Journal of Nanoparticle Research

, Volume 12, Issue 1, pp 301–305

A novel method for preparing lithium manganese oxide nanorods from nanorod precursor

Authors

    • Key Laboratory of Catalysis and Materials Science of Hubei Province, College of Chemistry and Materials ScienceSouth-Central University for Nationalities
  • Long Tan
    • Key Laboratory of Catalysis and Materials Science of Hubei Province, College of Chemistry and Materials ScienceSouth-Central University for Nationalities
Research Paper

DOI: 10.1007/s11051-009-9614-1

Cite this article as:
Liu, H. & Tan, L. J Nanopart Res (2010) 12: 301. doi:10.1007/s11051-009-9614-1

Abstract

Lithium manganese oxide nanorods were prepared from manganese dioxide nanorods precursor. The structure and morphology were confirmed by X-ray diffraction (XRD) and transmission electron microscope (TEM). The data of the Rietveld refinement indicate that the nanorods preferentially grow along the [111] direction. After charge–discharge test at 1.0 mA cm−2 in 3.0–4.4 V, the nanorods LiMn2O4 showed the 134.5 mAh g−1 initial discharge capacity and only lost 1.1% of initial capacity after 30 cycles, which is better than that of bulk particles LiMn2O4 prepared by traditional solid-state reaction method. This effective and simple route to synthesis nanorods LiMn2O4 from one-dimensional (1D) precursor could also be extended to prepare 1D other nanomaterials with special electrochemical properties.

Keywords

Lithium-ion batteriesNanorodsMnO2LiMn2O4Cycleability

Introduction

Over the past several decades, the spinel LiMn2O4 has been extensively studied as the most promising positive electrode material to replace LiCoO2 for lithium-ion batteries because of several advantages such as low cost, less toxicity, easy preparation, safe to handle, etc. (Blyr et al. 1998; Liu et al. 2007; Lim 2008). Nevertheless, the reversible capacity and the cycling stability of this compound have to be improved for the Jahn–Teller distortion, manganese dissolution, and decomposition of the electrolyte (Thackeray 1997; Amatucci et al. 1997).

Recently, nanostructured LiMn2O4 cathode shows better electrochemical properties than conventional electrodes because the distance over which Li+ diffuse is decreased dramatically (Kang and Goodenough 2000; Kang et al. 2001). Furthermore, the larger surface area of the nanostructured electrode makes higher experiment capacity (Che et al. 1998; Li et al. 2000). Among various nanomaterials with different morphologies, one dimensional (1D) nanostructures (nanotubes, nanowires, and nanorods) LiMn2O4 show a great advantage over that of other morphologies (Li et al. 2000; Nishizawa et al. 1997). One-dimensional nanostructure offers the shortest distance and the largest surface for Li+ transport in the solid state. In addition, by controlling the dimensions of the nanostructres, the unwanted oxidation of water during the charge process can be eliminated and good cyclability can be obtained (Li and Dahn 1995).

Much effort, such as the template method (Nishizawa et al. 1997), hydrothermal solid-state synthetic method (Li et al. 2000), a self-seeded, surfactant-directed growth process (Zhang and Yu 2003), has then been devoted to developing new approaches to prepare 1D LiMn2O4 cathodes. However, to the best of our knowledge, the synthesis of a 1D nanostructure of LiMn2O4 with nanorods structure has not been reported to date.

We here for the first time report a novel method for preparing lithium manganese oxide nanorods from nanorod precursor, which may provide the possibility of detecting the theoretical capacity limits of LiMn2O4 with the smallest dimension structures, and also could be extended to prepare other 1D nanomaterials with special electrochemical properties.

Experiments

The starting material of nanorods MnO2 used in this work can be prepared as described previously (Xun and Yadong 2002). LiOH · H2O with analytical grade and the as-prepared nanorods MnO2 in quantities corresponding to 0.1 mol stoichiometric LiMn2O4 were mixed thoroughly in a mortar and heated at 700 °C for 8 h, followed by air cooling to room temperature to obtain the final products. As a contrast to nanorods LiMn2O4 in this work, the bulk particles LiMn2O4 prepared by traditional solid-state reaction method were also characterized.

Phase identification and evaluation of lattice parameters of the product were carried out by powder X-ray diffraction (XRD, Bruker D8-advance, Germany). The XRD patterns were collected by steps of 0.02° in the range of 10° ≤ 2θ ≤ 70° with a constant counting time of 0.1 s per step at room temperature, and the Rietveld analysis used the TOPS R refinement software. The morphology was observed with a transmission electron microscope (TEM, FEI, TECNAI G220 S-Twin, America). The simulate cells were assembled by using lithium foil as anode and reference electrode in an argon–filled glove box, the as-prepared powders mixed with 12% acetylene black and 8% polytetrafluoroethylene (PTFE) as the cathode and 1 M LiPF6 in a 1: 1(V/V) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte, Celgard 2300 membrane as the cell separator. The charge–discharge cycle was performed at a constant current density of 1.0 mA/cm2 in a potential range of 3.0 and 4.4 V using the simulate cells. All the electrical measurements were carried out by a battery testing system (BTS-5 V/3A, Neware technology limited corporation, China) at room temperature.

Results and discussion

Figure 1 shows the powder XRD patterns of the as-prepared MnO2 and the prepared LiMn2O4 in this work. The diffraction peaks of the prepared MnO2 can be indexed in a tetragonal structure with a space group of I4/m (JCPDS44-0141) with the lattice parameter a = 12.0788 Å, c = 2.1915 Å. The a value is larger than that (9.7847) of the report (Fang et al. 2008), which showed that MnO2 might grow in a certain direction. The diffraction peaks of two LiMn2O4 powders are very similar to each other and characteristics of the good spinel structure of LiMn2O4 (space group Fd3m). No impurity peaks are observed. The X-ray diffraction profile of the nanorods LiMn2O4 is shown in Fig. 2. The refinement terminated with a convincible Rwp value (3.603). The cell parameter obtained from Rietveld refinement is a = 8.2308Å. The observed and calculated patterns match very well. It gives clear evidence for the smallest half width (0.042°) and the largest size (9999.9 Å) for the [111] reflection than the others, which indicates that the nanorods preferentially grow along the [111] direction.
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Fig. 1

XRD patterns of the as-prepared MnO2 and the prepared LiMn2O4 with different method

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Fig. 2

The Rietveld refinement of the nanorods LiMn2O4

The structure and morphologies of nanorods manganese oxides and LiMn2O4 were further examined by transmission electron microscopy. As shown in Fig. 3a, nanorods were clearly observed throughout the sample. No particles were found in the TEM images, confirming the high yield of nanorods in the sample. The diameters of the MnO2 rods were about 90 nm and the lengths were mostly more than 1.2 μm. The diameters of the LiMn2O4 nanorods in Fig. 3b were also about 90 nm while the lengths were diverse in the 0.1–1 μm range. These results were in agreement with the XRD data.
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Fig. 3

Typical TEM images of a the MnO2 precursor and b the nanorods LiMn2O4

The initial discharge/discharge curves of the prepared LiMn2O4 as cathode material of lithium-ion batteries are shown in Fig. 4. It can be seen obviously that there are two distinct potential plateaus at about 4.0 and 4.2 V, which is a typical characteristic of a well-defined LiMn2O4 spinel and indicates that the material underwent two distinct reversible oxidation and reduction processes. The initial charge–discharge capacity of the bulk particles LiMn2O4 reached 132.87 mAh g−1. However, the nanorods LiMn2O4 show the higher discharge capacity (134.5 mAh g−1) and the better efficiency than that of bulk particles, which can be ascribed to the shorter diffusion distances and the larger surface area of the nanorod morphology (Nishizawa et al. 1997). The experimental capacity obtained depends on the distance and the surface over which Li+ must diffuse. The smallest diameter is very convenient for Li+ diffusion, and the largest surface can offer the most of Li ions, which leads to the highest discharge capacity (Li et al. 2000).
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Fig. 4

The first charge–discharge curves of the prepared LiMn2O4 with different method in 3.0–4.4 V at a constant current density of 1.0 mA cm−2

The discharge capacity versus cycle number curves of the prepared LiMn2O4 at a constant charge–discharge density of 1.0 mA cm−2 is shown in Fig. 5. The discharge capacity of the bulk particles LiMn2O4 reached to 120.35 mAh g−1 after 30 cycles. The loss of capacity was about 9.43%. However, the capacity of the nanorods LiMn2O4 became relatively stable in the further cycles and finally reached a high capacity of 133.4 mAh g−1 after the 30th cycle. The discharge capacity decreased by 1.1% of the initial value. The cycle performance of nanorods electrode was much better than that of the bulk particles LiMn2O4. It might be ascribe to the shortest Li+ diffusion distance which is more convenient for the inserting and deinserting of Li+ ions, which was also speculated in the literature as well (Patzke et al. 2002). This also represents another unique advantage of the 1D nanostructured electrode concept.
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Fig. 5

The discharge capacity versus cycle number curves of the prepared LiMn2O4 with different method in 3.0–4.4 V at a constant charge–discharge density of 1.0 mA cm−2

Conclusions

Lithium manganese oxide nanorods were prepared from manganese dioxide nanorods precursor. The phase has been characterized by XRD. The structure refinement data indicated that the nanorods preferentially grew along the [111] direction. The product morphology was further confirmed using TEM. After charge–discharge test at 1.0 mA cm−2 in 3.0–4.4 V, the nanorods LiMn2O4 showed the 134.5 mAh g−1 initial discharge capacity and only lost 1.1% of initial capacity after 30 cycles.

Acknowledgment

This work was supported by Hubei Provincial Natural Science Foundation of China (No. 2007ABA345).

Copyright information

© Springer Science+Business Media B.V. 2009