Inorganic solid electrolytes

The conductivity of solid salts and oxides was first investigated by M. Faraday in 1833. It was not yet known at that time that the nature of conduction in solid salts is different from that in metals. A number of fundamental studies were performed between 1914 and 1927 by Carl Tubandt in Germany and from 1923 onward by Abram Ioffe and co-workers in Russia. These studies demonstrated that a mechanism of ionic migration in the lattice over macroscopic distances is involved. It was shown that during current flow in such a solid electrolyte, electrochemical changes obeying Faraday s laws occur at the metal-electrolyte interface. 

FIGURE 8.2 Conductivities of a number of solid electrolytes as functions of temperature (dashed lines the conductivities of 4 M H2SO4 and 8 M KOH solution). 

AU these features—low values of a, a strong temperature dependence, and the effect of impurities—are reminiscent of the behavior of p- and n-type semiconductors. By analogy, we can consider these compounds as ionic semiconductors with intrinsic or impurity-type conduction. As a rule (although not always), ionic semiconductors have unipolar conduction, due to ions of one sign. Thus, in compounds AgBr, PbCl2, and others, the cation transport number is close to unity. In the mixed oxide ZrOj-nYjOj, pure 0 anion conduction t = 1) is observed. 

In an ideal ionic crystal, all ions are held rigidly in the lattice sites, where they perform only thermal vibratory motion. Transfer of an ion between sites under the effect of electrostatic fields (migration) or concentration gradients (diffusion) is not possible in such a crystal. Initially, therefore, the phenomenon of ionic conduction in solid ionic crystals was not understood. 

Yakov Frenkel showed in 1926 that ideal crystals could not exist at temperatures above the absolute zero. Part of the ions leave their sites under the effect of thermaf vibrations and are accommodated in the interstitial space, leaving vacancies at the sites formerly taken up. Such point defects have been named Frenkel defects. These ideas were developed further by Walter Schottky in 1929, who pointed out that defects will also arise when individual ions or ion pairs are removed from the bulk 

---

Fig. 11.7 Schematic diagram of an all-solid state lithium-air battery using lithium anode, an inorganic solid electrolyte, and an air electrode composed of carbon nanotubes and solid electrolyte particles. Reprinted with permission from Hirokazu Kitaura etai, Energy Environ. Sci., 2012, 5,...

The structures and charge transport mechanisms for polymer electrolytes differ greatly from those of inorganic solid electrolytes, therefore the purpose of this chapter is to describe the general nature of polymer electrolytes. We shall see that most of the research on new polymer electrolytes has been guided by the principle that ion transport is strongly dependent on local motion of the polymer (segmental motion) in the vicinity of the ion. 

Suspension polymerization occurs in water with the liquid monomer dispersed by agitaliorL The polymer is produced as a dispersed solid phase fiom polymerization of initiator-containing, 10 to 500 pm droplets under kinetics that match those of the bulk reaction of the monomer (7). The suspension is stabilized by insoluble organic or inorganic solids, electrolytes to increase monomer-water interfacial tension, and water soluble polymers that increase aqueous viscosity. Suspension polymerization is commonly used to synthesize two polymers covered in this book, polystyrene and polyvinyl chloride. 

Another advantage is the improved safety of the battery because there is no risk of liquid electrolyte leaking and because of high thermal stability attributed to nonflammable and inorganic solid electrolytes. 

Much attention has been paid to a variety of inorganic solid electrolytes (Li7P3Sn [18] etc.) and its application to all-solid-state lithium-ion batteries. Since the transference number of the inorganic solid electrolyte is almost unity, the lithium-ion conductivity of the solid electrolyte is almost comparable to that of organic liquid electrolyte. However, in spite of the presence of highly lithium-ion conductive solid electrolytes, the all-solid-state batteries had not provided sufficient power densities until recendy. One of the critical reasons for the limited power density was due to the large lithium-ion transfer resistance at the interface between cathode and solid electrolyte. 

Inorganic solid electrolytes have been studied in two types of solids crystal and glass [1, 2]. Glass ion conductors have several advantages high conductivity based on so-called open structure... 

Much attention is now focused on whether or not solid electrolytes can be commercialized. Inorganic solid electrolytes have achieved ionic conductivity equivalent to current liquid electrolyte solutions. If such solid electrolytes can be commercialized, they are expected to revolutionize LIB electrode structure and battery characteristics. 

Cao C, Li Z-B, Wang X-L et al (2014) Recent advances in inorganic solid electrolytes for lithium batteries. Eront Energy Res 25 1-10... 

Though the conductivity of the PEO-alkali metal complexes (10 ohm -cm at 140°C) is fairly low in comparison with inorganic solid electrolytes such as 8-alumina(Na), RbCui6l7Cli3 and RbAg4ls at the same temperature, this can be compensated for by the facile production of thin films typically 25-500 pm thick. A cell may thus consist of a lithium or lithium-based foil as anode, an alkali metal salt-PEO complex, such as (PEO)9 LiCFsSOs (25-50 pm), as the electrolyte, and a composite cathode (50-75 pm) containing a vanadium oxide (VeOis) as the active ingredient. The vanadium oxide is one of a number of insertion compounds that permits the physical insertion lithium ions reversibly into their structure and thus allows recharging of the cell. 

Other inorganic solid electrolytes have also been employed in electrochemical capacitor applications, including hydrogen uranyl phosphate, (H3OUO2PO4) and Zr(HP04)2 xH20 (proton conductors) (65), Li and Na /9-alumina (66), LiNaSO (67). Clearly, these systems will only be satisfactory for relatively slow applications. 

The ideal inorganic solid electrolyte materials for lithium-ion batteries should meet the following conditions ... 

In terms of their practical application, the chemical stability of inorganic solid electrolytes, with both the positive and negative electrode materials, is very important besides the required high ionic conductivity. 

Another unique and interesting property of polymer electrolytes, which is not obtained with inorganic solid electrolytes, is the ability to include various kinds of electroactive molecules in them by dissolution or by chemical modification. The combination of this property with their high ionic conductivity will enable us to use polymer electrolytes as media for electrochemical reactions of electroactive molecules, just as we use ordinary electrolyte solutions for this purpose. 

---