Tioso4

The synthesis of KTiPO₄F was carried out by a hydrothermal route using a 50 ml PTFE reactor with a steel shell. Initial reagents were put in the reactor in the following weight proportion: 0.12 g of titanium powder (Ti, Sigma-Aldrich >99.9%), 0.40 g of titanyl sulfate (TiOSO₄, Sigma-Aldrich >99.9%), 2.04 g of monobasic potassium phosphate (KH₂PO₄, Sigma-Aldrich >99.9%), 0.38 g of potassium difluoride (KHF₂, Acros Organics > 99.9%), 30 g of deionized water. The reactor was maintained at 200 °C for 3 h under constant magnetic stirring and then air-cooled to room temperature. The resulting violet-colored powder was centrifuged with deionized water several times and then with acetone and dried under vacuum at room temperature. The resulting KTiPO₄F powder was stored under argon atmosphere (pO₂ < 0.1 ppm, pH₂O < 0.1 ppm). Hydrothermally prepared KTiPO₄F was carbon-coated using a solution of polyvinyl cyanide (C₃H₃N)_n in DMFA (Sigma-Aldrich, extra-pure) casted on the initial powder and then dried under Ar to yield the KTiPO₄F/C composite by further annealing at 600 °C for 2 h (3 K min⁻¹ heating rate) in the highly-dried and O₂-purified Ar atmosphere (titanium powder was used as an oxygen absorber).

Al@TiO₂ was synthesized using an ‘in situ water-shift’ method (See Full Method in Supplementary Methods). Specifically, 0.05 g TiOSO₄ (reagent grade, Sigma-Aldrich) and 3.0 g H₂SO₄ (ACS grade, 1.0 N, VWR) were dissolved in 100 ml deionized water. Then, 0.135 g Al powder (∼50 nm in diameter, 99.9%, US Research Nanomaterials, Inc.) was added to the saturated TiOSO₄ solution. After the 30 min vigorous agitation using an ultrasound cleaner (Symphony, VMR), the solution was stirred for 3.0–10.0 h until the colour changed from grey to light. Then the resultant solution was filtrated in a vacuum system and washed three times by ethanol. After drying at 70 °C for 7.0 h in a vacuum oven (Symphony, VMR), the sample was annealed at 450 °C for 1.0 h in an Ar filled quartz tube furnace (Lindberg Blue M, Thermo Scientific). Finally, the sample was collected for the characterization and battery test.

For the hydrothermal synthesis of TiO₂ nanoparticles, titanium (IV) oxysulfate hydrate (TiOSO₄, Sigma Aldrich, Saint-Quentin-Fallavier, France) and titanium tetrachloride (TiCL₄, Fluka, Saint-Quentin-Fallavier, France) precursors were used. All the chemicals were analytical grade, and used without further purification. The TiOSO₄ precursor solution was prepared by dissolving 6.4 g of TiOSO₄ (2.5 M) in 16 mL of distilled water (Milli Q System, electric resistivity 18.2 MΩ·cm, Merck, St Quentin en Yvelines, France) under constant stirring at 750 r/min and a temperature of 45 °C for 2 h to get a clear solution. Then, the solution of TiOSO₄ was transferred into a Teflon-lined stainless steel autoclave with a 25 mL capacity. The heating rate was 2.5 °C/min. During synthesis, the temperature was maintained at different temperatures—100, 150, 180, 190, 200 and 220 °C—for 6 h, depending on the aggregate size required. After synthesis in autoclave, a white TiO₂ powder was obtained, then washed 6 times in distilled water and 2 times in ethanol. Finally, the obtained powder was dried overnight in the oven and annealed in air at 500 °C for 30 min with the heating rate at 5 °C/min.