Micro-length anodic porous niobium oxide for lithium-ion thin film battery applications Jeong Eun Yoo, Jiyoung Park, Gihoon Cha, Jinsub Choi ⁎ Department of Chemical Engineering, Inha University, Inchon, 402-751, Republic of Korea abstract article info Article history: Received 3 February 2012 Received in revised form 4 January 2013 Accepted 7 January 2013 Available online 31 January 2013 Keywords: Anodic films Niobium oxide Nanoporous Thin film battery The anodization of niobium in an aqueous mixture of H 3 PO 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 com- position 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 3 PO 4 (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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Niobium pentoxide (Nb 2 O 5 ) is a white n-type transition metal oxide semiconductor with a wide bandgap (3.2–4 eV) and a large di- electric constant (~ 40) [1,2]. Due to its attractive physical properties, Nb 2 O 5 has been considered for use in fiber optics, gas sensors, cata- lysts, biocompatible materials, electrolytic capacitors, and dye sensi- tized solar cells [1–8]. Kodama et al. suggested that niobium oxide is one of the few cathode candidates for 2 V rechargeable lithium-ion batteries [9]. Le Viet et al. investigated the charge/discharge behavior of lithium-ion batteries based on niobium oxide with different crystal structures and found that the tetragonal and monoclinic structural niobium oxide demonstrated a high discharge capacity and good cy- cling stability [2]. Wei et al. reported that niobium oxide nanobelts demonstrate a high reversible charge/discharge capacity, high rate ca- pability, and excellent cycling stability for lithium-ion batteries [10]. The niobium oxide was primarily prepared using sol–gel, hydro- thermal synthesis and anodization [11–16]. Because anodization is a simple method of controlling the thickness and pore density, there have been many studies that have focused on the production of vari- ous niobium oxide films via anodization [14–17]. As reported in previ- ous articles, anodic niobium oxide films with a thickness of 200 nm and a pore diameter of 20 nm can be routinely prepared [14,17]. How- ever, the preparation of thicker porous niobium oxide in aqueous elec- trolytes has not been achieved because the surface of the porous niobium oxide is easily dissolved as a result of the attack of F - during long anodization times. In order to avoid the surface dissolution, pre-annealing or adding surfactant to the electrolyte were undertaken [18,19]. Even though the anodization of niobium in a non-aqueous electrolyte at high temperature produced 28 μm thick porous niobium oxide, the water content in the electrolyte should be controlled [15]. To date, anodic porous niobium oxide film has not been studied for the electrode of lithium-ion batteries. In this article, the electro- chemical conditions required to produce a niobium oxide film with a thickness of up to 2000 nm, which until now has not be achieved in an aqueous electrolyte, will be discussed. In addition, the prepared niobium oxides with different thicknesses with or without the surface dissolution are studied for electrodes in lithium-ion batteries. Note that porous type oxide should be produced in order to obtain thicker anodic film by anodization. Because the anodic nanoporous niobium oxide films prepared in this work do not require binding and conduc- tive agents for application in lithium-ion batteries, it is expected that it will provide a simple and routine process to prepare 2 V recharge- able thin film lithium-ion batteries. 2. Experimental details 2.1. Synthesis and characterization of niobium oxide High purity Nb foils with a thickness of 0.25 mm (99.9%; Goodfellow, England) were used as a starting material for fabricating the porous ni- obium oxide film. The Nb foils were washed in acetone, ethanol and de- ionized water (≥18 MΩ) with an ultrasonicator and were subsequently Thin Solid Films 531 (2013) 583–587 ⁎ Corresponding author. Tel.: +82 32 860 7476; fax: +82 32 866 0587. E-mail address: jinsub@inha.ac.kr (J. Choi). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.01.062 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf