Micro-length anodic porous niobium oxide for lithium-ion thin film battery applications

The anodization of niobium in an aqueous mixture of H₃PO ₄ and HF in the potential range from 2.5 to 30 V for 2 h at 5 C was performed, demonstrating that anodic porous niobium oxide film with a thickness of up to 2000 nm, including a surface dissolution layer, can be obtained by controlling the applied potential and composition of the electrolytes. Specifically, surface dissolution-free porous niobium oxide film with a thickness of 800 nm can be prepared in a low electrolyte concentration. The surface dissolution is observed when the concentration ratio of HF (wt.%):H ₃PO₄ (M) was more than 2:1. The discontinuous layers in the niobium oxide film were observed when the thickness was higher than 500 nm, which was ascribed to the large volume expansion of the niobium oxide grown from the niobium metal. The anodic porous niobium oxide film was used as the cathode for lithium-ion batteries in the potential range from 1.2 to 3.0 V at a current density of 7.28 × 10 -⁶ A cm^{- 2}. The first discharge capacity of ca. 53 μA h cm^{- 2} was obtained in 800 nm thick niobium oxide without a surface dissolution layer.