Open-Structured Oxyfluorinated Titanium Phosphate Nanosheets Synthesized with the Assistance of an Ionic Liquid and Their Use as an Anode in Lithium-Ion Batteries

Two-dimensional Ti2(PO4)2F nanosheets were synthesized with an ionic liquid (IL) containing PF6 ([Bmim]PF6) via a hydrothermal method. In comparison, only anatase titanium dioxide (TiO2) nanoparticles were obtained when changing the type of anion ([Bmim]Br or [Bmim]BF4) under the same conditions, suggesting that the anion type could greatly affect the crystalline phase, particle size, specific surface, and morphology of the final products; two-dimensional nanosheets were obtained in [Bmim]PF6, whereas nanocrystals with a relatively large surface were formed in the latter two ILs, as revealed by X-ray diffraction, nitrogen gas sorption, scanning electron microscopy, and transmission electron microscopy measurements. 1H and 19F nuclear magnetic resonance examinations of the fresh and recycled ILs suggest that the IL with the PF6 anion experienced complete hydrolysis, which could be the key factor for the formation of such unique Ti2(PO4)2F nanosheets. The electrochemical performances of such Ti2(PO4)2F nanosheets were evaluated as anode materials in lithium-ion batteries (LIBs) in comparison to TiO2 nanomaterials. The rate capability and cycling performance of the Ti2(PO4)2F electrode outperformed the other two electrodes studied, as illustrated by cyclic voltammetry curves, charge/discharge behavior, and long cycling test, which is mainly due its unique layered structure and low surface area, which resulted in low volume change. The Ti2(PO4)2F electrode derived from [Bmim]PF6 displays a steady reversible capacity of 170 mAh/g and high cycling stability with over 70% capacity retention after 100 cycles. The synthesis of the controllable crystalline phase and morphology of Ti2(PO4)2F nanomaterials with such ILs paves a useful route to prepare such alternative anode materials for LIBs.