Solid Electrolytes Materials and Applications

Chem 754 - Solid State ChemistryChem 754 - Solid State Chemistry

Solid ElectrolytesSolid ElectrolytesMaterials and Materials and ApplicationsApplications

Chemistry 754Chemistry 754Solid State Chemistry Solid State Chemistry

Lecture #25Lecture #25May 29, 2002May 29, 2002


ReferencesReferencesA. Manthiram & J. KimA. Manthiram & J. Kim – “Low Temperature – “Low Temperature

Synthesis of Insertion Oxides for Lithium Synthesis of Insertion Oxides for Lithium Batteries”, Batteries”, Chem. MaterChem. Mater. . 1010, 2895-2909 (1998)., 2895-2909 (1998).

J.C. Boivin & G. MairesseJ.C. Boivin & G. Mairesse – “Recent Material – “Recent Material Developments in Fast Oxide Ion Conductors”, Developments in Fast Oxide Ion Conductors”, Chem. Mater.Chem. Mater. 1010, 2870-2888 (1998)., 2870-2888 (1998).

J.C. Boivin J.C. Boivin – “Structural and Electrochemical – “Structural and Electrochemical Features of Fast Oxide Ion Conductors”, Features of Fast Oxide Ion Conductors”, Int. J. Int. J. Inorg. Mater.Inorg. Mater. 33, 1261-1266 (2001)., 1261-1266 (2001).

S.C. SinghalS.C. Singhal – “Science and Technology of Solid- – “Science and Technology of Solid-Oxide Fuel Cells”, Oxide Fuel Cells”, MRS BulletinMRS Bulletin, 16-21 (March, , 16-21 (March, 2000).2000).

M.M. Thackeray, J.O. Thomas & M.S. M.M. Thackeray, J.O. Thomas & M.S. WhittinghamWhittingham – “Science and Applications of – “Science and Applications of Mixed Conductors for Lithium Batteries”, Mixed Conductors for Lithium Batteries”, MRS MRS BulletinBulletin, 39-46 (March, 2000)., 39-46 (March, 2000).


Schematic of Rechargable Li Schematic of Rechargable Li BatteryBattery

Taken from A. Manthiram Taken from A. Manthiram & J. Kim, & J. Kim, Chem. MaterChem. Mater. . 1010, 2895-2909 (1998)., 2895-2909 (1998).

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.


Cathode Materials Cathode Materials ConsiderationsConsiderations

1. The transition metal ion should have a large work function 1. The transition metal ion should have a large work function (highly oxidizing) to maximize cell voltage.(highly oxidizing) to maximize cell voltage.

2. The cathode material should allow an insertion/extraction of a 2. The cathode material should allow an insertion/extraction of a large amount of lithium to maximize the capacity.large amount of lithium to maximize the capacity.

High cell capacity + high cell voltage = high energy High cell capacity + high cell voltage = high energy densitydensity

3. The lithium insertion/extraction process should be reversible 3. The lithium insertion/extraction process should be reversible and should induce little or no structural changes.and should induce little or no structural changes.

4. The cathode material should have good electronic and Li4. The cathode material should have good electronic and Li++ ionic conductivities.ionic conductivities.

5. The cathode should be chemically stable over the entire 5. The cathode should be chemically stable over the entire voltage range and not react with the electrolyte.voltage range and not react with the electrolyte.

6. The cathode material should be inexpensive, environmentally 6. The cathode material should be inexpensive, environmentally friendly and lightweight.friendly and lightweight.


LiLixxTiSTiS22

•Structure type is CdIStructure type is CdI22, hcp packing , hcp packing of anions, octahedral Tiof anions, octahedral Ti

•Li intercalates between the ILi intercalates between the I-- layers layers

•Pure TiSPure TiS22 is a semi-metal, is a semi-metal, conductivity increases upon conductivity increases upon insertion of Li (high electronic insertion of Li (high electronic conductivity)conductivity)

•Lithium insertion varies from 1 Lithium insertion varies from 1 x x 0 0

•10% expansion, TiS10% expansion, TiS22 LiTiS LiTiS22

•Capacity ~ 250 A-h/kgCapacity ~ 250 A-h/kg

•Voltage ~ 1.9 Volts (This is the Voltage ~ 1.9 Volts (This is the major limitation of the TiSmajor limitation of the TiS22 cathode)cathode)

•Energy density ~ 480 W-h/kgEnergy density ~ 480 W-h/kg

Li Inserts in Li Inserts in this layerthis layer

Li Inserts in Li Inserts in this layerthis layer


LiLi1-x1-xCoOCoO22

•LiMOLiMO22 s structures are ordered tructures are ordered derivatives of rock salt (ordering derivatives of rock salt (ordering occurs along alternate 111 layers)occurs along alternate 111 layers)

•Li intercalates into octahedral sites Li intercalates into octahedral sites between the edge sharing CoObetween the edge sharing CoO22 layerslayers

•Good electrical conductorGood electrical conductor

•Lithium de-intercalation varies from 0 Lithium de-intercalation varies from 0 x x 0.5 and is reversible 0.5 and is reversible


•Voltage ~ 3.7 VoltsVoltage ~ 3.7 Volts

•Energy density ~ 165 W-h/kg Energy density ~ 165 W-h/kg

•Cobalt is expensive (relative to Ti, Ni Cobalt is expensive (relative to Ti, Ni and Mn).and Mn).


LiLi1-x1-xMnMn22OO44

•Structure type is defect spinelStructure type is defect spinel•Mn ions occupy the octahedral Mn ions occupy the octahedral sites, while Lisites, while Li++ resides on the resides on the tetrahedral sites.tetrahedral sites.

•Rather poor electrical conductivityRather poor electrical conductivity

•Lithium de-intercalation varies Lithium de-intercalation varies from 0 from 0 x x 1, comparable to 1, comparable to LiLi1-x1-xCoOCoO22

•Presence of MnPresence of Mn3+3+ gives a Jahn- gives a Jahn-Teller distortion that limits cycling. Teller distortion that limits cycling. High Li content stabilizes layer like High Li content stabilizes layer like structure. structure.


•Voltage ~ 3.8 VoltsVoltage ~ 3.8 Volts

•Energy density ~ 137 W-h/kgEnergy density ~ 137 W-h/kg

•Mn is cheap and non-toxic.Mn is cheap and non-toxic.


Solid Oxide Fuel CellsSolid Oxide Fuel Cells

A fuel cell generates electricity and heat by electrochemically A fuel cell generates electricity and heat by electrochemically combining a gaseous fuel and an oxidizing gas, via an combining a gaseous fuel and an oxidizing gas, via an ion ion conductingconducting electrolyteelectrolyte, typically at elevated temperatures (eg , typically at elevated temperatures (eg 800-1000 ºC) 800-1000 ºC)

Typical Fuels -Typical Fuels -

2H2H22 + O + O22 (from the air) (from the air) H H22OO

2CO + O2CO + O22 (from the air) (from the air) 2CO 2CO22

Advantages vs. Conventional Power Generation Advantages vs. Conventional Power Generation MethodsMethods (e.g. (e.g. Steam Turbines)Steam Turbines)

Higher conversion efficiencyHigher conversion efficiency

Lower COLower CO22 emissions emissionsSee http://www.spice.or.jp/~fisher/sofc.html for more details


Schematic of a Solid Oxide Fuel Schematic of a Solid Oxide Fuel CellCell

Taken from http://www.spice.or.jp/~fisher/sofc.html


Materials Issues (SOFC)Materials Issues (SOFC)Cathode (Air Electrode) & Anode (HCathode (Air Electrode) & Anode (H22/CO Electrode)/CO Electrode)

–High electronic conductivityHigh electronic conductivity–Chemical and mechanical stability (at 600-900 ºC in oxidizing conditions for Chemical and mechanical stability (at 600-900 ºC in oxidizing conditions for the cathode and in highly reducing conditions for the anode) the cathode and in highly reducing conditions for the anode)

–Thermal expansion coefficient that matches electrolyteThermal expansion coefficient that matches electrolyte

–Sufficient porosity to facilitate transport of OSufficient porosity to facilitate transport of O22 from the gas phase to the from the gas phase to the electrolyteelectrolyte

Electrolyte (Air Electrode)Electrolyte (Air Electrode)–Free of porosityFree of porosity–High oxygen ion conductivityHigh oxygen ion conductivity–Very low electronic conductivityVery low electronic conductivity

Interconnect (between Cathode and Anode)Interconnect (between Cathode and Anode)–Free of porosityFree of porosity–High electronic conductivity and negligible ionic conductivityHigh electronic conductivity and negligible ionic conductivity–Stable in both oxidizing and reducing atmospheresStable in both oxidizing and reducing atmospheres–Chemical and thermal expansion compatibility with other componentsChemical and thermal expansion compatibility with other components


Favored Materials (SOFC)Favored Materials (SOFC)Cathode (Air Electrode)Cathode (Air Electrode)

–(La(La1-x1-xCaCaxx)MnO)MnO33 (Perovskite)(Perovskite)

•(La(La1-x1-xSrSrxx)(Co)(Co1-x1-xFeFexx)O)O3 3 (Perovskite)(Perovskite)

•(Sm(Sm1-x1-xSrSrxx)CoO)CoO3 3 (Perovskite)(Perovskite)

•(Pr(Pr1-x1-xSrSrxx)(Co)(Co1-x1-xMnMnxx)O)O3 3 (Perovskite)(Perovskite)

Anode (HAnode (H22/CO Electrode)/CO Electrode)–Ni/ZrNi/Zr1-x1-xYYxxOO22 Composites Composites

Electrolyte (Air Electrode)Electrolyte (Air Electrode)–ZrZr1-x1-xYYxxOO22 (Fluorite) (Fluorite)

•CeCe1-x1-xRRxxOO22 , R = Rare Earth Ion (Fluorite) , R = Rare Earth Ion (Fluorite)

•BiBi2-x2-xRRxxOO33 , R = Rare Earth Ion (Defect Fluorite) , R = Rare Earth Ion (Defect Fluorite)

•GdGd1.91.9CaCa0.10.1TiTi22OO6.956.95 (Pyrochlore) (Pyrochlore)

•(La,Nd)(La,Nd)0.80.8SrSr0.20.2GaGa0.80.8MgMg0.20.2OO2.82.8 (Perovskite (Perovskite))

Interconnect (between Cathode and Anode)Interconnect (between Cathode and Anode)–LaLa1-x1-xSrSrxxCrOCrO33 (Perovskite) (Perovskite)


OO22 Gas Sensor 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.

See http://www.cambridge-sensotec.co.uk/sensors_explained.htm for details

V = (RT/4F) ln[{(PV = (RT/4F) ln[{(PO2O2(ref.)}/{(P(ref.)}/{(PO2O2(sample)}](sample)}]


Design Principles: ODesign Principles: O2-2- ConductorsConductors

•High concentration of anion vacanciesHigh concentration of anion vacancies–necessary for Onecessary for O2-2- hopping to occur hopping to occur

•High SymmetryHigh Symmetry–provides equivalent potentials between occupied and vacant sitesprovides equivalent potentials between occupied and vacant sites

•High Specific Free Volume (Free Volume/Total High Specific Free Volume (Free Volume/Total Volume)Volume)

–void space/vacancies provide diffusion pathways for Ovoid space/vacancies provide diffusion pathways for O2-2- ions ions

•Polarizable cations (including cations with Polarizable cations (including cations with stereoactive lone pairs)stereoactive lone pairs)

–polarizable cations can deform during hopping, which lowers the polarizable cations can deform during hopping, which lowers the activation energyactivation energy

•Favorable chemical stability, cost and thermal Favorable chemical stability, cost and thermal expansion characteristicsexpansion characteristics

–for commercial applicationsfor commercial applications


Phase Transitions in ZrOPhase Transitions in ZrO22

Room TemperatureRoom TemperatureMonoclinic (P2Monoclinic (P211/c) /c)

7 coordinate Zr7 coordinate Zr4 coord. + 3 coord. O4 coord. + 3 coord. O2-2-

High TemperatureHigh TemperatureCubic (Fm3m) Cubic (Fm3m)

cubic coordination for cubic coordination for ZrZr

tetrahedral coord. for tetrahedral coord. for OO2-2-


Effect of Dopants: ZrOEffect of Dopants: ZrO22, CeO, CeO22

•Doping ZrODoping ZrO22 (Zr (Zr1-x1-xYYxxOO2-x/22-x/2, Zr, Zr1-x1-xCaCaxxOO2-x2-x) fulfills two purposes) fulfills two purposes

–Introduces anion vacancies (lower valent cation needed)Introduces anion vacancies (lower valent cation needed)–Stabilizes the high symmetry cubic structure (larger cations are Stabilizes the high symmetry cubic structure (larger cations are most effective)most effective)

•We can also consider replacing Zr with a larger cation (i.e. CeWe can also consider replacing Zr with a larger cation (i.e. Ce4+4+) in ) in order to stabilize the cubic fluorite structure, or with a lower valent order to stabilize the cubic fluorite structure, or with a lower valent cation (i.e. Bication (i.e. Bi3+3+) to increase the vacancy concentration.) to increase the vacancy concentration.

CompoundCompound r r4+4+ Specific Free Conductivity Specific Free Conductivity

(Angstroms) Volume @ 800 ºC(Angstroms) Volume @ 800 ºC

ZrZr0.80.8YY0.20.2OO1.91.9 0.86 0.31 0.03 S/cm 0.86 0.31 0.03 S/cm

CeCe0.80.8GdGd0.20.2OO1.91.9 1.01 0.38 0.15 S/cm 1.01 0.38 0.15 S/cm

-Bi-Bi22OO33 1.17 1.17 0.50 0.50 1.0 S/cm 1.0 S/cm (730 C)(730 C)

BiBi22OO33 is only cubic from 730 ºC to it’s melting point of 830 ºC. Doping is necessary to 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.stabilize the cubic structure to lower temps.


GdGd22TiTi22OO77 Pyrochlore PyrochloreThe pyrochlore structure can be derived The pyrochlore structure can be derived from fluorite, by removing 1/8 of the from fluorite, by removing 1/8 of the oxygens, ordering the two cations and oxygens, ordering the two cations and ordering the oxygen vacancies. ordering the oxygen vacancies.

By replacing some of the GdBy replacing some of the Gd3+3+ with Ca with Ca2+ 2+

oxygen vacancies in the Aoxygen vacancies in the A22O network O network are created, significantly increasing the are created, significantly increasing the ionic conductivity (at 1000 ionic conductivity (at 1000 ºC)ºC)::GdGd22TiTi22OO77

= 1 = 1 10 10-4-4 S/cm, E S/cm, EAA = 0.94 eV = 0.94 eV

GdGd1.81.8CaCa0.20.2TiTi22OO6.956.95

= 5 = 5 10 10-2-2 S/cm, E S/cm, EAA = 0.63 eV = 0.63 eV

There is an opportunity to obtain There is an opportunity to obtain mixed electronic-ionic conductivity in mixed electronic-ionic conductivity in the pyrochlore structure. the pyrochlore structure.

MM22OO66 Network Network AA22O NetworkO Network


BaBa22InIn22OO55 Brownmillerite Brownmillerite

The brownmillerite structure can be The brownmillerite structure can be derived from perovskite, by removing derived from perovskite, by removing 1/6 of the oxygens and ordering the 1/6 of the oxygens and ordering the vacancies so that 50% of the smaller vacancies so that 50% of the smaller cations are in distorted tetrahedral cations are in distorted tetrahedral coordination.coordination.

In BaIn Ba22InIn22OO55 at 800 ºC the oxygen at 800 ºC the oxygen vacancies disorder throughout the vacancies disorder throughout the tetrahedral layer, and the ionic tetrahedral layer, and the ionic conductivity jumps from 10conductivity jumps from 10-3-3 S/cm to S/cm to 1010-1-1 S/cm. S/cm.

BaZrOBaZrO33-Ba-Ba22InIn22OO55 solid solutions absorb solid solutions absorb water to fill oxygen vacancies and water to fill oxygen vacancies and become good proton conductors over become good proton conductors over the temperature range 300-700 ºC. the temperature range 300-700 ºC.

Tetrahedral Tetrahedral LayerLayer

Octahedral Octahedral LayerLayer


Aurivillius and BIMEVOX phasesAurivillius and BIMEVOX phasesBiBi22WOWO66 is a member of the Aurivilius structure is a member of the Aurivilius structure family. The structure contains 2D perovskite-like family. The structure contains 2D perovskite-like sheets made up of corner sharing octahedra, sheets made up of corner sharing octahedra, stacked with Bistacked with Bi22OO22

2+2+ layers. layers.

BiBi44VV22OO1111 is a defect Aurivillius phase, better is a defect Aurivillius phase, better written as (Biwritten as (Bi22OO22)VO)VO3.53.5, where 1/8 of the oxygen , where 1/8 of the oxygen sites in the perovskite layer are vacant. sites in the perovskite layer are vacant. Conductivity at 600 ºC is the highest ever Conductivity at 600 ºC is the highest ever reported for an Oreported for an O2-2- conductor ~ 0.2 S/cm. conductor ~ 0.2 S/cm.

Only the perovskite oxygens are mobile.Only the perovskite oxygens are mobile.

Normally BiNormally Bi44VV22OO1111 undergoes phase transitions undergoes phase transitions upon cooling that lower it’s ionic conductivity, upon cooling that lower it’s ionic conductivity, but doping onto the V site stabilizes the HT but doping onto the V site stabilizes the HT phase. These phases are generally called phase. These phases are generally called BIMEVOX phases. BIMEVOX phases. (Bi(Bi22OO22)V)V0.90.9CuCu0.10.1OO3.353.35 has a has a conductivity of 0.01 S/cm at 350 ºC !!conductivity of 0.01 S/cm at 350 ºC !!


Summary OSummary O2-2- Conductors Conductors

•It is generally true that dopants have to be added either to It is generally true that dopants have to be added either to introduce vacancies, or to stabilize the high introduce vacancies, or to stabilize the high temperature/high symmetry phasetemperature/high symmetry phase

•Among fluorite based OAmong fluorite based O2-2- conductors both doped CeO conductors both doped CeO22 and and BiBi22OO33 have higher conductivities than stabilized ZrO have higher conductivities than stabilized ZrO22, but , but both are less chemically stable. In particular they are prone both are less chemically stable. In particular they are prone to reduction. This limits their use.to reduction. This limits their use.

•Brownmillerite conductors show high conductivity, but are Brownmillerite conductors show high conductivity, but are prone to become electrically conducting under mildly prone to become electrically conducting under mildly reducing conditions. They show promise as proton reducing conditions. They show promise as proton conductors.conductors.

•Ionic conductors based on BiIonic conductors based on Bi44VV22OO1111 (BIMEVOX) show very (BIMEVOX) show very high conductivity for low temperature applications.high conductivity for low temperature applications.

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Solid Electrolytes Materials and Applications