Electrochemical cointercalation of propylene carbonate with alkali metals in SnS2 single crystals

The cathodic reduction of SnS2 single crystals in Li and Na cells was studied by step potential electrochemical spectroscopy, using propylene carbonate (PC) as the organic electrolyte. The potential-composition curves obtained at the beginning of the discharge process exhibit an extended plateau that is resolved as an asymmetric and a symmetric reduction peak for Li and Na, respectively, in the corresponding voltammograms. The unusual shapes of the current relaxation curves suggest that the electrochemical reduction is a multiphase process that results in the formation of new phases with basal spacings of about 18 Angstrom (12 Angstrom greater than that of the host). Based on the elemental analysis and thermogravimetric and X-ray photoelectron spectroscopy results, these compounds are interpreted as ternary phases of the formula A(x)(PC)(0.5)SnS2 (A = Li, Na) where the alkali ions are solvated by PC molecules via oxygen atoms. No peaks suggestive of reversible solvent cointercalation were observed in the oxidation process. Owing to this cointercalation reaction, the capacity of the cells fades on cycling. Preliminary investigations by atomic-force microscopy revealed that the intercalation reaction clearly deteriorates the crystal surface. The well-ordered two-dimensional hexagonal array of the surface of the pristine compound, which is preserved when SnS2 crystals are exposed to air or immersed in PC for several days, was virtually completely lost after the electrochemical reduction process. (C) 1999 The Electrochemical Society. S0013-4651(98)05-087-3. All rights reserved.