Cubic and octahedral Cu 2 O nanostructures as anodes for lithium-ion batteries Min-Cheol Kim, Si-Jin Kim, Sang-Beom Han, Da-Hee Kwak, Eui-Tak Hwang, Da-Mi Kim, Gyu-Ho Lee, Hui-Seon Choe and Kyung-Won Park * Well-defined nanostructured electrodes are known to have improved lithium ion reaction properties for lithium-ion batteries. Herein, we prepared shape-controlled Cu 2 O nanostructures as an anode material using ascorbic acid as a reducing agent with and without polyvinylpyrrolidone (PVP) as a surfactant. Using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction methods, we observed that the sample prepared in the absence of PVP exhibited cubes with dominant {100} facets, whereas octahedral Cu 2 O nanostructures with dominant {111} facets were formed in the presence of PVP. During the charge–discharge process, an octahedron-shaped Cu 2 O nanostructured electrode having {111} facets favourable for lithium ion transport revealed an enhanced conversion reaction mechanism with high reversible capacity and high rate cycling performance, due to its low charge transfer resistance and high lithium ion diffusion coefficient. 1. Introduction Lithium ion batteries (LIBs) are excellent power sources for portable devices and transportation applications because of their high energy density, high power density, and long oper- ating life. 1–4 In particular, the development of high performance LIBs requires a remarkably increased capacity of electrode materials. 5–8 In LIBs, the cathodes such as LiCoO 2 , LiMn 2 O 4 , and LiMnPO 4 consist of transition metal oxides containing lithium sources, which show a higher voltage and lower capacity than the anodes. On the other hand, carbon-based anode materials having a relatively higher initial capacity and lower voltage than the cathodes have been typically utilized in lithium ion batteries as anodes. 9–13 Recently, transition metal oxides, such as Cu 2 O, NiO, SnO 2 , and TiO 2 , have been regarded as alternative anodes for carbon-based materials with a low production cost, high abundance, non-toxicity, and high capacity 14,15 among diverse transition metal oxides. In the lith- iation/delithiation process with Cu 2 O as an anode, the Cu nanocrystals that form upon lithiation are dispersed in a Li 2 O matrix and restored to the oxide upon delithiation. However, during the charge/discharge process, the volume expansion of the electrode can result in a reduced electrochemical perfor- mance in LIBs. To address this issue, Cu 2 O nanostructured electrodes with controlled shapes or other complex conducting phases have been prepared using advanced synthesis methods. 16–19 Shape-controlled nanostructures have various facets that can determine the physical and chemical properties of the as- prepared electrodes for LIBs. 20–22 Many studies have reported that electrode nanomaterials with specic morphologies exhibit excellent electrochemical performance in LIBs. 23–31 Herein, we synthesized Cu 2 O nanostructured anodes with different morphologies by adopting a reducing agent with and without polyvinylpyrrolidone (PVP) as a surfactant. The electrochemical performances of cubic and octahedral Cu 2 O with {100} and {111} facets, respectively, were studied as anode materials for LIBs. The structural analysis of the as-prepared samples was performed using eld-emission transmission electron micros- copy (FE-TEM), eld-emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD). To evaluate the perfor- mance of the samples in LIBs, the charge/discharge curves, cyclic voltammograms (CVs), and electrochemical impedance spectra of the electrodes were measured using lithium coin cells. 2. Experimental 2.1. Synthesis of octahedral and cubic Cu 2 O anodes For an octahedral Cu 2 O sample, 9 g of PVP (Aldrich, M w ¼ 55 000) as a surfactant was dissolved in 500 mL of 0.01 M CuCl 2 $2H 2 O (Aldrich, 99%) with constant stirring at 30  C for 30 min (Fig. 1(a)). 50 mL of 2 M NaOH aqueous solution was then added to the solution mixture (Fig. 1(b)). Aer stirring for 30 min, 50 mL of 0.6 M ascorbic acid solution was added to the solution mixture which was maintained for 3 h (Fig. 1(c)). The resulting precipitate was separated from the solution by centrifugation and washed several times with distilled water Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea. E-mail: kwpark@ssu.ac.kr Cite this: J. Mater. Chem. A, 2015, 3, 23003 Received 17th July 2015 Accepted 6th October 2015 DOI: 10.1039/c5ta05455a www.rsc.org/MaterialsA This journal is © The Royal Society of Chemistry 2015 J. Mater. Chem. A, 2015, 3, 23003–23010 | 23003 Journal of Materials Chemistry A PAPER Published on 07 October 2015. Downloaded by SUNGSIL UNIVERSITY on 11/11/2015 13:12:26. View Article Online View Journal | View Issue