Ionic Conductivity and Solid Electrolytes II: Materials and Applications

1. Ionic Conductivity and Solid Electrolytes II: Materials and Applications Chemistry 754 Solid State Chemistry Lecture #27 June 4, 2003 Chem 754 - Solid State Chemistry

2. References A. Manthiram & J. Kim – “Low Temperature Synthesis of Insertion Oxides for Lithium Batteries”, Chem. Mater. 10, 2895-2909 (1998). J.C. Boivin & G. Mairesse – “Recent Material Developments in Fast Oxide Ion Conductors”, Chem. Mater.10, 2870-2888 (1998). J.C. Boivin – “Structural and Electrochemical Features of Fast Oxide Ion Conductors”, Int. J. Inorg. Mater.3, 1261-1266 (2001). S.C. Singhal – “Science and Technology of Solid-Oxide Fuel Cells”, MRS Bulletin, 16-21 (March, 2000). M.M. Thackeray, J.O. Thomas & M.S. Whittingham – “Science and Applications of Mixed Conductors for Lithium Batteries”, MRS Bulletin, 39-46 (March, 2000). Chem 754 - Solid State Chemistry

3. 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

4. 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

5. 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) • 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 Li Inserts in this layer Chem 754 - Solid State Chemistry

6. 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

7. 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 Jahn-Teller 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

8. Solid Oxide Fuel Cells • A fuel cell generates electricity and heat by electrochemically combining a gaseous fuel and an oxidizing gas, via an ion conductingelectrolyte, 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

9. Schematic of a Solid Oxide Fuel Cell Taken from http://www.spice.or.jp/~fisher/sofc.html Chem 754 - Solid State Chemistry

10. 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

11. 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

12. 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.htmfor details Chem 754 - Solid State Chemistry

13. 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

14. 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

15. 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 r4+ Specific Free Conductivity (Angstroms) Volume @ 800 ºC Zr0.8Y0.2O1.9 0.86 0.31 0.03 S/cm Ce0.8Gd0.2O1.9 1.01 0.38 0.15 S/cm d-Bi2O3 1.17 0.50 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

16. 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

17. 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. 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. Tetrahedral Layer Octahedral Layer Chem 754 - Solid State Chemistry

18. Aurivillius and BIMEVOX phases Bi2WO6 is a member of the Aurivilius structure family. The structure contains 2D perovskite-like 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

19. 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