The hydrothermal synthesis of new oxide materials

The development of advanced secondary lithium batteries depends on cathode structures that can reversibly intercalate lithium ions. The initial prototypical system Li/TiS2 showed the feasibility of high energy density systems with extensive reversibility. However, with the advent of the safe lithium-carbon anodes by Sony there is a need for higher voltage cathodes. LiCoO2 presently fulfills this need in small cells. However, for large systems where cost is an issue new oxides are needed. One approach to the formation of such oxide is low temperature hydrothermal synthesis. Mild hydrothermal reactions lead to the formation of new metastable transition metal oxide structures, not accessible by conventional high temperature methods, which have relatively open crystal structures. The nature of the cations present in solution (the "templating ion") has a dramatic effect on the crystal structure of the phase formed, as also does the pH of the reaction medium and the particular transition metal. Thus, by appropriate choice of reaction medium new structures containing large tunnels or channels, similar to those found in aluminosilicate zeolites, can be formed that will offer unique properties for the Materials Scientist. In particular, it is expected to be possible to realize enhanced diffusion in such materials. Here, the hydrothermal synthesis of tungsten, molybdenum and vanadium oxides is considered. The role of the cation in the synthesis is described, as well as the key interfacial reactions that are taking place. Cases are described where the cation controls the structure formed and is retained in the structure, to instances where the cation critically controls the reactions occurring but is not retained in the lattice.