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

Abstract

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 (Fe₅(PO₄)₄(OH)₃·2H₂O; edge length: ∼1.0 μm; a known orthorhombic phase) and its dehydrated counterpart (Fe₅(PO₄)₄(OH)₃; edge length: ∼0.9 μm; 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 Fe₅(PO₄)₄(OH)₃·2H₂O 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.