Ionic solid electrolytes

Takahashi T, Yamamoto O (1970) Solid ionics-solid electrolyte cells. J Electrochem Soc 4117 1-5... 

Ceramic electrochemical reactors are currently undergoing intense investigation, the aim being not only to generate electricity but also to produce chemicals. Typically, ceramic dense membranes are either pure ionic (solid electrolyte SE) conductors or mixed ionic-electronic conductors (MIECs). In this chapter we review the developments of cells that involve a dense solid electrolyte (oxide-ion or proton conductor), where the electrical transfer of matter requires an external circuitry. When a dense ceramic membrane exhibits a mixed ionic-electronic conduction, the driving force for mass transport is a differential partial pressure applied across the membrane (this point is not considered in this chapter, although relevant information is available in specific reviews). 

A. Goetzberger, C. Hebling, and H.-W. Schock. Photovoltaic materials, history, status and outlook. Mater. Set Eng., R, 40(1) 1-46, January 2003. F.-C. Chen, Q. Xu, and Y. Yang. Enhanced efficiency of plastic photovoltaic devices by blending with ionic solid electrolytes. Appl. Phys. Lett., 84 3181-3183,2004. 

The physical chemistry of electrolytic solutions is a special area of physical chemistry with a large number of reference literatures. Classical descriptions are given in the books of Hamed and Owen or Robinson and Stokes. A more recent advanced treatment is found in the book of Barthel, Krienke, and Kunz. The special problems of ionic-conducting polymers and ionic solid electrolytes are described in various other reviews. Grajd described polymer electrolytes. A classical treatment of ionic solid electrolytes is the book by Rickert or the Kudo and Fueki compilation. Because these materials are used in batteries and fuel cells, there is much hterature for this research field including such detailed reviews in the book by Julien and Nazri. Another source for details and data compilations is the CRC Handbook of Solid State Electrochemistry f... 

Chen FC, Xu Q, Yang Y. Enhanced efficiency of plastic photovoltaic devices by blending with ionic solid electrolytes. Appl Phys Lett 2004 84 3181-3. 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

Solid Electrolytes. Of the salt-like nitrides, only Li N has attracted technical interest. Lithium nitride has an uncommonly high ionic... 

A completely separate family of conducting polymers is based on ionic conduction polymers of this kind (Section 11.3.1.2) are used to make solid electrolyte membranes for advanced batteries and some kinds of fuel cell. 

Increasing numbers of advanced batteries for all purposes depend on ionically conducting solid electrolytes, so it will be helpful to discuss these before continuing. It should be remembered that any battery can be described as an electron pump, and the role of the electrolyte is to block the passage of electrons, letting ions through instead. 

The scope of the term corrosion is continually being extended, and Fontana and Staehle have stated that corrosion will include the reaction of metals, glasses, ionic solids, polymeric solids and composites with environments that embrace liquid metals, gases, non-aqueous electrolytes and other non-aqueous solutions . 

When an ionic solid such as NaCl dissolves in water the solution formed contains Na+ and Cl- ions. Since ions are charged particles, the solution conducts an electric current (Figure 2.12) and we say that NaCl is a strong electrolyte. In contrast, a water solution of sugar, which is a molecular solid, does not conduct electricity. Sugar and other molecular solutes are nonelectrolytes. 

The ions in an electrolyte solution can arise in two major ways. They may already be present in the pure compound, as in ionic solids. When such a solid is placed in water, the ions separate and move throughout the solution. However, some compounds that form ions in water are not considered to contain ions when pure, whether in the solid, liquid, or gas phase. Hydrochloric acid, HQ, and sulfuric acid, H2S04, are good examples of the second type of compound. They form molecular liquids (or solids, if cold enough). But in water they form ions HC1 gives hydrogen ion, H+(aq), and chloride ion, G (aq) H2SO ... 

In Chapter 6 we saw that the chemistry of sodium can be understood in terms of the special stability of the inert gas electron population of neon. An electron can be pulled away from a sodium atom relatively easily to form a sodium ion, Na+. Chlorine, on the other hand, readily accepts an electron to form chloride ion, Cl-, achieving the inert gas population of argon. When sodium and chlorine react, the product, sodium chloride, is an ionic solid, made up of Na+ ions and Cl- ions packed in a regular lattice. Sodium chloride dissolves in water to give Na+(aq) and C (aq) ions. Sodium chloride is an electrolyte it forms a conducting solution in water. 

The equivalent circuit of a section of this SEI is presented in Fig. 13(b). It was recently found [123, 124] that at temperatures lower than 90 °C, the grain-boundary resistance of composite polymer electrolytes and composite solid electrolytes based on Lil-A Ojis many times larger than their ionic resistance. At 30 °C / GB is several orders of magnitude larger than (the ionic resistance) and for 100 pm-thick CPE foils or Lil-A Oj pellets it reaches [125] 105-106Qcm2 (depending on CPE composition). 

A value of Rqb for an SEI lOnm thick can be estimated from its values for CPE and CSE by assuming that these solid electrolytes consist of nanometer-sized particles. Thus the expected value for / GB at 30 °C for a lOnm SEI is in the range 10-lOOQcm2, i.e., it cannot be neglected. In some cases it may be larger than the ionic (bulk) resistance of the SEI. This calculation leads us to the conclusion that 7 GB and CGB must be included in the equivalent circuits of the SEI, for both metallic lithium and for LixC6 electrodes. The equivalent circuit for a mosaic-type... 

Fabrication techniques, especially the preparation of thin films of functional materials, have made major progress in recent years. Thin-film solid electrolytes in the range of several nanometers up to several micrometers have been prepared successfully. The most important reason for the development of thin-film electrolytes is the reduction in the ionic resistance, but there is also the advantage of the formation of amorphous materials with stoichiometries which cannot be achieved by conventional techniques of forming crystalline compounds. It has often been observed that thin-film electrolytes produced by vacuum evaporation or sputtering provide a struc-... 

Ionic transport in solid electrolytes and electrodes may also be treated by the statistical process of successive jumps between the various accessible sites of the lattice. For random motion in a three-dimensional isotropic crystal, the diffusivity is related to the jump distance r and the jump frequency v by [3] ... 

This relationship makes it possible to calculate the maximum ionic conductivity of solid electrolytes. Assuming that the mobile ions are moving with thermal velocity v without resting and oscillating at any lattice site, this results in a jump frequency... 

Generally, solid electrolytes for battery applications require high ionic conductivities and wide ranges of appropriate thermodynamic stability. 

Though solid electrolytes for multivalent ions offer the advantage of a larger charge transfer, their conductivities are much lower than those of monovalent ions at ambient temperature because of a higher activation enthalpy for the ionic motion... 

Another way of looking at high ionic conductivities of solid electrolytes is to consider the activation enthalpy as illustrated in Fig. 8. Generally, the activation enthalpy is strongly correlated with the room-temperature ionic conductivity the higher the room-temperature ionic conductivity, the lower the activation enthalpy. The straight lines in the Arrhenius... 

Traditionally, the chemical stability of the electrode/electrolyte interface and its electronic properties have not been given as much consideration as structural aspects of solid electrolytes, in spite of the fact that the proper operation of a battery often depends more on the interface than on the solid electrolyte. Because of the high ionic conductivity in the electrolyte and the high electronic conductivity in the electrode, the voltage falls completely within a very narrow region at the electrolyte/electrode interface. 

Improvement of the ionic current by fast transport in the electrodes. High electronic mobility and low electronic concentration favor fast chemical diffusion in electrodes by building up high internal electric fields [14]. This effect enhances the diffusion of ions toward and away from the solid electrolyte and allows the establishment of high current densities for the battery. 

Thin-film solid electrolytes in the range of lpm have the advantage that the material which is inactive for energy storage is minimized and the resistance of the solid electrolyte film is drastically decreased for geometrical reasons. This allows the application of a large variety of solid electrolytes which exhibit quite poor ionic conductivity but high thermodynamic stability. The most important thin-film preparation methods for solid electrolytes are briefly summarized below. 

The substitution of the two-sided tape with a film of an ionic conductor gives (Fig. 24) a triple-layered muscle working in air.114 The tape now acts as a solid electrolyte. Nevertheless, the system only works if the relative humidity in air surpasses 60%. Under these conditions, movements and rates similar to those shown by a triple layer working in aqueous solution were obtained. This device was developed in cooperation with Dr. M. A. De Paoli from the Campinnas University (Campinnas, Brazil). At the moment several groups are developing actuators, muscles, and electrochemomechanical devices based on bilayer or multilayer structures.115-125... 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

Chapter 3 discusses solid electrolytes and some of their early applications in fuel cells and catalysis. This material is quite familiar to the solid state ionics community but may be helpful to surface scientists, aqueous electrochemists and chemical reaction engineers. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Detailed information about the conductivity of solid electrolytes can be found elsewhere.2,3,6 8,10,11 As shown in Fig. 3.1, the temperature dependence of the ionic conductivity o can, in general, be described by the semiempirical equation ... 

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