Lithium Conduction in the Perovskite Structure

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- 

By considering the relative sizes of the ions in the structure and the necessity of maintaining cation-anion contacts it can be seen that the ideal cubic structure will form if the ionic radii are such that the tolerance factor, t, is equal to unity  

The reason that perovskites are so widespread is that the structure is capable of accommodating deviations from ideal ion sizes. By undergoing one of an array of distortions the structure is capable of adjusting the coordination environment in order to provide the bond lengths necessary to stabilise cations that are other than the optimal size. These distortions can involve tilting of the BOg octahedra around various axes of 

The structure is named after the mineral perovskite, calcium titanate, that contains Ti in the octahedrally coordinated positions and in the large, 12-coordinate site in the centre of the unit cell. Lithium conductivity has been reported in closely related compounds that replace Ca with various combinations of Li , La and cation vacancies that maintain an overall divalent charge per formula unit. The 

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 

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Good results are obtained with oxide-coated valve metals as anode materials. These electrically conducting ceramic coatings of p-conducting spinel-ferrite (e.g., cobalt, nickel and lithium ferrites) have very low consumption rates. Lithium ferrite has proved particularly effective because it possesses excellent adhesion on titanium and niobium [26]. In addition, doping the perovskite structure with monovalent lithium ions provides good electrical conductivity for anodic reactions. Anodes produced in this way are distributed under the trade name Lida [27]. The consumption rate in seawater is given as 10 g A ar and in fresh water is... 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

Lithium can be inserted into the material up to at least 0.08 Li" " per formula unit. This level of intercalation is insufficient for the number of lithium and lanthanum cations to exceed unity and so the A sites of the perovskite structure still contain some vacancies at this stoichiometry. Whilst this intercalation process is reversible, experiments using this electrolyte in conjunction with a graphite electrode show that an irreversible oxidation process occurs. The reduction of Ti" " narrows the band gap and leads to electronic conductivity of 0.01 S cm at room temperature. This reactivity and electronic conduction would lead to a rapid discharge via short circuit of a stored battery and so makes these materials unsuitable for use as an lithium electrolyte in these applications. 

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]. 

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. 

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