Synthesis, Characterization And Research On The Electrochenmical Properties Of The Nanoparticles Of Titanium Mixed-anion Oxyfluorides And The Lithiated Product

In recent years, research on lithium-ion batteries and sodium-ion batteries have gain great improvement in practical application because of the advantages such as practicability, cycling ability and the high energy density etc. The discovery of new electrode materials, as well as the nanocrystallization of exist electrode materials, plays a significant role in the development of lithium-ion batteries and sodium-ion batteries. Metal oxyfluorides, which have been studied and applied as functional materials in the field of optics, electrochemistry, electrochromism and catalysis/photocatalysis and so on, are more and more attractive as electrode materials for lithium-ion batteries and sodium-ion batteries. Compared with metal oxides, metal oxyfluorides possess advantages such as high specific capacity, higher discharge plateau and cycling stability. TiOF2 (space group:Pm-3m) was studied as anode material and the lithium intercalation/de-intercalation mechanism was revealed recently. However, the application of TiOF2 as cathode material is restricted and the disadvantages such as low specific capacity and dissatisfactory cycling stability still existed. To solve these problems structurally, we synthesized hexagonal TiOF2 (space group:R-3c) material with a nanocube morphology and LiTiOF2 (space group:R3c), and the electrochemical properties were studied in detail, which further revealed lithium intercalation/de-intercalation mechanism and the same frame of hexagonal TiOF2 and LiTiOF2. The structural details of LiTiOF2 were first studied by using Rietveld refinement of the XRD pattern. The primary contents of this paper are:1. Two different metal oxyfluorides were synthesized via a facile one-step solvothermal method using TiF4, deionized water and ethanol as reagents:(1) hexagonal TiOF2 nanocubes, and (2) TiOo.9(OH)0.9F1.2·0.59H2O nanoparticles with the hexagonal tungsten bronze structure (space group:P63mmc) and different morphologies. In addition, pure LiTiOF2 nanocubes were obtained by using the lithium-liquid ammonia method (lithiation of hexagonal TiOF2). According to the results of our survey and investigation of research papers, this was the first synthesis and report of hexagonal TiOF2 nanoparticles, TiOo.9(OH)0.9F1.2·0.59H2O (titanium oxyhydroxy-fluorides) nanoparticles and pure LiTiOF2. By analyzing the Rietveld refinement of the XRD result of LiTiOF2, we found that the structure refinement based on LiNbO3 model converged very well (Rp=6.35%, Rwp=8.24%), the detailed features of the LiTiOF2 structure were further investigated and discussed. We discussed the controllability of the structure and morphology of the products by adjusting the ratio among the precursors and the reaction time, and by analyzing the results of XRD and SEM; the reaction mechanism and growth process of the products were also investigated by analyzing the contents of the left reagents; the structure and morphology were confirmed again by analyzing the results of TEM and HRTEM. The same frame of hexagonal TiOF2 and LiTiOF2, the purity of the products were confirmed by Rietveld refinement of the XRD pattern and TGA, respectively.2. The electrochemical properties of hexagonal TiOF2 were tested by using galvanostatic method. The half-cells were assembled by using hexagonal TiOF2 as cathode, lithium metal as counter and reference electrode. Hexagonal TiOF2 exhibited a plateau which started at 2.6 V and end at 0.8 V, the plateau was broader than cubic TiOF2. Hexagonal TiOF2 exhibited a stabilized discharge capacity of 200 mAh g-1 at 1 C after 500 cycles without observed fading; a discharge capacity of 120 mAh g-1 was displayed at 20 C after 1000 cycles without observed fading, and the discharge capacity was still above 100 mAh g-1 after 2000 cycles. All the aforementioned electrochemical properties were only attributed to lithium intercalation/de-intercalation reaction while no converting reaction occurred and the host structure was maintained. This was a great improvement compared with the electrochemical properties of cubic TiOF2 (cubic TiOF2 exhibited the best capacity of 100 mAh g-1 at 0.1-1 C). In consideration of the XRD results of the original electrode slices and cycled electrode slices and the test results of half-cells (the galvanostatic test results of hexagonal TiOF2, cubic TiOF2 and LiTiOF2), the mechanism of irreversible capacity and electrochemical activation process which was first reported for TiOF2 were analyzed and confirmed. And above all, the lithium intercalation/de-intercalation mechanism of hexagonal TiOF2, the reaction phenomena, the electrochemical properties and the corresponding relationship among them were studied and discussed by analyzing the ex situ XRD results of hexagonal TiOF2 electrode slices and using LiTiOF2 as a contrast.3. It is aforementioned that, TiO0.9(OH)0.9F1.2·0.59H2O nanoparticles has the hexagonal tungsten bronze structure, and there is one-dimension channels in the structure which can support lithium intercalation/de-intercalation and sodium intercalation/de-intercalation. So it is considered that TiOo.9(OH)0.9F1.2·0.59H2O nanoparticles can been used as electrode material for both lithium-ion batteries and sodium-ion batteries. Hexagonal TiOF2 is not suitable because of the insufficient place for sodium ions. We studied the controllable synthesis of the products with specific structure and morphology by using different ratios between deionized water and ethanol; the structures, morphologies and chemical composition of the products were characterized by XRD, HRTEM and corresponding FFT, ICP, ion chromatography, EDX and so on; we also tested the half-cells using TiO0.9(OH)0.9F1.2·0.59H2O nanoparticles as cathode and lithium metal/sodium metal as counter and reference electrode. For lithium-ion batteries, the discharge plateau of TiO0.9(OH)0.9F1.2·0.59H2O started at 2.5 V and end at 1.2 V; the stabilized specific capacity of 200,170,150,140 mAh g-l was exhibited at 25,50,125,250 mA g-1 after 100-500 cycles, respectively. For sodium-ion batteries, the discharge plateau started at 1.4 V and end at 0.5 V; the specific capacity of 100 mAh g-1 was exhibited after 115 cycles at 25 mA g-1; a stabilized capacity of 90 mAh g-1 was exhibited at 125 mA g-1.