A study of the mechanism of cobaltocene intercalation in Cd₂P₂S₆

Important details of the mechanism of cobaltocene [CoCp₂ (Cp = C₅H₅)] intercalation into single crystals of the layered trichalcogenophosphate Cd₂P₂S₆ were deduced. These details were obtained experimentally by the intercalation of lithium, ruthenocene (RuCp₂) and ferrocene (FeCp₂) into partially intercalated Cd₂P₂S₆ (CoCp₂)_x (0.01 ≤ x ≤ 0.69) and the subsequent analysis of these compounds by Fourier-transform infrared spectroscopy, mass spectroscopy and chemical analysis. Results indicate that intercalation is initiated by the oxidation of cobaltocene at the crystal edges, followed by the movement of cobaltocenium ions into the crystal, with the outermost galleries of the host filling most rapidily. Propagation of the intercalation front proceeds by the diffusion of neutral cobaltocene from the crystal edge, through the intercalated area to the reaction front, where it further reduces the Cd₂P₂S₆ crystal. Additional experiments indicate that it is possible to lithium-intercalate a crystal which was previously non-receptive to the reaction by first intercalating it with an easily oxidized species such as cobaltocene. Potential applications include the use of trichalcogenophosphate crystals pre-intercalated with cobaltocene as cathode materials in solid-state lithium batteries. Such a route could lead to improvements in the energy density of existing cathode materials. Another possibility is the use of CoCp₂ as a pillaring agent in the cathode material to reduce the swelling during intercalation.