - Solid State Chemistry