Tandem molecular intercalation exfoliated TiSe2 nanosheets for enhanced sodium-ion storage

Although layered transition metal dichalcogenides (TMDs) possess relatively high electrical and moderate ionic conductivities, their inclination to stack layered structure inevitably leads to a sluggish ion diffusion and limited cyclic life when used as anode materials. Therefore, the realization of few-layered structure with widened layer distance would facilitate the ion migration and thus help TMDs to gain enhanced performance. In this study, tandem molecular intercalation method was applied to fabricate few-layered TiSe2 nanosheets (ef-TiSe2) with pronounced expanded layer spacing (∼10 Å) using propylamine and hexylamine as intercalators. Density functional theory calculations disclose that there is a continuous charge accumulation between the TiSe2 interlayers, which enables Na+ to exhibit a strong adsorption tendency, thus forming a certain Coulombic force. Furthermore, the lowest Na+ diffusion barrier between interlayers guarantees TiSe2 to achieve rapid reaction kinetics. The effective exfoliation strategy renders the obtained ef-TiSe2 rich exposed reaction active sites, sufficient electrolyte penetration, shortened diffusion distance of ion/electron, suppressed stress caused by structural changes, and increased effective utilization of the material. Interestingly, compared with bulk TiSe2, few-layered ef-TiSe2 nanosheets exhibited a shorter activation cycling period, lower reaction polarization, and expedited reaction kinetics, therefore providing a high specific capacity (417 mA h g−1 at 1 A g−1) and cyclic stability (with a high-capacity retention of 95.36 % after 2000 cycles at 5 A g−1). In addition, the combined mechanism of intercalation and conversion in TiSe2 was closely elucidated by in situ X-ray diffraction and in situ electrochemical impedance spectroscopy.