Conductor, lithium cation structures

This structural model suggests that the limited lithium mobility that does occur in these compounds proceeds via a percolation pathway for lithium migration. This path is composed of portions of the material that contain either lithium cations or vacancies in the large interstitial void. The presence of a lanthanide cation in 55% of the subcells prevents the occupation of the immediately adjacent tetrahedrally coordinated lithium positions and so blocks the passage of Li. Thus the lithium conductivity in these structures will be limited first by the ability of lithium cations to exit the tetrahedrally coordinated site and secondly, by the availability of an empty interstitial site in an adjacent subcell. Given these limitations on ion movement through the structure, it would be surprising if these materials were fast lithium conductors. 

Ti,Nb)-0, and La-deficient La2-02 layers (Fig. 6.7(a)). Two-dimensional lithium cation conduction has also been reported in the orthorhombic layered perovskite-type compound Lao.62Lio.i6Ti03 [55], in which the Li cation exists and migrates only near the La-deficient La2-02 layer. This work has thus revealed that oxide ion diffusion in an ionic conductor with a double perovskite structure is two dimensional. 

In the case of NiO which is p-type metal-deficient material we have a structure with Ni2+, O2- and Ni3+ in nickel vacancy sites. Addition of lithium ion impurity to NiO makes it a p-type conductor and oxide growth rate is reduced compared with the Ni-02 system alone. On the other hand addition of Cr to Ni results in greater ease of oxidation than nickel alone due to higher cation diffusivity. 

Tlie aim of this chapter is to provide an overview of materials where fast transport of alkali metal cations and protons is observed. A general discussion of factors affecting conductivity and techniques used to study ion migration paths is followed by a review of the large number of cation conductors. Materials with large alkali ions (Na-Cs) are often isostructural and therefore examined as a group. Tire lithium conductors with unique crystal structure types and proton conductors with unique conduction mechanisms are also discussed. 

By far the largest class of ternary lithium nitrides are those with the antifluorite structure, prepared largely by Juza, and reviewed by him [42], The ternaries are more thermally stable than the binaries which would seem to indicate a relaxation of the internal strain (both cation- cation and anion-anion repulsions) of binary nitrides. Many of these compounds are in fact ordered superstructures of antifluorite and it is remarkable that most of the transition metals are observed in high oxidation states--much higher than those in the binaries (e.g. Li7Mn + N4 vs Mn5 + N2). They are Li+ conductors at elevated... 

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. 

La2/3Ti03 is a perovskite-type oxide in which one third of the A-site cations is deficient. When lithium is partially substituted for La in A sites of this oxide, lithium ions become mobile [46]. The lithium ion conductivity of Lao.51Lio.34TiO2.94 is about 10 S cm at room temperature [47]. This value belongs to the highest among lithium ion conductors that are chemically stable in an atmospheric environment. As La and Li ions are randomly distributed in the A-site position in the perovskite-type structure and, therefore, A-site vacancies are also distributed randomly, it is considered that the lithium ions can easily move through the vacancies. The relationship between the conductivity and content of lithium ions obeys so-called percolation theory [48]. 

Single-ion conductors can be obtained by the intercalation of PEO on clay due to the presence of cation charge at the silicate surface. The conductivity values of electrolytes based on POEM with the addition of 2 and 5 wt% clay were found to be around 4 x 10 S/cm at 70 °CF The conductivity obtained can be anisotropic. Molecular dynamic simulation has shown that the Li" ions are solvated preferentially by the silicate oxygen atom rather than PEO. The conductivity is too low for practical applications, even with a cationic transference number equal to one. In order to increase conductivity, but with a cationic transference number different from one, lithium salts were added to PEO/clay nanocomposites. At room temperature, the nanocomposite electrolyte exhibited higher ionic conductivity than unfilled polymer due to the larger content of the PEO amorphous phase. The improvement in conductivity depends on the nature of the clay. Fan et al. have shown that 250-Li-MMT, i.e. Li-MMT heated to 250°C, was more effective in enhancing the conductivity of (PE0)i6LiC104 than Org-MMT, dodecylamine modified Li-MMT, and Li-MMT, since 250-Li-MMT forms an exfoliated structure in the PEO matrix. 

---