Effect of Cobalt on Structural and Electrochemical Properties of the TiSe2 System and Its Sodiation

This work focuses on investigating the phase equilibria in NanCoxTiSe2 (0 <= n <= 3; x = 0.1, 0.2, 0.25, 0.33, 0.4, and 0.5) systems. For this purpose, TiSe2, Co0.1TiSe2, and Co0.33TiSe2 compounds were synthesized with subsequent electrochemical intercalation of sodium. The obtained compounds were investigated by electrochemical (coulometric titration), scanning electron microscopy, and diffraction methods (ex situ and in situ/operando X-ray diffraction). By utilizing electrochemical and X-ray data, this study successfully determined the boundaries of structural phases, thereby significantly enhancing the comprehension of the NanTiSe2 system. The investigation revealed that the introduction of cobalt through preintercalation into the TiSe2 matrix leads to notable simplification in the phase regions of the system. Specifically, the NanTiSe2 system exhibited five single-phase regions, while NanCo0.1TiSe2 and NanCo0.33TiSe2 exhibited two single-phase regions each. This effect is associated with stabilization of the trigonal crystal structure of the host lattice due to the presence of a strong covalent bond between Co and Ti within the host lattice. Additionally, the effective sodium capacity of the studied systems was observed to be 236 mA h g(-1) (Na2.3TiSe2), 172 mA h g(-1) (Na1.6Co0.1TiSe2), and 197 mA h g(-1) (Na2.0Co0.33TiSe2).