Ionic Conductivity and Solid Electrolytes II: Materials and Applications

Transcription

1 Ionic Conductivity and Solid Electrolytes II: Materials and Applications Chemistry 754 Solid State Chemistry Lecture #27 June 4, 2003 References A. Manthiram & J. Kim Low Temperature Synthesis of Insertion Oxides for Lithium Batteries, Chem. Mater. 10,, (1998). J.C. Boivin & G. Mairesse Recent Material Developments in Fast Oxide Ion Conductors, Chem. Mater. 10,, (1998). J.C. Boivin Structural and Electrochemical Features of Fast Oxide Ion Conductors, Int. J. Inorg.. Mater. 3,, (2001). S.C. Singhal Science and Technology of Solid-Oxide Fuel Cells, MRS Bulletin,, (March, 2000). M.M. Thackeray,, J.O. Thomas & M.S. Whittingham Science and Applications of Mixed Conductors for Lithium Batteries, MRS Bulletin,, (March, 2000). 1

2 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, (1998). 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. 2

3 Li x TiS 2 Structure type is CdI 2, hcp packing of anions, octahedral Ti Li intercalates between the I - layers Pure TiS 2 is a semi-metal, metal, conductivity increases upon insertion of Li (high electronic conductivity) Lithium insertion varies from 1 x 0 10% expansion, TiS 2 LiTiS 2 Capacity ~ 250 A-h/kgA Voltage ~ 1.9 Volts (This is the major limitation of the TiS 2 cathode) Energy density ~ 480 W-h/kgW Li Inserts in this layer Li Inserts in this layer Li 1-x CoO 2 LiMO 2 structures are ordered derivatives of rock salt (ordering occurs along alternate 111 layers) Li intercalates into octahedral sites between the edge sharing CoO 2 layers Good electrical conductor Lithium de-intercalation varies from 0 x 0.5 and is reversible Capacity ~ 45 A-h/kgA Voltage ~ 3.7 Volts Energy density ~ 165 W-h/kgW Cobalt is expensive (relative to Ti, Ni and Mn). 3

4 Structure type is defect spinel Mn ions occupy the octahedral sites, while Li + resides on the tetrahedral sites. Li 1-x Mn 2 O 4 Rather poor electrical conductivity Lithium de-intercalation varies from 0 x 1, comparable to Li 1-x CoO 2 Presence of Mn 3+ gives a Jahn- Teller distortion that limits cycling. High Li content stabilizes layer like structure. Capacity ~ 36 A-h/kgA Voltage ~ 3.8 Volts Energy density ~ 137 W-h/kgW Mn is cheap and non-toxic. 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( ºC) Typical Fuels - 2H 2 + O 2 (from the air) 2CO + O 2 (from the air) H 2 O 2CO 2 Advantages vs. Conventional Power Generation Methods (e.g. Steam Turbines) Higher conversion efficiency Lower CO 2 emissions See for more details 4

5 Schematic of a Solid Oxide Fuel Cell Taken from Materials Issues (SOFC) Cathode (Air Electrode) & Anode (H 2 /CO Electrode) High electronic conductivity Chemical and mechanical stability (at 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 O 2 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 5

6 Favored Materials (SOFC) Cathode (Air Electrode) (La 1-x Ca x )MnO 3 (Perovskite) (La 1-x Sr x )(Co 1-x Fe x )O 3 (Perovskite) (Sm 1-x Sr x )CoO 3 (Perovskite) (Pr 1-x Sr x )(Co 1-x Mn x )O 3 (Perovskite) Anode (H 2 /CO Electrode) Ni/Zr 1-x Y x O 2 Composites Electrolyte (Air Electrode) Zr 1-x Y x O 2 (Fluorite) Ce 1-x R x O 2, R = Rare Earth Ion (Fluorite) Bi 2-x R x O 3, R = Rare Earth Ion (Defect Fluorite) Gd 1.9 Ca 0.1 Ti 2 O 6.95 (Pyrochlore) (La,Nd 0.8 Sr 0.2 Ga 0.8 Mg 2.8 (Perovskite) Nd) 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 2.8 Interconnect (between Cathode and Anode) La 1-x Sr x CrO 3 (Perovskite) O 2 Gas Sensor The partial pressure of oxygen in the sample gas, P O2 (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[{(p O2 (ref.)}/{(p O2 (sample)}] See for details 6

7 Design Principles: O 2- Conductors High concentration of anion vacancies necessary for O 2 - 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 O 2 - 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 Phase Transitions in ZrO 2 Room Temperature Monoclinic (P2 1 /c) 7 coordinate Zr 4 coord coord.. O 2- High Temperature Cubic (Fm3m) cubic coordination for Zr tetrahedral coord.. for O 2-7

8 Effect of Dopants: : ZrO 2, CeO 2 Doping ZrO 2 (Zr 1-x Y x O 2-x/2 x/2,, Zr 1-x Ca x O 2-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. Ce 4+ ) in order to stabilize the cubic fluorite structure, or with a lower valent cation (i.e. Bi 3+ ) to increase the vacancy concentration. Compound r Specific Free Conductivity (Angstroms) 800 ºC Zr 0.8 Y 0.2 O S/cm Ce 0.8 Gd 0.2 O S/cmS δ-bi 2 O S/cm (730 C) Bi 2 O 3 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. Gd 2 Ti 2 O 7 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 Gd 3+ with Ca 2+ oxygen vacancies in the A 2 O network are created, significantly increasing the ionic conductivity (at 1000 ºC): Gd 2 Ti 2 O 7 σ = S/cm, E A = 0.94 ev Gd 1.8 Ca 0.2 Ti 2 O 6.95 σ = S/cm, E A = 0.63 ev There is an opportunity to obtain mixed electronic-ionic ionic conductivity in the pyrochlore structure. M 2 O 6 Network A 2 O Network 8

9 Ba 2 In 2 O 5 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 Ba 2 In 2 O 5 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. BaZrO 3 -Ba 2 In 2 O 5 solid solutions absorb water to fill oxygen vacancies and become good proton conductors over the temperature range ºC. Octahedral Layer Aurivillius and BIMEVOX phases Bi 2 WO 6 is a member of the Aurivilius structure family. The structure contains 2D perovskite- like sheets made up of corner sharing octahedra, stacked with Bi 2 O 2+ 2 layers. Bi 4 V 2 O 11 is a defect Aurivillius phase, better written as (Bi 2 O 2 )VO 3.5, where 1/8 of the oxygen sites in the perovskite layer are vacant. Conductivity at 600 ºC is the highest ever reported for an O 2 - conductor ~ 0.2 S/cm. Only the perovskite oxygens are mobile. Normally Bi 4 V 2 O 11 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. (Bi 2 O 2 )V 0.9 Cu 0.1 O 3.35 has a conductivity of 0.01 S/cm at 350 ºC!! 9

10 Summary O 2- 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 O 2 - conductors both doped CeO 2 and Bi 2 O 3 have higher conductivities than stabilized ZrO 2, 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 Bi 4 V 2 O 11 (BIMEVOX) show very high conductivity for low temperature applications. 10