Hollow single-crystalline octahedra of hydrated/dehydrated hydroxyl ferric phosphate and crystal-water-enhanced electrochemical properties of the hydrated sample for reversible lithiation–delithiation

The solution-based preparation of a polyanionic inorganic compound sometimes introduces structural water into its chemical composition; however, to date, the positive impact of this structural water on the electrochemical properties of the hydrated sample is still ambiguous. Herein, hollow single-crystalline octahedra of hydrated hydroxyl ferric phosphate (Fe-5(PO4)center dot(OH)(3)center dot 2H(2)O; edge length: similar to 1.0 mu m; a known orthorhombic phase) and its dehydrated counterpart (Fe-5(PO4)4 center dot OH)(3); edge length: similar to 0.9 mu m; a fitting orthorhombic phase) were uniformly prepared for the first time, facilitating comparative studies on their structural and electrochemical properties. As a lithium-ion battery cathode within 1.5-4.5 V vs. Li+/Li, the hydrated sample can deliver the initial discharge capacity of 176.6 mA h g(-1) at 5 mA g(-1), which is close to the theoretical value of 180.0 mA h g(-1). By comparison, regardless of the charge-discharge current rate, the reversible capacity of the hydrated microcrystallites in each cycle was much higher than that of the dehydrated counterparts. In other words, the 1,2-propanediol solvent mainly determines the hydro-/solvothermal formation of Fe-5(PO4)(4)center dot OH)(3)center dot 2H(2)O hollow single-crystalline octahedra, and it is the presence of crystal water that modifies the cell parameters of the orthorhombic lattice and promotes the reversible lithiation-delithiation capability of microcrystallites.