KINETIC AND STRUCTURAL CHARACTERISTICS OF 3R-NIOBIUM DISULFIDE AS A POSITIVE MATERIAL FOR SECONDARY LITHIUM BATTERIES

The kinetic and thermodynamic studies of the intercalation process of lithium ions in the layered structure of niobium disulfide with a 3R-type modification were made. The standard free energy of lithium intercalation into the van der Waals gaps, DELTA-G(I)0, was -218 kJ mol-1 at 298 K in the range of x = 0-1 in Li(x)NbS2. Applying the thermodynamic model by Nagelberg and Worrell [Nagelberg and Worrell, J. Solid St. Chem. 38, 321 (1981)] the interaction energy between the intercalated lithium ions was estimated as a model parameter. The chemical diffusion coefficients, D approximately, of lithium in the disulfide were measured as functions of the lithium composition and temperature by using several kinds of electrolytes and an ac impedance method. The obtained values of D approximately, based on a geometrical surface area, were dependent on the lithium composition and the kind of electrolytes. The values of D approximately obtained in 1 M LiClO4 and 1 M LiAsF6 solutions of propylene carbonate were found to be of the order of 10(-8) cm2 s-1 at 298 K for 0.2 < x < 0.6 in Li(x)NbS2, corresponding to a lithium self-diffusion coefficient of 10(-9) cm2 s-1 order, while in the 1 M LiBF4 solution a considerably lower values of D approximately, 10(-10)-10(-11) cm2 s-1, were obtained for 0.1 < x < 0.5. The activation energies for the diffusion of lithium in the range of 20-30 kJ mol-1 in the LiClO4 and LiAsF6 solutions, and in the range of 80-90 kJ mol-1 in the LiBF4 solution; the values in the former electrolytes are typical of diffusion in layered insertion materials, whereas in the latter electrolyte the kinetics may be controlled by the surface layer.