Applicable Solid Electrolytes for Batteries

Making use of Eq. (25), the maximum conductivity of a solid electrolyte with monovalent mobile species is given by 

The experimental value for Agl is 1.97 Q cm [16, 3], which indicates that the silver ions in Agl are mobile with nearly a thermal velocity. Considerably higher ionic transport rates are even possible in electrodes, by chemical diffusion under the influence of internal electric fields. For Ag2S at 200 °C, a chemical diffusion coefficient of 0.4cm s , which is as high as in gases, has been measured [17]. 

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

Currently, there is great interest in the application of solid electrolytes for high-performance secondary lithium batteries... 

Some further important aspects for the design of solid electrolytes and solid electrodes for battery-type applications are the following ... 

For the application of solid electrolytes in batteries it is essential to characterize the materials carefully. These measurements may be easily misleading and are often incorrectly interpreted. It is therefore vital that the correct arrangements and procedures are applied. 

The use of solid electrolytes in batteries and fuel cells is another important application. Examples are zirconia based fuel cells and sulphur batteries with Na-/ -A1203 as electrolyte. Many other interesting and practical aspects of solid electrolytes are worth mentioning, for example, the possibility to detect stresses, to build up high pressures, or to monitor mass accelerations. Also, solid electrolytes have recently been used to investigate the interface kinetics in crystals (Section 10.4.2). 

Because all electrochemical devices such as batteries, fuel cells, sensors, and electrochromics require an electrolyte, the potential applications for ionic conductors are enormous. In addition to these more conventional applications, solid electrolyte materials are investigated for use as electrochemical memory devices, oxygen pumps, gas phase electrolyzers, and thermoelectric generators. ... 

Solid electrolytes for lithium-ion batteries are expected to offer several advantages over traditional, nonaqueous liquid electrolytes. A solid electrolyte would give a longer shelf life, along with an enhancement in specific energy density. A solid electrolyte may also eliminate the need for a distinct separator material, such as the polypropylene or polyethylene microporous separators commonly used in contemporary liquid electrolyte-based batteries. Solid electrolytes are also desirable over liquid electrolytes in certain specialty applications where bulk lithium-ion batteries as weU as thin-film lithium-ion batteries are needed for primary and backup power supplies for systems, devices, and individual integrated circuit chips. 

One of the most important practical applications of lithium compounds is as fast ion conductors with potential electronic applications such as solid electrolytes for lithium batteries. Li20 is a fast ion conductor in which the Li ions occupy a simple cubic sublattice with the antifluorite structure. Both MAS and static Li NMR spectra of Li20 have been reported, the former recorded as a function of temperature up to 1000 K (Xie et al. 1995). The effect of introducing vacancies on the Li sites by doping with LiF has been studied by high-temperature static Li NMR, which reveals the interaction of the Li defects > 600 K and the appearance of 2 distinct quadrupolar interactions at about 900 K. Measurements of the relative intensities of the satellite peaks as a function of temperature have provided evidence of thermal dissociation of an impurity-vacancy complex (Xie et al. 1995). 

Appreciable ionic conductivity is found in open framework or layered materials containing mobile cations see Ionic Conductor. Several phosphates have been found to be good ionic conductors and are described above NASICON (Section 5.2.1), Q -zirconium phosphates (Section 5.3.1), HUP (Section 5.3.3), and phosphate glasses (Section 5.4). Current interest in lithium ion-conducting electrolytes for battery applications has led to many lithium-containing phosphate glasses and crystalline solids such as NASICON type titanium phosphate being studied. 

It is now well established that solvent-free films can be cast from solutions of polyethers (such as poly(ethylene oxide)) and alkali metal salts, and that these films can display high ionic conductivity. Most of the effort devoted to this field has been based on the potential of such materials as solid-state electrolytes for battery applications. In this context, from viewpoints of both ionic mobility and weight, lithium salts in PEO have attracted the most intensive research and appear to offer the most promise such materials are discussed elsewhere. The preparation of materials displaying both electronic and ionic conductivity raises interesting possibilities both in the field of batteries and sensors and is beginning to attract attention (16). 

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