Fast lithium ion conduction

FIGURE 12.5 (a) Fast lithium-ion conducting phase of Li3xLa2/3 ,Ti03 (0.07 grain boundary) ionic conductivity derived by artificial neural network (ANN). 

O. Bohnke, The Fast Lithium-Ion Conducting Oxides Li3x-La2/3 xTi03 from Fundamentals to Application , Solid State Ionics, 2008,179, 9. 

The compounds Li3Cr2(P04)3 and Li3Fe2(P04)3 both show the same sequence of structural transitions as Li3Sc2(P04)3. These compounds form orthorhombic fast lithium ion conducting phases above 265 °C and 312 °C respectively and show conductivities ca 10 S cm at these temperatures. These compounds contain trivalent chromium and iron and so are susceptible to reduction. Whilst this is associated with an increase in the electronic conductivity that would render these materials unsuitable for use as an electrolyte in lithium batteries it does offer potential for use as an electrode material for lithium storage. The related compound Li3V2(P04)3 has been studied as a possible intercalation host for lithium cations based on the redox couple and transforma-... 

This structural model has features which can be seen to favour fast lithium ion conductivity, especially when compared with the relatively poorly conducting Nd and Pr analogues that contain tetrahedrally coordinated lithium. The presence of Li in pseudo square-planar coordination in Lio.5Lao.5Ti03 provides a large aperture for ion migration movement of the Li perpendicular to the plane defined by the four... 

Bohnke O (2008) The fast lithium-ion conducting oxides Li3 La2/3-j Ti03 from fundamentals to application. Solid State Ionics 179 9-15... 

Thangadurai V, Kaack H, Weppner WJP (2003) Novel fast lithium ion conduction in garnet-type Ll5La3M20i2 (M = Nb, Ta). J Am Ceram Soc 86 4374-40... 

Thangadurai V, Weppner W (2005) Investigations on electrical conductivity and chemical compatibility between fast lithium ion conducting gamet-like Li4BaLa2Ta20i2 and lithium battery cathodes. J Power Sourc 142 339-344... 

Awaka J, Takashima A, Kataoka K, Kijima N, Idemoto Y, Akimoto J (2011) Crystal structure of fast lithium-ion-conducting cubic LiyLa3Zr20i2. Chem Lett 40 60-62... 

Table 1. Conductivities, activation enthalpies, and other aspects of fast lithium-ion solid conductors...

The NASICON structure is capable of accommodating considerable compositional variation and a large number of related compounds have been studied in order to try an improve the lithium ion conducting properties. There have been two distinct approaches to this. One approach has been to try and reduce the temperature of the structural transition and reduce the barrier to ion mobility and so access a compound that shows fast lithium conductivity under ambient conditions. An alternative strategy is to adjust the number of mobile cations and vacant sites in order to increase the conductivity of the cations in the higher temperature, disordered phase. Both approaches have had considerable success in both illuminating the mechanism for ion motion in the structure and in changing the physical properties towards those of a useful fast ion conductor. 

The presence of lithium on the large interstitial site in lithium lanthanum titanate perovskites gives rise to exceptional ionic mobility. The lithium conductivity in this system can be as high as 10 S cm at room temperature, i.e. several orders of magnitude higher than many other fast lithium ion conductors. However, it must be noted that this is the value of conductivity within a crystallite and the presence of grain... 

The material must have high electronic conductivity and high lithium ion conductivity to facilitate fast charge transferring and then deliver a high power density. 

Quite a large variety of interesting fast lithium-ion solid conductors is now known, as compiled in Figure 19.9 and Table 19.1. In the case of sodium-and potassium-ion conductors only the /l/)3"-alumina family, and for sodium the NASICON structure, were considered for practical application, due to the high ionic conductivities of these materials which are unmatched by any other sodium- or potassium-ion conductor. However, for 73/73"-alumina, NASICON and structurally related ionic conductors, the ionic conductivities and activation enthalpies are... 

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. 

Table 6.5 D.c. conductivity activation barriers in the fast ion conducting lithium iodide-lithium thioborate glasses (After Burckhardt et al., 1984).

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