Density Functional Theory Study on Aqueous Aluminum−Fluoride Complexes: Exploration of the Intrinsic Relationship between Water-Exchange Rate Constants and Structural Parameters for Monomer Aluminum Complexes

Density functional theory (DFT) calculation is carried out to investigate the structures, 19F and 27Al NMR chemical shifts of aqueous Al−F complexes and their water-exchange reactions. The following investigations are performed in this paper: (1) the microscopic properties of typical aqueous Al−F complexes are obtained at the level of B3LYP/6-311+G**. AlOH2 bond lengths increase with F− replacing inner-sphere H2O progressively, indicating labilizing effect of F− ligand. The Al−OH2 distance trans to fluoride is longer than other AlOH2 distance, accounting for trans effect of F− ligand. 19F and 27Al NMR chemical shifts are calculated using GIAO method at the HF/6-311+G** level relative to F(H2O)6− and Al(H2O)63+ references, respectively. The results are consistent with available experimental values; (2) the dissociative (D) activated mechanism is observed by modeling water-exchange reaction for [Al(H2O)6-iFi](3−i)+ (i = 1−4). The activation energy barriers are found to decrease with increasing F− substitution, which is in line with experimental rate constants (kex). The log kex of AlF3(H2O)30 and AlF4(H2O)2− are predicted by three ways. The results indicate that the correlation between log kex and AlO bond length as well as the given transmission coefficient allows experimental rate constants to be predicted, whereas the correlation between log kex and activation free energy is poor; (3) the environmental significance of this work is elucidated by the extension toward three fields, that is, polyaluminum system, monomer Al-organic system and other metal ions system with high charge-to-radius ratio

Dual Design on Hierarchically Hollow MoTe₂/C with Ion/Electron Channel Engineering for High-Performance Sodium Storage

Transition-metal tellurides have been investigated as novel anode materials for application in sodium-ion batteries (SIBs) due to their rich active sites and unique and controllable layered nanostructures. However, the weak structural strength and inferior intercalation/deintercalation kinetics inhibit the development of transition-metal tellurides. In this work, MoTe2/C composites with two different hollow nanostructures are designed and prepared. By adjustment of the precursor structure, MoTe2/C-2 exhibits superior sodium-storage performance because of its uniquely hollow nanostructure with self-assembled 2D flexible nanosheets grown on the external surface. MoTe2/C-2 delivers a higher specific capacity (276 mAh g–1 at 0.1 A g–1 after 300 cycles), much more than MoTe2/C-1 (201 mAh g–1 at 0.1 A g–1 after 300 cycles), and exhibits a long-time cycling performance (131 mAh g–1 at 1 A g–1 after 2000 cycles). The excellent sodium-storage performance derived from the rational structure design is beneficial for shortening the ion paths, facilitating the sodiation/desodiation process, and reinforcing the intrinsic structural stability, thus boosting the reaction kinetics and prolonging the cycling life. Meanwhile, the assembled full-cell maintains 101 mAh g–1 at 0.1 A g–1 after 50 cycles and lights an electric watch. The findings provide several new views for preparation of more transition-metal tellurides with multi-ion/electron migration channel engineering