Lithium-ion conductors

Figure 3. Schematic representation of the lithium-ion conductor LiAICl4. The A1C14 may be considered as tetrahedral anions, as indicated by green. The lithium ions are located between them.

The structure of the perovskite-type lithium ion conductor Li0 29La0 57Ti03 is represented in Fig. 6. The small gray circles depict the lithium ions, the big gray circles the lanthanum ions. These are randomly distributed over the A sites 14 per-... 

Figure 6. Structure of the perovskite-type lithium-ion conductor Li 2yLa057TiO3. The lithium ions (small, gray) and the lanthanum ions (large, gray) are randomly distributed over the A sites, of which 14 percent are vacancies, enabling the lithium ions to be mobile. Titanium forms TiOh octahedra, as shown in yellow. The unit cell is indicated.

In practice, for a ternary system, the decomposition voltage of the solid electrolyte may be readily measured with the help of a galvanic cell which makes use of the solid electrolyte under investigation and the adjacent equilibrium phase in the phase diagram as an electrode. A convenient technique is the formation of these phases electrochemically by decomposition of the electrolyte. The sample is polarized between a reversible electrode and an inert electrode such as Pt or Mo in the case of a lithium ion conductor, in the same direction as in polarization experiments. The... 

Since the realization in the early 1980s that poly (ethylene oxide) could serve as a lithium-ion conductor in lithium batteries, there has been continued interest in polymer electrolyte batteries. Conceptually, the electrolyte layer could be made very thin (5im ) and so provide higher energy density. Fauteux et al. [31] have recently reviewed the present state of polymer elec-... 

At this time the only commercially available all-solid-state cell is the lithium battery containing Lil as the electrolyte. Many types of solid lithium ion conductors including inorganic crystalline and glassy materials as well as polymer electrolytes have been proposed as separators in lithium batteries. These are described in the previous chapters. A suitable solid electrolyte for lithium batteries should have the properties... 

Difluorides such as PbF2 with the fluorite structure exhibit fast ion conduction due to facile F ion transport (Section 6.4.5). An interesting structure showing Li" conduction is that of LijN (Rabenau, 1978). Conduction is two-dimensional. Cooperative basal plane excitations involving the rotation of six Li ions by 30 about a common ion to edge positions (positions midway between ions in the Li2N layer) seem to be responsible for conduction in this nitride. In the fluorite structure, a rotation by 45 of a single cube of F ions seems to be involved. The Zintl alloy LiAl is also a lithium-ion conductor. 

For using lithium batteries (which generally have high energy densities) under extreme conditions, more durable and better conducting electrolytes are necessary. Salt-in-polymer electrolytes discovered by Angell et al. (1993) seem to provide the answer. Polypropylene oxide or polyethylene oxide is dissolved in low melting point mixtures of lithium salts to obtain rubbery materials which are excellent lithium-ion conductors at ambient temperatures. 

Table 2 Conductivity characteristics of selected lithium ion conductors ...

Following a period of intense research on NASICON (Na3Zr2Si2POi2) three-dimensional sodium-ion conductors, research interests drifted toward identifying similar LlSlCON-type three-dimensional lithium-ion conductors. A direct analogue compound (Li3Zr2Si2PO,2) was a stable phase, but exhibited poor ionic conductivity." Researchers also analyzed the ionic conductivity of various... 

Irvine, J. and West, A., Crystalline lithium ion conductors, in High Conductivity Solid Ionic Conductors, T. Takahashi, Ed., World Scientific, Singapore, 1989. 

Thangadurai, V., Shukla, A., and Gopalakrishnan, J., New lithium-ion conductors based on the NASICON structure, J. Mater. Chem., 9, 739, 1999. 

Lithium ion conductors are very much desired in commercial applications because of the relatively high open circuit voltages (up to 4 V) that can be achieved in electrochemical devices employing lithium-based anodes with high chemical activities (or chemical potentials). Many of the polycrystalline lithium-based solid electrolytes, that have been studied to date have ionic resistivities at 300°C in the range between 20 and 200 fl cm. While thin-film applications for these materials are possible, the biggest drawback associated with lithium ion conductors is their chemical and electrochemical instability over time at temperatures of interest in environments very high in lithium chemical activity. 

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