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Recent progress in solid oxide and lithium ion conducting electrolytes research

Abstract Recent material developments of fast solid oxide and lithium ion conductors are reviewed. Special emphasis is placed on the correlation between the composition, structure, and electrical transport properties of perovskitetype, perovskite-related, and other inorganic crystalline materials in terms of the required functional properties for practical applications, such as fuel or hydrolysis cells and batteries. The discussed materials include Sr- and Mg-

(Li,La,)TiO3,L i3La3La3Nb2O12 (M=Nb, Ta), and Na super-ionic conductor-type phosphate. Critical problems with regard to the development of practically useful devices are discussed.

Keywords Ionic transport . Oxide ion conductors . Solid lithium ion conductors . Lithium batteries . Fuel cells . SOFCs


After Walter Nernst’s pioneering discovery [1] of various solid ceramic oxides being “conductors of second kind,” as electrolytes were called at the time of this discovery at the end of the 19th century, fast ionic transport in solids was observed for a variety of metal halides and sulfides during the early decades of the 20th century. However, fast progress in understanding and developing novel materials for practical applications has only been achieved during the last few decades. From this point of view, solid electrolytes should have high ionic conductivity based on a single predominantly conducting anion or cation species and should have negligibly small electronic conductivity. Typically, useful solid electrolytes exhibit ionic conductivities in the range of 10−1–10−5 S/cm at room temperature. This value is in-between that of metals and insulators and has the same order of magnitude as those of semiconductors and liquid electrolytes (Fig. 1)[ 2–6].

Research on solid electrolytes has recently drawn much attention due to the wide range of potential important technological applications in solid-state devices, including solid oxide fuel cells (SOFCs), proton exchange membrane fuel cells, water hydrolysis cells, chemical sensors (e.g., for

H2,O 2,N H3, CO, CO2,N Ox,S Ox,C H4,C nH2n+2,S Ox, etc.), high-energy-density rechargeable (secondary) bat- teries, electrochromic displays, chemotronic elements (e.g., nonvolatile memory elements) thermoelectric converters, and photogalvanic solar cells [7–25]. Furthermore, solid electrolytes turned out to be very useful for the determination of fundamental thermodynamic quantities, such as Gibbs energies, entropies, enthalpies, activity coefficients, and nonstoichiometries of solids at elevated temperatures and kinetic parameters, such as chemical diffusion coefficients, diffusivities, and Wagner factors, as functions of the stoichiometry and temperature [16].

A broad variety of materials is known today to exhibit ionic conduction, including single-crystalline, polycrystalline, and amorphous ceramic materials, composites, and polymer salt mixtures. Especially, several solid electrolytes are known for monovalent protons and lithium, sodium, silver, potassium, copper, and fluoride ions, as well as divalent oxide and metal ions [1–6]. Among those types of ions, protons, oxide and lithium ions play a key role for future technologies. Recent progress in the development of new materials, especially compounds based on perovskite and related structures are discussed in this review.

V. Thangadurai (*) Department of Chemistry, University of Calgary, 2500 University Dr NW, Calgary, Alberta, T2N 1N4, Canada e-mail: Tel.: +1-403-2108649 Fax: +1-403-2899488

W. Weppner Faculty of Engineering, Chair for Sensors and Solid State Ionics, University of Kiel, 24143 Kiel, Germany e-mail: Tel.: +49-431-8806201 Fax: +49-431-8806203

Oxide ion electrolytes General considerations

Doping parent metal oxides with oxides of lower-valent cations than the host cation produces anion deficiencies, i.e., oxygen stoichiometries less than that of the mother phase. This is an essential requirement for oxide ion conduction [2, 26, 27], because the transport of oxide ions occurs generally by a hopping process between an occupied and a vacant oxide ion site. It is generally accepted that high ionic conduction of oxides is based on the following structural properties:

1. An optimum number of oxide ion vacancies which should be structurally distributed 2. Vacant and occupied sites should have nearly the same potential energies for low-activation-energy conduction processes 3. Weak frame-work and 3D structures 4. Weak covalent bonding between the metal and oxygen (M–O) 5. Host and guest metal ions should be stable with different coordination numbers of oxygen.

Solid oxide ion electrolytes are notably of interest for energy conversion and environmental applications, including SOFCs, oxygen sensors, oxygen pumps, and oxygen permeable membranes [9–14, 17, 23–25]. Besides solid electrolytes, suitable predominantly electronically (mixed ionic–electronic) conducting anode and cathode materials have to be developed. Present SOFCs are based on fluorite- type Y2O3-doped ZrO2 (YSZ), perovskite-type, Sr-doped

LaMnO3 (LSM) cathodes and composite Ni-YSZ cermet anodes [9–14, 17]. A critical problem is the high operating temperature (≈1,0 °C) that is required for reasonably high oxide ion conduction of YSZ. The high operating temperature leads to several materials problems [25]. These are:

1. Interfacial diffusion of Sr and La between the electrode (LSM) and the electrolyte, leading to the formation of poor conducting reaction products, e.g., SrZrO3 and La2Zr2O7

2. Mechanical stress due to different thermal expansion coefficients, which destroys good electrical electrolyte and electrode contacts 3. High cost of the bipolar separators, which are required for long-term mechanical and chemical stability in oxidizing and reducing atmospheres 4. Lack of appropriate sealing materials that are chemically inert against chemical reactions with all components of the fuel cell.

Accordingly, there is strong, currently ongoing research for intermediate temperature (IT; 400–800 °C) solid oxide conductors. In this regard, a set of functional material properties has tobe fulfilledfor practicalSOFCdevelopment:

1. High oxide ion conductivity at the operating temperatures (400–800 °C) without structural phase transition and decomposition 2. Electrochemical stability up to at least 1.2 Vagainst air or pure O2 at the operating temperature 3. Negligible electronic conductivity over the entire or most of the employed range of oxygen partial pressures and temperature 4. Dense, gas-tight, pore-free preparation of the material with good adhesion to both anode and cathode materials, with matching thermal expansion coefficients (including the sealing material) 5. Stability against chemical reactions with anode, cathode, and sealing materials over an extended period of operation time 6. Negligible electrolyte–electrode interface and chargetransfer resistances 7. Low-cost, nontoxicity, easiness of preparation, chemical and kinetic stability under ambient conditions, especially conditions of moisture and CO2 content in the atmosphere.

Various transition inorganic metal oxides possessing fluorite, pyrochlore, perovskite, and structurally perovskite-related brownmillerite, Aurivillius, and Ruddlesden–

Popper layered perovskites (e.g., K2NiF4-type) have been considered for the development of intermediate tempera- ture solid oxide fuel cell (IT-SOFC) electrolytes. Table 1 lists the potential oxide electrolytes and comments on their technological problems with regard to their application in

Fig. 1 Regime of electrical conductivity of solid electrolytes at room temperature as shown by the broken lines and compared with the electrical conductivities of typical metals, semiconductors, and insulators

fuel cells [9–14, 17–25]. Figure 2 shows the Arrhenius plots for the oxide ion conductivities of selected metal oxides [13].

Oxide ion electrolytes based on ABO3 perovskite

Takahashi and Iwahara were the first to work on oxide

ion conductors based on the perovskite structure ABO3 (A = Ca, Sr, La; B = Al, Ti) with aliovalent substitution of the metals [21]. The idealized crystal structures of perovskite and related structures are shown in Fig. 3. Onlyi nr ecent years, perovskite and related crystal structures have been investigated more intensively with regard to the possibility of oxide ion conduction. Figure 4 summarizes the electrical conductivities of perovskite and structurally related materials. Partial substitution of Ca2+ by Nd3+ and Ga3+ by Al3+ in the perovskite-type NdAlO3 yields an aniondeficient compound with the chemical formula of

S/cm at 900 °C [29]. The highest bulk oxide ion conductivity value of 0.17 S/cm at 800 °C was observed for x = 0.2

Fig. 2 Arrhenius plot of the oxide ion conductivity of selected metal oxides in ambient atmosphere [13]

Table 1 Potential oxide ion electrolytes and their problems when employed in IT-SOFCs [9–14, 17–25]

Oxide ionelectrolyte Structure type Critical materials issues when used in IT-SOFCs

Y2O3-doped ZrO2 Fluorite Poor ionic conductor, incompatible with perovskite-type cathode materials (e.g., Sr-doped LaMO3 (M = Mn, Co) at elevated temperatures and long period of operation time

Fluorite Expensive, long-term performance is not known

Rare-earth-doped CeO2

Fluorite Not stable in the low-oxygen partial pressure, poor mechanical stability, large grain boundary resistance at lower temperature Sr + Mg-doped


Perovskite Not stable at low oxygen partial pressures, forms carbonates in CO and CO2 atmospheres, Gaevaporates in H2 atmosphere, incompatible with Ni anode at elevated temperatures

Ba2In2O5 Brownmillerite Not stable at low oxygen partial pressures, poor ionic conductor at low temperature, shows first-order phase transition accompanied by structural change, degradation in CO2 atmosphere with the formation of BaCO3

Doped Bi4V2O11 Aurivillius Stable over a limited range of oxygen partial pressures BaBi4Ti3InO14.5 Aurivillius Moderate ionic conductor, electrochemical stability at low and high oxygen partial pressures is not known, may form carbonates in CO2 atmosphere

Gd2Ti2O7 Pyrochlore Poor ionic conductor and not stable at low oxygen partial pressures at elevated temperatures

Doped BaCeO3 Perovskite Chemically not stable in CO2-containing atmospheres, exhibits hole (p-type) and electronic (n-type) conduction at high and low oxygen partial pressures, respectively, at elevated temperatures

Sr3Ti1.9Al0.1O7−x Ruddlesden–Popper Poor ionic conductor, p-type electronic conduction at high oxygen partial pressures

YSZ (3.6×10−2 S/cm at 800 °C). Accordingly, LSGM- based materials are presently considered as potential candidates for IT-SOFCs.

Effect of doping on the ionic conductivity of LSGM perovskites

Effect of A-site substitution Figure 5 shows the chemistry of metal ion substitution in the case of perovskite-type LSGM. Partial substitution (10 mol %) of La by other rare earth ions at the A-site (Nd, Sm, Gd, Y, Yb) in

La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) decreases the total electrical conductivity [34]. Unlike other rare earth substitutions of LSGM, Pr substitution (50 at. %) of La does not affect the electrical conductivity, except with a slight decrease at high temperature, while the conductivity corresponds to that of LSGM at low temperature [35]. Among the alkaline earth ions occupying the A-site, Sr provides the highest ionic conductivity enhancement, compared to Ca and Ba. K and Pr substitutions for La/Sr in LSGM result in a simple cubic perovskite structure.

Replacement of Sr by K in La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) shows a lower electrical conductivity (8.56×10−3

S/cm at 700 °C) with high activation energy of 1.42 eV, compared to 1.07 eV for LSGM.

Effect of B-site substitution Partial substitution of Ga by Al or In in LSGM decreases the electrical conductivity. The decrease in electrical conductivity may be attributed to the changes in the crystal structure and lattice parameter. The complete replacement of trivalent Ga3+ by other isovalent metal ions such as Al3+,I n3+, and Sc3+ in LSGM decreases the total ionic conductivity [36]. Oxygenpartial-pressure-dependent electrical measurements of

La0.9Sr0.1In0.8Mg0.2O2.85 (LSIM) show p-type electronic conduction at high oxygen partial pressures in the range

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