High Li storage capacity of poorly crystalline porous d-MnO 2 prepared by hydrothermal route Prasant Kumar Nayak, Tirupathi Rao Penki, N. Munichandraiah ⇑ Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India article info Article history: Received 1 August 2012 Received in revised form 9 May 2013 Accepted 18 May 2013 Available online 13 June 2013 Keywords: Porous manganese dioxide Poor crystallinity Morphology High capacity Cathode material abstract Poorly crystalline porous d-MnO 2 is synthesized by hydrothermal route from a neutral aqueous solution of KMnO 4 at 180 °C and the reaction time of 24 h. The as-synthesized sample and also the sample heated at 300 °C have nanopetals morphology with large surface area. On heating at temperatures P400 °C, there is a decrease in BET surface area and also a change in morphology from nanopetals to clusters of nanorods. Furthermore, the poorly crystalline d-MnO 2 converts into well crystalline a-MnO 2 phase. The electrochemical lithium intercalation and de-intercalation studies in a non-aqueous electrolyte pro- vide a high discharge specific capacity (275 mAh g 1 ) at a specific current of 40 mA g 1 for the poorly crystalline d-MnO 2 samples. The rate capability is also high. There is a decrease in capacity on repeated charge–discharge cycling. The specific capacity values of the crystalline a-MnO 2 samples are considerably less than the values of poorly crystalline d-MnO 2 samples. Thus, the hydrothermal route facilitates prep- aration of poorly crystalline electrochemically active porous MnO 2 . Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction There has been an increasing interest on rechargeable Li-ion batteries as power sources for various applications because of their high working voltage, high energy density and good cyclability. Various intercalation compounds such as LiCoO 2 , LiMn 2 O 4 and LiFePO 4 are studied extensively as cathode materials for Li-ion bat- teries [1–6]. MnO 2 is investigated as a cathode material because of its diverse structures (tunnels or layers), which facilitate easy transportation of Li-ions. On the basis of Mn 4+ /Mn 3+ redox reaction, a theoretical specific capacity of 308 mAh g 1 is expected from MnO 2 . Also, it is inexpensive, non-toxic, easy to synthesize, envi- ronmentally compatible, and its resources are plentiful. MnO 2 exists in various crystallographic forms such as a, b, c, d, and k, depending on how MnO 6 octahedra are inter-linked. The electrochemical performance of different crystallographic forms of MnO 2 as cathode materials in Li-ion batteries was reviewed by Thackeray [7]. a-MnO 2 with 2  2 channels provided an initial dis- charge capacity of about 200 mAh g 1 [8–10]. Mesoporous b-MnO 2 provided a stable discharge capacity of about 150 mAh g 1 [11,12]. A crystalline macroporous b-MnO 2 synthesized by hydrothermal route exhibited a stable discharge capacity of 150 mAh g 1 at a specific current of 10 mA g 1 [13]. Layered d-MnO 2 synthesized by decomposition of KMnO 4 at 300 °C provided a specific capacity of 130–140 mAh g 1 for 20 cycles [14]. A high capacity of 375 mAh g 1 was obtained in the first cycle in the voltage range of 1.0–4.8 V for d-MnO 2 nanobelts synthesized by hydrothermal method [15]. However, a capacity loss of 85 mAh g 1 was observed in the second cycle [15]. Recently, d-MnO 2 prepared by hydrothermal method delivered a discharge specific capacity of 144 mAh g 1 in the potential range of 2.0–4.0 V [16]. For achieving a good performance, the cathode materials of Li-ion cells need to possess well ordered crystalline phase [17–19]. However, amorphous Li 1.5 Na 0.5 MnO 2.85 I 0.12 delivered a discharge capacity of 260 mAh g 1 [20]. Xu et al. reported a specific capacity of 278 mAh g 1 for amorphous MnO 2 [21]. A discharge specific capac- ity of 270 mAh g 1 was reported by Liu et al. for electrochemically prepared amorphous hydrous MnO 2 [22]. Synthesis of MnO 2 is known by several routes, which include hydrothermal [23,24], co-precipitation [25–27], sol–gel [28,29], electrochemical methods [20,30–34] and reduction of KMnO 4 by various reducing agents [28,35,36]. Hydrothermal technique is useful for synthesizing various nano-structured metal oxides [37,38]. In this technique, an aqueous solution containing the reac- tants is subjected to a temperature usually between 100 and 200 °C under auto-generated pressure in a sealed container. The advan- tage with the hydrothermal method is that the particle character- istics such as morphology and particle size can be controlled by controlling the reaction temperature and time. This is due to the rapidly changing properties of water with temperature and pres- sure [3]. From the study of above literature, it is known that hydrother- mal route is versatile for the synthesis of Li-ion battery electrode 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.05.016 ⇑ Corresponding author. Tel.: +91 80 22933183. E-mail address: muni@ipc.iisc.ernet.in (N. Munichandraiah). Journal of Electroanalytical Chemistry 703 (2013) 126–134 Contents lists available at SciVerse ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem