solid_electro_lect27

solid_electro_lect27 - Ionic Conductivity and Solid...

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Unformatted text preview: Ionic Conductivity and Solid Electrolytes II: Materials and Applications Chemistry 754 Solid State Chemistry Lecture #27 June 4, 2003 Chem 754 - Solid State Chemistry References A. Manthiram & J. Kim "Low Temperature Synthesis of Insertion Oxides for Lithium Batteries", Chem. Mater. 10, 2895-2909 (1998). 10, 2895J.C. Boivin & G. Mairesse "Recent Material Developments in Fast Oxide Ion Conductors", Chem. Mater. 10, 2870-2888 (1998). 10, 2870J.C. Boivin "Structural and Electrochemical Features of Fast Oxide Ion Conductors", Int. J. Inorg. Mater. 3, 1261-1266 (2001). Inorg. 1261S.C. Singhal "Science and Technology of Solid-Oxide SolidFuel Cells", MRS Bulletin, 16-21 (March, 2000). 16M.M. Thackeray, J.O. Thomas & M.S. Whittingham Thackeray, "Science and Applications of Mixed Conductors for Lithium Batteries", MRS Bulletin, 39-46 (March, 392000). Chem 754 - Solid State Chemistry 1 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, 10, 2895-2909 (1998). 2895- 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. voltage. 2. The cathode material should allow an insertion/extraction of a large amount of lithium to maximize the capacity. 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 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 voltage range and not react with the electrolyte. 6. The cathode material should be inexpensive, environmentally friendly and lightweight. Chem 754 - Solid State Chemistry 2 LixTiS2 Structure type is CdI2, hcp packing of anions, octahedral Ti Li intercalates between the I- layers Pure TiS2 is a semi-metal, conductivity semiincreases upon insertion of Li (high electronic conductivity) Lithium insertion varies from 1 x 0 10% expansion, TiS2 LiTiS2 Capacity ~ 250 A-h/kg A Voltage ~ 1.9 Volts (This is the major limitation of the TiS2 cathode) Li Inserts in this layer Li Inserts in this layer Energy density ~ 480 W-h/kg W- 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 dex 0.5 and is reversible Capacity ~ 45 A-h/kg A Voltage ~ 3.7 Volts Energy density ~ 165 W-h/kg W Cobalt is expensive (relative to Ti, Ni and Mn). Mn). Chem 754 - Solid State Chemistry 3 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 defrom 0 x 1, comparable to Li1-xCoO2 Presence of Mn3+ gives a JahnJahnTeller distortion that limits cycling. High Li content stabilizes layer like structure. Capacity ~ 36 A-h/kg A Voltage ~ 3.8 Volts Energy density ~ 137 W-h/kg W Mn is cheap and non-toxic. nonChem 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 C) (eg 800Typical 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 4 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 C in oxidizing 600conditions 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 Free of porosity High oxygen ion conductivity Very low electronic conductivity Electrolyte (Air Electrode) 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 components Chem 754 - Solid State Chemistry 5 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 Zr1-xYxO2 (Fluorite) Electrolyte (Air Electrode) Ce1-xRxO2 , R = Rare Earth Ion (Fluorite) Bi2-xRxO3 , R = Rare Earth Ion (Defect Fluorite) Gd1.9Ca0.1Ti2O6.95 (Pyrochlore) Pyrochlore) (La,Nd)0.8Sr0.2Ga0.8Mg0.2O2.8 (Perovskite) (La,Nd) 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 C. V = (RT/4F) ln[{(PO2(ref.)}/{(PO2(sample)}] ln[{(P See http://www.cambridge-sensotec.co.uk/sensors_explained.htm for details Chem 754 - Solid State Chemistry 6 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 void space/vacancies provide diffusion pathways for O2- ions High Specific Free Volume (Free Volume/Total Volume) 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. O2coord. coord. High Temperature Cubic (Fm3m) cubic coordination for Zr tetrahedral coord. for O2coord. Chem 754 - Solid State Chemistry 7 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 -Bi2O3 Specific Free r4+ (Angstroms) Volume 0.86 0.31 1.01 0.38 1.17 0.50 Conductivity @ 800 C 0.03 S/cm 0.15 S/cm S/cm 1.0 S/cm (730 C) Bi2O3 is only cubic from 730 C to it's melting point of 830 C. Doping is necessary to Doping 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 oxygens, 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 C): Gd2Ti2O7 = 1 10-4 S/cm, EA = 0.94 eV Gd1.8Ca0.2Ti2O6.95 = 5 10-2 S/cm, EA = 0.63 eV There is an opportunity to obtain mixed electronic-ionic conductivity in electronicthe pyrochlore structure. M2O6 Network A2O Network Chem 754 - Solid State Chemistry 8 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 C 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 C. 300Octahedral Layer Tetrahedral Layer Chem 754 - Solid State Chemistry Aurivillius and BIMEVOX phases Bi2WO6 is a member of the Aurivilius structure family. The structure contains 2D perovskiteperovskitelike 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 C 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 C !! Chem 754 - Solid State Chemistry 9 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. 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 10 ...
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