1. Field of the Invention
This invention relates to layered inorganic compounds and, more particularly, to transition metal dichalcogenides and to intercalation complexes thereof.
2. Brief Description of the Prior Art
Certain transition metal dichalcogenides, notably titanium disulphide, have been proposed for use as active cathodic materials in electrochemical cells in view of the ease with which they may be intercalated, and de-intercalated, with ions derived from the active anodic material.
The physical effect of intercalation on the crystal structure of certain transition metal dichalcogenides has been reported in a paper by R. R. Chianelli in the Journal of Crystal Growth 34(1976) 239-244 and by M. S. Whittingham in the Journal of the Electrochemical Society 12, No. 3, (1976) 315-320. Both authors visualise intercalation to involve the entry of intercalating ions between the layers of the dichalcogenide structure accompanied by an increase in the spacing between the layers. In the case of titanium disulphide intercalated with lithium ions or with n-butyl lithium ions, Whittingham, in the above identified paper, reports x-ray diffraction measurements as indicating an increase of about 10% in the hexagonal lattice parameter on the c-axis. Chianelli also reports a lattice expansion of titanium disulphide intercalated with lithium of approximately 10%.
It may be anticipated that repeated expansion and contraction of the transition metal dichalcogenide crystal lattice during repeated intercalation and de-intercalation could cause disruption of the lattice. Such disruption could result in a decrease in the efficiency of the system in which the dichalcogenide is to be utilised. Chianelli reports observing cracking of crystals of titanium disulphide during intercalation and theorises that this may be due to the existence of twinning faults in the crystals.
Despite the physical stresses to which a crystal lattice is subjected during intercalation and deintercalation and the tendency of the crystals to crack, certain transition metal dichalcogenides, notably titanium disulphide, tend to retain their efficiency in use in an electrochemical cell relatively well. It is reported however, in U.S. Pat. No. 4,007,055 that the mobility of ions between the layers of titanium disulphide may be drastically lowered by the presence of interstitial titanium between the sulphur layers and that the presence of this interstitial titanium may be due to crystal imperfections due in turn to the crystal growth conditions. It is clear, therefore, that both crystal morphology and the avoidance of interstitial titanium are important properties in titanium disulphide intended for use as an active cathodic material.