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

Abstract

This work focuses on investigating the phase equilibria in Na n Co x TiSe 2 (0 ≤ n ≤ 3; x = 0.1, 0.2, 0.25, 0.33, 0.4, and 0.5) systems. For this purpose, TiSe 2 , Co 0.1 TiSe 2 , and Co 0.33 TiSe 2 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 Na n TiSe 2 system. The investigation revealed that the introduction of cobalt through preintercalation into the TiSe 2 matrix leads to notable simplification in the phase regions of the system. Specifically, the Na n TiSe 2 system exhibited five single-phase regions, while Na n Co 0.1 TiSe 2 and Na n Co 0.33 TiSe 2 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 (Na 2.3 TiSe 2 ), 172 mA h g −1 (Na 1.6 Co 0.1 TiSe 2 ), and 197 mA h g −1 (Na 2.0 Co 0.33 TiSe 2 ).